Revisions: The Developer, by separate instrument, grants to the Home Owners Association the right and responsibilities regarding the maintenance of swales, pond banks and all the associated drainage easements to the original design conditions

HILLSBOROUGH COUNTY

STORMWATER MANAGEMENT TECHNICAL MANUAL

Copy Registration Number:

PREFACE

The following package contains all current information for designing and submitting construction plans for all proposed stormwater management facilities to be constructed in unincorporated Hillsborough County.

This Technical Manual contains Criteria and Procedures for designing stormwater management systems. The Criteria includes both design criteria and material specifications which shall be adhered to when submitting plans for approval by Hillsborough County. The Procedures in this Manual are not mandatory, but are suggested for meeting the County Criteria. However, the Procedures are not to be construed as Criteria. The following chapters and/or sections describe the County Criteria which must be met in the design of stormwater management systems. Anything that is not within these chapters and/or sections is not to be construed as criteria:

* Chapter 3 - Sections 1.B, 1.C, 2, 3 and 4.
* Chapter 4 - Sections 1, 2, 4.B.3.b, 4.C.3.d, 6.E, 7.B, and 8.
* Chapter 5 - Sections 2, 3.A.1, 4, 5 and 6.
* Chapter 6 - Section 2.
* Chapter 7 - All (except 1.A)
* Chapter 8 - All
* Chapter 9 - All
* Chapter 10 - All (except 1)

STORMWATER MANAGEMENT TECHNICAL MANUAL REVISION PROCEDURE

As methodologies, material specifications, and technical criteria change to meet new needs and changing technology, it will become necessary to revise and update this manual. These revisions will be handled administratively under the direction of the County Administrator.

Any proposed revisions to this manual will be mailed to registered recipients of this technical manual for comment. Recipients of the proposed revisions will have twenty-one calendar days to provide written comments to the Director of Engineering Services. An informal workshop attended by staff and any interested parties will be held to discuss the proposed revisions. If staff is of the opinion that additional review and/or workshops are necessary, all registered recipients of this technical manual will receive written notification of this action. If, after a proposed revision has been thoroughly reviewed and discussed in at least one informal workshop, staff holds the opinion that the proposed revision is in the best interest of Hillsborough County and should be incorporated in this manual, then a recommendation with dissenting viewpoints, if any, will be forwarded to the County Administrator for action.

All revisions approved by the County Administrator will be mailed in a timely manner to registered recipients of this technical manual. The effective date of such changes shall be no sooner than 180 days from the date of approval of the revision by the County Administrator.

The preparers of this manual have gone to great lengths to develop a manual that is accurate, consistent, and free from errors and conflicts. However, there is no fool proof way to assure that a technical document of this type is totally free from imperfections. Therefore, in the unlikely event that a conflict exists in this manual or is created by an approved revision to this manual, the Director of Engineering Services shall send a written letter of clarification of the conflict to all registered recipients of this manual. A recommendation will be forwarded to the County Administrator for approval of a revision to resolve or remove the conflict. The effective data of "conflict revisions" will be the date of the approval by the County Administrator. The resolution of design problems directly related to criteria conflicts in this manual will be handled on a case-by-case basis by the Director of Engineering Services. Public health, safety and welfare, economic impact, and due diligence by the site designer will all be considered in the resolution of these design problems.

Finally, correction of any typographical errors contained herein that do not materially and significantly affect criteria will not require approval by the County Administrator. All corrections of this nature will be handled by written notification of registered recipients and will be effective immediately.

THE STORMWATER PRODUCT REVIEW COMMITTEE

The Stormwater Product Review Committee evaluates new and existing products for efficient and economical utilization within the County Stormwater Management system. The Committee is charged with the development of a fair and reasonable methodology to systematically evaluate products for use through academic research and field evaluation.

The Committee is comprised of representatives of the Engineering, Road and Street Maintenance, Planning & Development Management, and Capital Projects Departments. The representatives have technical and/or management positions and either supervise maintenance or construction personnel or have a background in maintenance and/or construction.

CHAPTER 1

INTRODUCTION

This document has been adopted by reference in a number of County Ordinances. Unless specifically stated otherwise, any proposed designs in conflict with the criteria portion of this Manual will require either a waiver of the applicable County Ordinance or approval of the County Administrator, whichever is applicable. In addition, the County Administrator shall be solely responsible for interpreting any criteria in this Manual which may be deemed vague or uncertain. Furthermore, the interpretation shall be in the best interest of the citizens of Hillsborough County.

1.1 PURPOSE

The purpose of this Manual is to guide engineers, architects, planners and developers in the design of stormwater management systems in Hillsborough County. The Manual integrates recommended methodologies, design procedures, standards and County Stormwater Criteria into a single-source document. The intent of the Manual is to (1) standardize criteria and present suggested procedures and design aides, (2) make it compatible to the Hillsborough County Capital Improvement Program and the Stormwater Management Element of the Comprehensive Plan. This manual represents a coordinated effort to bring water resource managers, developers and designers up-to-date with the regulations and criteria applicable to stormwater management design in Hillsborough County. As an integral part of the Hillsborough County Stormwater Management Master Plan, this manual will be utilized by Hillsborough County for permitting, study, review and design.

1.2 OBJECTIVES

Design criteria presented in this manual have been established to enable architects, engineers, planners and developers to accomplish the following objectives:

1. Protect human life, health and welfare.

2. Minimize private and public property damage resulting from erosion, sedimentation and flooding in and adjacent to the proposed developments and drainage networks.

3. Provide a technically efficient drainage system.

4. Provide a cost-effective drainage design.

5. Provide an information source to alert owners, builders and developers and the general public of potential flood hazards.

6. Maintain or enhance the quantity and quality of groundwater supplies.

7. Present guidelines to design for the maximum beneficial use of water resources.

8. Reduce the occurrence and loading of contaminants into natural water bodies.

9. Harmonize, to the greatest practical extent, Hillsborough County design criteria with those of other agencies.

10. Provide for the least possible disturbance to community welfare and to the environment during construction.

11. Develop Best Management Practices (BMPs) to utilize public open space for water storage while producing an aesthetically pleasing appearance.

Cooperation between the developers and Hillsborough County is necessary since many of the above objectives are attainable only when a balance between profit and social acceptability is reached. This is especially true since many of the objectives are controlled by the specific design.

1.3 ORGANIZATION OF THE MANUAL

The Hillsborough County Stormwater Management Technical Manual integrates into a single source: criteria, recommended methodologies, design procedures, tables, and graphs, to guide Site Designers in a concise and informative manner. Discussion focuses upon three major topics, the contents of which are summarized below:

A. RELATIONSHIP BETWEEN COUNTY ORDINANCES, REGULATIONS AND POLICIES

Familiarizes the designer/manager/developer with Hillsborough County's applicable ordinances, regulations, and policies. Concise summaries focus upon the purpose and scope of policies pertinent to stormwater management.

B. COUNTY STORMWATER CRITERIA AND STANDARDS FOR DESIGN

Incorporates drainage criteria presented in the County's regulatory documents into a single source. Standards for design are presented in a concise, informative manner.

C. APPENDICES

A Glossary, Referral Index and Bibliography are included as appendices for definitions and referencing.

Tables and figures presented in the Manual represent guidelines for commonly encountered situations. A vast array of additional design aides can be found in the referenced literature. The designer is encouraged to refer to the original sources for additional reference materials. A bibliography is presented in Appendix C.

1.4 ROLE OF HILLSBOROUGH COUNTY IN STORMWATER MANAGEMENT

In order to effectively deal with development pressures, Hillsborough County developed a planning document referred to as The Future of Hillsborough Comprehensive Plan, 1989. The Hillsborough County Stormwater Management Master Plan (PLAN) is the direction the County is taking to achieve the stormwater goals of the Comprehensive Plan. In order to implement and develop the PLAN, County staff is presently responsible for the following:

A. REVIEW OF PERMITS AND REZONINGS

Staff reviews all permit applications and rezonings to ensure that they meet all County criteria related to drainage and to ensure that other properties are not impacted by flooding as a direct result of development. The types of applications reviewed relate to subdivisions, commercial sites, parks and recreation and any land alterations. The Hillsborough County Environmental Protection Commission (HCEPC) regulates water quality and environmental concerns for the County. Included in this report is a bibliography of applicable ordinances, regulations and policies relevant to stormwater (Appendix C).

B. OVERSEEING COUNTY FLOOD DAMAGE CONTROL ORDINANCE

Staff reviews all proposed developments and rezonings to assure that requirements of the Federal Emergency Management Agency are being met relative to flood zones. The Hillsborough County Flood Damage Control Ordinance (Hillsborough County, 1978 and subsequent revisions) is the legislation utilized to evaluate proposed developments and rezonings. Hillsborough County is an Agent of FEMA. The County encourages Federal Insurance Rate Map amendments to be filed by the Site Designer as part of the design process when the flood zone designation of portions of the development are changed.

Staff also assists the public in determining the proper flood zone designation for residential properties.

County staff will indicate to the developer if a 100-year flood elevation has been determined previously. Otherwise, if the property is located in a Flood Zone A, it will be the responsibility of the Site Designer to set an acceptable 100-year flood elevation. In order for the County to determine a Flood Zone and 100-year elevation, a Flood Zone/Elevation Determination Request form is required from the applicant. A legal description of the property in question is also necessary.

C. DEVELOPMENT, REVIEW AND INSPECTION OF PLANS, DESIGNS AND CONSTRUCTION OF DRAINAGE WORKS

There are numerous drainage projects being developed throughout the County; some are included in the Capital Improvement Program and others are out-of-cycle projects. The out-of-cycle projects are the result of unforeseen local drainage problems.

Complex projects are evaluated and designed in the following sequence:

1. Preliminary design, including hydrologic and hydraulic evaluations.

2. Final design, including right-of-way and easement requirements, if any.

3. Construction of project. The County is responsible for overall project management, including inspection during construction and contract management.

Staff also analyzes, designs and coordinates permits on smaller projects and assists all roadway projects by either designing or reviewing the drainage portions of the projects.

D. CITIZEN REPORTED DRAINAGE PROBLEMS

Hillsborough County is responsible for evaluation and recommendation of corrective action for citizen reported drainage problems. Flooding problems can be caused by overgrown ditches, inadequate outfalls, nonpermitted obstructions that hinder conveyance of runoff, inadequate storm sewer systems and catastrophic rainfall events.

Reports of drainage problems are received via Action Orders, the County Administrator's Office, or by direct mail or by direct calls to staff. Reported problems are handled in the following manner: The County Road and Street Maintenance Department screens the reports. If design assistance is needed, the report is forwarded to the Stormwater Design staff of the Engineering Services Department or to the Planning & Development Management Department Stormwater Team, if a project (subdivision or commercial) less than two (2) years old is involved.

E. IMPACTS OF MINING, LAND EXCAVATION AND LANDFILL ACTIVITIES

Mining, land excavations and landfills all have an effect on the natural drainage of the land. Consequently, steps must be taken to provide for adequate drainage when the land is altered for these activities.

Staff evaluates the impact of the alterations by reviewing land excavation applications, including on-site inspections. When necessary, recommendations are made to the Planning & Development Management Department if there is a drainage problem. Under such circumstances Planning & Development Management staff meets with the Board of County Commissioners (BOCC) during Land Use Public Hearings. Presently, staff reviews the drainage element (other than water quality) of mining activities (e.g. phosphate industry).

F. IMPLEMENTATION OF A DITCH MAINTENANCE PROGRAM

Ditch maintenance is essential in reducing flooding problems throughout the County. There are two factors that are important in conducting this maintenance -- manpower and permits. The Maintenance Units are responsible for clearing ditches of shoaling, herbaceous growths and debris.

The Florida Department of Environmental Regulation (FDER) is the state agency that issues permits for the clearing of ditches. There are two categories of ditches that FDER evaluates: (1) jurisdictional and maintainable and (2) not maintainable. The County is working with FDER in obtaining ditch maintenance permits.

G. PRIORITIZATION AND SCHEDULING OF FUTURE PROJECTS

A comprehensive prioritization and scheduling program for future projects will be recommended to the BOCC for implementation as a part of the County 5-Year Capital Improvement Program. The program has been implemented within budgetary constraints.

H. DATA COLLECTION PROGRAM

Major sources of data are federal agencies (e.g. United States Geological Survey, hereafter referred to as U.S.G.S.), state agencies (e.g. Southwest Florida Water Management District, hereafter referred to as District) and local government agencies (e.g. City of Tampa, hereafter referred to as City). This information is being obtained and cataloged by the County.

I. EXISTING BASELINE INFORMATION

There is presently a significant volume of information available on water quantity and quality related to stormwater management in the County. This information which is in/on reports, books, journals, maps, plans, microfiche and electronic media is available for viewing at County department offices, the District and the University of South Florida Library.

J. INTERAGENCY REGULATION

Drainage related activities may be subject to a variety of regulations and/or permitting requisitions in addition to those required by Hillsborough County. Stormwater management design criteria which may differ from Hillsborough County Criteria are specified by various jurisdictional agencies at federal, state and local levels.

Site designers are responsible for assuring that all appropriate permits or approvals are obtained prior to construction of the site.

CHAPTER 2

COUNTY LAND DEVELOPMENT CODE (LDC), REGULATIONS AND POLICIES

2.1 INTRODUCTION

The purpose of this chapter is to briefly describe County regulations and policies as they pertain to stormwater. Detailed description of each will not be included in this Manual as a result of the frequency of rule revisions. Alterations could outdate this Manual frequently and require constant revisions. Therefore, one should refer to the specific rule to determine up-to-date requirements.

2.2 SUBDIVISION REGULATIONS (LDC, DIV. 3.3)

The purpose of the Hillsborough County Subdivision Regulations (applicable to stormwater standards) is to establish procedures and standards which (1) insure that development is responsive to the environment and (2) prevent periodic and seasonal flooding through properly designed flood control systems and drainage facilities. Standards and design criteria presented in the Manual supersede those listed in the Subdivision Regulations. Criteria has been assembled into a single source, and modified to allow effective stormwater management.

2.3 LAND ALTERATION AND LANDSCAPING REGULATIONS (LDC, DIV. 3.4 AND 3.5)

Regulations and standards are specified for land alteration, vegetational protection and the installation and maintenance of landscaping. Regulation of land alteration includes standards and guidelines for the protection of soil and water resources, flooding control, and for the protection of environmentally sensitive areas. Landscaping requirements include specific provisions for land use compatibility and stormwater detention ponds among other factors.

2.4 FLOOD DAMAGE CONTROL REGULATIONS (LDC, DIV. 3.14)

The Hillsborough County Flood Damage Control Regulations provide standards for "all areas of special flood hazards" within Hillsborough County. Standards outlined in this ordinance are intended to (1) restrict or prohibit uses which cause excessive water or erosion due to flood heights and velocities, (2) to protect construction areas from flooding, (3) to control the alteration of natural floodplains, channels and barriers and (4) to regulate developments which increase erosion or flood damage.

Requirements are specified for residential and commercial construction, mobile homes, floodways, and coastal high hazard areas.

2.5 LAND EXCAVATION REGULATIONS (LDC, DIV. 3.7)

Land excavation permits are required by the County. In terms of drainage standards, the County is primarily concerned with proximity to environmentally sensitive areas, water depth, and the impact that the excavation would cause to the surrounding community. In addition to specifying standards, this regulation specifies permit application requirements, reclamation and reuse plans, public hearing considerations, and permit application procedures.

2.6 SITE DEVELOPMENT REGULATIONS (LDC, DIV. 3.4)

A drainage review pursuant to this regulation is required for all commercial, industrial, office and residential developments which are not subject to the drainage requirements of the Hillsborough County Subdivision Regulations.

2.7 ZONING CODE (LDC, ARTICLE 2)

The Hillsborough County Zoning Code affects all buildings, structures, lands and waters within the unincorporated portions of Hillsborough County. Stormwater management is not dealt with directly in the County's Zoning Code. However, Division 2.6 establishes a Special Use Permit system for uses, occupancies, and activities of a temporary nature or those which may potentially cause adverse impacts. Special Use Permit applications may require a site plan which includes storm drainage and sanitary sewage disposal plans. According to Paragraph 2.6.2.6 of the LDC:

"Due consideration shall be given to provision for drainage, with particular reference to effect on adjoining and nearby properties and on general drainage systems in the area. Where major drainage volumes appear likely and capacity of available systems is found marginal or inadequate, consideration shall be given to possibilities for recharge of groundwater supply on the property, temporary retention with gradual discharge, or other remedial measures."

In addition, drainage standards and criteria may become an issue within Planned Development Districts.

2.8 PHOSPHATE MINING REGULATIONS (LDC, DIV. 3.8)

This regulates activities associated with phosphate mining and processing to insure that resource development is compatible with the overall needs and development of Hillsborough County. This Ordinance applies to all phosphate mining activities conducted within the boundaries of Hillsborough County. Drainage requirements related to phosphate mining are set forth in the specification for permit submittal application and are to be included within the applicant's Mining and Reclamation Plan.

2.9 BUILDING AND CONSTRUCTION CODE

Building code provisions do not directly address stormwater drainage issues except that finish floor elevations may be required to be at or above a specific height above adjacent roadways. Such issues, however, may become important if building in a flood zone and/or tidal area. Under such circumstances, the Hillsborough County Subdivision Regulations and Flood Damage Control Regulations would apply.

2.10 FUTURE OF HILLSBOROUGH COMPREHENSIVE PLAN

Hillsborough County's Comprehensive Plan offers a guide for future land development to the year 2010. Goals, objectives and policies are classified into five categories: General Development, Residential, Commercial, Industrial and Agricultural. Policies included in the plan address future growth issues and are used for planning and development in unincorporated Hillsborough County. The Horizon 2000 Revised Land Use Element defines and discusses water bodies and environmentally sensitive areas in the context of determining land use densities. Discussion of stormwater management is revised from the Stormwater Management Element.

2.11 FUTURE OF HILLSBOROUGH COMPREHENSIVE PLAN - STORMWATER MANAGEMENT ELEMENT

The drainage element examines hydrologic modification caused by drainage activities, quantity and quality control strategies, flooding problems, and the cost of drainage facilities to both the public and private sectors. Attention is focused primarily upon retaining runoff on-site, through the incorporation of retention and/or maintained percolation, minimizing public expenditures on future drainage activities, ensuring wise floodplain management techniques, and ensuring that a proper hydrologic balance is maintained (Board of County Commissioners of Hillsborough County).

2.12 RULES OF THE HILLSBOROUGH COUNTY ENVIRONMENTAL PROTECTION COMMISSION - WATER POLLUTION (CHAPTER 1-5)

The Hillsborough County Environmental Protection Commission (HCEPC) specifies water quality criteria for Public Water Supplies, Shellfish Harvesting, Recreation - Propagation and Management of Fish and Wildlife, Agricultural and Industrial Water Supplies and Navigation, Utility and Industrial Uses (Classes 1 to 5 Waters, respectively). In addition, the HCEPC specifies minimum standards for earthen dams. According to these standards, "drainage facilities shall be provided to maintain the water level on the outside of the dam within design limitations" (Hillsborough County Environmental Protection Agency, 1987).

2.13 RULES OF THE HILLSBOROUGH COUNTY ENVIRONMENTAL PROTECTION COMMISSION - WETLANDS (CHAPTER 1-11)

County policy for wetland identification, delineation and mitigation is regulated by the Hillsborough County Environmental Protection Commission (HCEPC) in this rule. Discussion of surface water is limited to the HCEPC's review of proposed development within the wetlands. Stormwater design criteria is not included in this chapter.

CHAPTER 3

GENERAL STORMWATER MANAGEMENT SYSTEM DESIGN STANDARDS/CRITERIA

3.1 REQUIREMENTS APPLICABLE TO ALL DEVELOPMENTS REQUIRING REVIEW

A. STORMWATER MANAGEMENT SYSTEM REVIEW

1. Stormwater management systems shall include water quality treatment in accordance with the rules and regulations of the Southwest Florida Water Management District, and any other federal/state/local agency having jurisdiction.

2. Provisions shall be included in the site development construction plans to control soil erosion and sedimentation both during and after the construction phase of the development.

3. A stormwater management system review will be required regardless of size of project for all of the following types of developments:

a. Residential/Subdivision

b. Commercial

c. Industrial

d. Multi-family/Non-subdivision

4. Impervious areas shall include, but not be limited to:

a. Buildings

b. Asphalt surfaces

c. Concrete

d. Shell

e. Limerock

f. Any other material either temporary or permanent which will shed over 70% of the water falling upon it.

5. These standards, unless otherwise noted, shall apply to all developments requiring review.

B. STORMWATER POND DESIGN CRITERIA

The capacity of a "receiving waters" is classified by the County as more than adequate, adequate, peak sensitive or volume sensitive. It is the responsibility of the Site Designer to classify the capacity of the receiving waters for a particular project site based upon the following

criteria; and to utilize the corresponding design standards defined in the following criteria:

1. More Than Adequate Capacity:

a. These receiving waters are sizable water bodies (i.e. Tampa Bay) which have inbank storage and/or conveyance capacity significantly in excess of that required to accommodate the 100-year event, and for which an increase in flow will not cause a measurable increase in water surface elevation. The requirements for any proposed construction or upgrading of connecting facilities to these sizable water bodies will be established on a case-by-case basis by the County Administrator and will include a minimum design event of at least the 100-year return frequency assuming post-development conditions. It is strongly recommended that the Site Designer have a pre-design meeting with the Engineering Review Manager prior to submitting the site design so that specific design frequencies and freeboard requirements can be established.

b. The Developer must submit a stormwater management plan along with his design which demonstrates, via calculation, that direct discharge to the receiving waters without attenuation of flow will not measurably raise the water surface elevation.

c. This design standard in no way precludes the stormwater treatment requirements of any other local, state or federal agency.

d. Additional stormwater pond design criteria is contained in Chapter 7.

2. Adequate Capacity:

a. These receiving waters have no known inadequacies or flooding problems during the 25-year, 24-hour storm, but could experience an increase in water surface elevation if the inflow rate is increased.

b. A development discharging into this type of receiving waters must be designed with a post-development peak discharge rate, due to a 25-year, 24-hour storm event, not to exceed the pre-development peak discharge rate for the 10-year, 24-hour storm event.

c. Projects which do not discharge directly into a well-defined conveyance system, (i.e. ditch, storm sewer, etc.) in the pre-developed state must utilize the design standard defined in Section 3.1.B.4, unless otherwise specifically approved by the County Administrator.

d. Additional stormwater pond design criteria is contained in Chapter 7.

3. Peak Sensitive Capacity

a. These receiving waters generally have histories of flooding problems related to resistance and restrictions within the channel and/or inadequate conveyance structures, and thus have inadequate flow capacities. Information on areas designated as having peak sensitive capacity can be obtained from the County Administrator. In addition, all storm sewer collection systems which are used to convey discharge from stormwater ponds are considered to have peak sensitive capacity.

b. Developments discharging into this type of receiving waters must be designed to meet the more stringent of the following conditions:

1. The post-development peak discharge rate for the 25-year, 24-hour storm event is not to exceed the pre-development peak discharge rate for the 10-year, 24-hour storm event (the maximum allowable flow rate).

2. The post-development flow rate is not to exceed the area-weighted capacity of the receiving waters unless the Site Designer can demonstrate, via detailed calculations, that a larger discharge rate can be used for the design.

c. The receiving waters area-weighted capacity shall be determined according to the following criteria:

1. Identify the potential locations of the most restrictive portions of the receiving waters downstream from the project site. This investigation should proceed downstream to the point where the receiving waters no longer has documented flow restriction problems.

2. Compute the flow capacities at the identified restriction locations as the flow corresponding to the respective elevation below which there should be no adverse flooding or excessive erosion. Typically, these critical flooding elevations correspond to a roadway or finished floor or top-of-bank elevation, whichever is most appropriate at a location.

3. Divide the computed flow capacities by the respective upstream drainage areas which contribute to the flows. The lowest of these "flow per acre" values will correspond to the "critical restriction" and the area-weighted capacity of the receiving waters relative to the project location.

4. The Site Designer is encouraged to contact the Engineering Review Manager to obtain the area-weighted capacity data for those receiving waters designated as having peak sensitive capacity.

d. If the Site Designer wishes to demonstrate that a flow, greater than that computed by application of the appropriate area-weighted capacity but less than the 10-year 24-hour pre-development peak flow for the site, can be used for design, an analysis of the timing of post-development peak flows will be required. The analysis shall show that the flow (which includes the proposed post-development contribution from the project site) at the critical restriction at the time of arrival of the proposed post-development peak flow from the project site, will be less than the capacity of the critical restriction (with capacity as defined in Section 3.1.B.3.c.2).

e. A minimum allowable peak discharge of 1 cfs from the stormwater pond may be assumed if the project site exceeds 2" acres and the 10-year, 24-hour pre-development peak flow for the site exceeds 1" cfs.

f. As an alternative to the evaluation of the peak sensitive capacity criteria, the Site designer can choose to design in accordance with the criteria contained in Section 3.1.B.4.a.2.b.

g. Additional stormwater pond design criteria is contained in Chapter 7.

4. Volume Sensitive Capacity:

a. These receiving waters, also referred to as "blinds", do not have positive outfall for storm events less than or equal to the 25-year, 24-hour event. In addition, sites which do not directly discharge into a well-defined conveyance system (i.e. ditch, storm sewer, etc.) are considered to have volume sensitive capacity since they do not have positive outfall.

1. Positive outfall is defined as the ability to discharge directly into a manmade or natural channel, waterway or pipe system which is part of a receiving waters which has more than adequate, adequate or peak sensitive capacity.

2. In such areas the site shall be designed in accordance with either of the following criteria:

a. The difference between the pre-development and post-development runoff volumes, due to the 100-year, 24-hour rainfall event, shall be retained on-site. This design storage volume must be again available within 72 hours after the end of the design storm event. The Site Designer must demonstrate that percolation, alone, is sufficient to discharge the design storage volume within the 72-hour period. Runoff volume in excess of the design storage volume is not to be discharged from the site during the 100-year, 24-hour storm event, at a rate greater than that corresponding to the pre-development 10-year 24-hour event.

b. The total post-development runoff volume due to the 100-year, 24-hour rainfall event shall be retained or detained on site. The pond shall be designed to bleeddown during the 100-year/24-hour design event, at a rate which will result in a discharge of approximately one inch of runoff from the total area contributing to the pond, within 24 hours of the inception of inflow to the pond. An exception to this bleeddown criteria can be made by the County Administrator when other agency bleeddown criteria is more stringent. When percolation is the only means of discharge from the pond, one inch of runoff is the minimum volume to be discharged in the 24-hour period described above.

3. In those cases where discharge from a site under the pre-development condition is via sheetflow, final disposal of allowable discharge under the post-development condition shall be via a spreader-swale (or some similar mechanism) to reestablish this sheet flow. Exceptions to this criteria are:

a. If the adjacent, downgradient development has provided a proper point of direct entry into a well-defined conveyance system (i.e. ditch, storm sewer, etc.) within which the necessary capacity is available, the point discharge will be permitted.

b. If the adjacent, downgradient land is existing residential property and the required stormwater pond is to be designed according to 3.1.B.4.a.2.b, proof of an appropriate drainage easement(s) from the affected property owner(s) will be required for either a sheetflow discharge or a point discharge, whichever is most practical. If Hillsborough County is to maintain the site's stormwater management system, the appropriate drainage easement(s) shall be for a point discharge and shall be donated to the County.

C. CAPITAL IMPROVEMENTS PROGRAM COORDINATION

The Site Designer will verify that his design is compatible with the design of any capital improvement projects that may impact the site or that the site may impact. If design conflicts are encountered, the design constraints imposed by the capital improvement project shall take precedence.

3.2 DEVELOPMENTS IN FLOODPLAINS

A. CRITERIA FOR DEVELOPMENT

1. The criteria for development in floodplains pertains to all floodplains and is not limited to those defined on FEMA maps. The Site Designer is responsible for determining the on-site 100-year flood elevations if not defined by a FEMA detailed study. The Site Designer is encouraged to submit a Letter of Map Amendment or Map Revision to FEMA for any changes in flood zone designations as determined by a detailed study of the area.

2. No development (structures and/or fill) shall be allowed in the conveyance portion of any 100-year frequency floodplain associated with a freshwater stream, channel, lake or waterway unless provisions are made to compensate for any reduction in conveyance caused by the development.

3. No development (structures and/or fill) shall be allowed in any 100-year frequency non-tidal floodplain unless provisions are made to compensate for the reduction in storage volume due to the proposed development.

a. Any compensation storage volumes must be provided in addition to stormwater detention or retention volumes otherwise required to reduce peak runoff rates from the development. Credit for partial usage of stormwater pond storage volume for both compensation storage volume and peak discharge attenuation storage volume shall be appropriate if the Site Designer can show that it is applicable by a detailed analysis of the hydrologic timing of the water surface elevations within the watershed. County Administrator approval is required when this partial or dual credit concept for storage is utilized in the stormwater system. For developed conditions, the controlled seasonal high groundwater table elevation will be used in the analysis and design. Controlled seasonal high groundwater is the elevation of the seasonal high groundwater after site modifications are completed.

b. No earth fill may be placed within a 100-year floodplain area unless an equal amount of flood storage volume is created by excavation below the 100-year flood elevation and above the seasonal high groundwater table elevation or controlled seasonal high ground water table elevations, whichever is appropriate.

c. Exceptions are allowed if the floodplain is associated with a landlocked waterbody and is under one ownership.

4. If the development is a highway or similar facility requiring a "causeway" type encroachment across a floodplain, a small cumulative increase (0.1" ft.) in upstream off-site elevations will be allowed. The 0.1" foot off-site elevations allowance shall be waived if all upstream affected property owners agree, via deed restrictions, to accept the increased flooding.

5. No encroachment will be allowed in a regulatory floodway, as designated on the FEMA Floodway Maps, unless approved by FEMA and subsequently accepted by Hillsborough County.

B. INTERAGENCY JURISDICTION

If a conflict exists between these requirements and the Hillsborough County Flood Damage Control Ordinance, or Southwest Florida Water Management District criteria, etc., the most restrictive requirements shall apply.

3.3 STORMWATER MANAGEMENT DATA REQUIREMENTS

A. STORMWATER MANAGEMENT PLAN MAPS

The Site Designer shall include with the construction plan submittal a Master Stormwater Management Plan Map showing all existing and proposed land features. An appropriate scale shall be used to adequately represent the information on the map. A Master Stormwater Management Report shall be prepared as the technical backup for the Master Stormwater Management Plan. The following information shall be included in either the Master Stormwater Management Report or on the Master Stormwater Management Plan:

1. Vicinity sketch and legal description.

2. Basin and sub-basin boundaries, including all on-site and off-site areas contributing to the site, and the breakdown of the subarea(s) contributing to each inlet in the internal stormwater collection system.

3. Topographic Site Data showing existing contours and spot elevations based on NGVD 1929. Show contours to a minimum one (1) foot interval to 25' outside the project boundaries. The Engineer of Record must give the source of such topographic data and certify as to its currency.

4. Flow paths used to determine the basin and sub-basin times-of-concentration.

5. Existing drainage features (ditches, ponds, etc.). Existing features are to be shown within and downstream of the proposed development. The downstream distance is to be determined from the following, whichever is applicable:

a. The Site Designer shall field investigate drainage patterns and stormwater management facilities within at least 1000 feet of the site, and shall include this information in the stormwater management system documents.

b. The Site Designer shall demonstrate that the assumed design tailwater conditions are appropriate.

6. Highwater data upstream and downstream of the proposed development.

7. Notes pertaining to standing water, springs, areas of seepage and sources of highwater data.

8. Proposed development layout with horizontal and vertical controls.

9. Proposed stormwater management system features including the locations of inlets, swales, ponds, conveyance systems, easements, etc.

10. General soil characteristics obtained from the Hillsborough County Soil Survey and existing land uses/ground cover.

11. Flood zone designation determined from the Flood Insurance Rate Maps. Elevations of the flood zone along with the Flood Hazard boundary shall be delineated on the drainage plans.

B. STORMWATER DESIGN CALCULATIONS

1. Report

Stormwater design calculations shall be submitted in a bound report. The report shall contain all hydrologic and hydraulic calculations and assumptions used to design the proposed development. A soils report shall be included per Section 7.2 and 7.3 requirements. The stormwater design calculations report shall be signed and sealed by a professional engineer registered in the state of Florida. When a separate geotechnical report is included with the stormwater design calculations, it shall be signed and sealed by the responsible professional engineer.

2. Computer Programs

All reports containing computer generated information shall include input and output data. Any calculation generated by a program not recognized by Hillsborough County may be checked by accepted programs. The Site Designer shall be responsible for clarifying any discrepancies between programs. A list of acceptable computer programs are available from the County Administrator.

3.4 DRAINAGE EASEMENT CRITERIA

A. GENERAL CRITERIA

Drainage easements shall be provided to Hillsborough County for all stormwater management facilities to be maintained by Hillsborough County. Off-site drainage easements may be required in cases where the performance of minimum maintenance activities associated with roads and stormwater management facilities to be dedicated to Hillsborough County would not be practical without such easements. In the case of subdivisions which are to be privately maintained, the design and maintenance requirements are the same as those for stormwater management facilities that are to be dedicated to Hillsborough County.

B. ENCLOSED STORMWATER CONVEYANCE SYSTEMS

1. Enclosed stormwater conveyance systems shall be located in drainage easements or road right-of-ways dedicated to Hillsborough County if they are to be maintained by Hillsborough County.

2. For enclosed stormwater conveyance systems not within road right-of-ways, the drainage easement width shall be sufficient to encompass a work trench having 1:1 sideslopes (measured from the proposed ground surface to the proposed invert of the enclosed stormwater conveyance system) and a bottom width two (2) feet wider than the total width of the installed conveyance system.

3. The drainage easement width shall not be less than twenty (20) feet unless otherwise approved by the County Administrator.

C. CANALS AND DITCHES

Canals and ditches shall have sufficient drainage easement dedicated to Hillsborough County to allow for installation of the canal or ditch including an unobstructed twenty (20) foot wide maintenance area on both sides, measured from the top of the bank, unless otherwise approved by the County Administrator.

D. DETENTION AND RETENTION PONDS

If the pond is to be maintained by Hillsborough County, sufficient drainage easements shall be dedicated to Hillsborough County to include the area of the pond within the perimeter of the inside top of bank, and an unobstructed twenty (20) foot wide maintenance area around the entire perimeter of the inside top of bank. Alternatives to the width and/or extent of the maintenance area may be accepted by the County Administrator if it can be demonstrated that proper maintenance practices will not be impaired. If the maintenance area is on an embankment, the drainage easement shall extend to the external toe of slope of the embankment.

E. INGRESS/EGRESS

Sufficient perpetual, legal access shall be conveyed to Hillsborough County to provide ingress/egress to a drainage easement. This access shall be unobstructed and at least twenty (20) feet in width.

3.5 SWFWMD AS-BUILT CERTIFICATION AND OPERATION PERMIT REQUIREMENTS

Upon completion of all stormwater ponds constructed in Hillsborough County, the owner/developer shall comply with the SWFWMD As-Built Certification requirements prior to issuance of Certificates of Occupancy for Site Development Projects or Acceptance of Improvement Facilities for Subdivision Projects. For facilities to be maintained by Hillsborough County, acceptance of the maintenance responsibilities by the respective maintenance entities is required subsequent to County acceptance of the Improvement Facilities. The intent of this paragraph is not to withhold final approval (C.O.), but to insure that the process to convert drainage facilities from the construction to operations and maintenance phase of the S.W.F.W.M.D. permit is begun.

CHAPTER 4

DETERMINATION OF STORM RUNOFF

4.1 GENERAL

This chapter outlines the approved methods available to the Site Designer for estimating storm runoff. Of the many methods available, this manual makes use of four (listed below), which have proved convenient and reliable. In addition, other recognized methods may be used if their applicability can be demonstrated to the County Administrator.

* THE RATIONAL METHOD (for areas of ten (10) acres or less)

* THE MODIFIED RATIONAL METHOD VOLUME AND HYDROGRAPH GENERATION (for areas of ten (10) acres or less)

* SCS SYNTHETIC UNIT HYDROGRAPHY METHOD (SCS)

* SANTA BARBARA URBAN HYDROGRAPH METHOD (SBUH)

These methods should be used to calculate the discharge and runoff volumes resulting from rainfall events of specified frequency and duration.

4.2 RAINFALL CRITERIA

A. RATIONAL AND MODIFIED RATIONAL METHODS

The F.D.O.T. Zone 6 IDF curves (Fig. 4-2) shall be used in Hillsborough County when designing by the Rational or Modified Rational Methods.

B. SCS SYNTHETIC UNIT AND SANTA BARBARA URBAN HYDROGRAPH METHODS

1. The SWFWMD rainfall depths found in Figures 4-4 through 4-10 are to be used in Hillsborough County when designing by the SCS or SBUH Methods.

2. The only allowable rainfall distributions for design use in Hillsborough County are the SCS Type III (SCS Type II Florida Modified) and Hillsborough County (Delaney Creek) rainfall distributions. These distributions can be found in Tables 4-6a and 4-6b, respectively. Since the Hillsborough County (Delaney Creek) rainfall distribution is based on data specific to Hillsborough County, it is the recommended rainfall distribution. The same design rainfall distribution shall be used in the calculation of both the pre- and post- development runoff hydrographs.

4.3 TIME OF CONCENTRATION

The Time of Concentration is a common parameter of the above methods. It is recommended that the time of concentration be determined by the Velocity Method.

The velocity method is a segmental approach that can be used to account for overland, shallow channel, and main channel flows by considering the average velocity for each flow segment, and by calculating a travel time using the following equation:

t_i = L_i (4-1)

(60) V_i

Where:

t_i = Travel time for velocity segment (i), in minutes

L_i = Length of the flow path for segment (i), in feet

V_i = Average velocity for segment (i), in ft/sec

The time of concentration is then calculated by summing the individual segment travel times as follows:

t_c = t₁ + t₂ + t₃ + . . . . . . + t_i (4-2)

Where:

t_c = Time of concentration, in minutes

t₁ = Overland flow travel time, in minutes

t₂ = Shallow channel travel time (typically rill or gutter flow), in minutes

t₃ = Main channel travel time (typically storm sewer, swale, ditch, canal, etc.), in minutes

t_i = Travel time for the i^th segment, in minutes

A. OVERLAND FLOW

The Kinematic Wave Equation developed by Ragan (1971) should be used for calculating the travel time for overland flow conditions. The Kinematic Wave method is the recommended method for calculating the time of concentration for overland flow. Other methods may be used, however, the design will be checked with the Kinematic Wave Equation. Any significant differences in the calculation of the time of concentration for overland flow by methods other than the Kinematic Wave Equation must be approved by the County Administrator. Figure 4-1

presents a nomograph that can be used to solve this equation, which is expressed as:

t₁ = 0.93 L^0.6 N^0.6 (4-3)

I^0.4 S^0.3

Where:

t₁ = Overland flow travel time, in minutes

L = Overland flow length, in feet

N = Surface roughness coefficient for overland flow (See Table 4-1)

I = Rainfall intensity, in inches/hr, corresponding to the design storm frequency

S = Average slope of the overland flow path, in ft/ft

It should be noted that the surface roughness coefficient values shown in Table 4-1 were determined specifically for overland flow conditions and are not to be used for conventional open channel flow calculations.

TABLE 4-1

ACCEPTABLE "N" VALUES FOR USE IN THE KINEMATIC WAVE FORMULA

(Orange County, Florida, Subdivision Regulations June 3, 1985)

Land Use N-Value

Pavement 0.015

Bare Soil (Average Roughness) 0.05

Poor Grass Cover 0.2

Average Grass Cover or Lawns 0.4

Dense Grass Cover of Woodlands 0.6

Forest with Thick Humus/Litter 0.8

Layer and Dense Undergrowth

Equation 4-3 is solved by a trial and error process as follows:

a. Assume a trial value of rainfall intensity (I).

b. Determine the corresponding overland travel time (t₁), using Figure 4-1 or Equation 4-3.

c. Use t₁ from Step 2 as the critical storm duration and determine the actual rainfall intensity for this critical storm duration. Select the appropriate intensity-duration-frequency (IDF) curve from Figure 4-2.

d. Compare the trial and actual rainfall intensities. If they are different, select a new trial rainfall intensity and repeat the process.

B. SHALLOW CHANNEL FLOW (e.g. shallow swale, gutter, etc.)

Average velocities for shallow channel flow can be calculated using Manning's Equation or Figure 4-3.

C. MAIN CHANNEL (e.g. ditch, pipes, etc.)

Average velocities for main channel flow should be calculated using Manning's Equation.

4.4 RATIONAL METHOD

A. GENERAL

1. The Rational Method is an empirical method which is used to estimate peak discharge and has gained wide acceptance because of its simplicity.

2. This method relates peak rate of runoff or discharge to rainfall intensity, surface area and surface characteristics by the equation:

Q = CiA (4-4)

Where:

Q = Peak rate of runoff, in cubic feet per second

C = Rational coefficient representing a ratio of runoff rate to rainfall intensity (dimensionless)

i = Average rainfall intensity, in inches/hour, which is expected to occur for a duration equal to the basin time of concentration

A = Area of the drainage basin, in acres

B. ASSUMPTIONS

1. The peak rate of runoff at any point is a direct function of the average uniform rainfall intensity which occurs for a duration equal to the time of concentration to that point in the drainage basin.

2. The time of concentration of the drainage basin refers to the travel time required for the runoff to flow along the representative basis flowpath which is typically defined to be from the most hydraulically remote point of the drainage basin to the point of interest. Overland flow, storm sewer or gutter flow, and channel flow are commonly used in computing travel time.

3. The storm duration equals or exceeds the time of concentration of the basin.

a. The Rational Method, in general, tends to over estimate the rates of flow for larger areas, therefore the application of a more sophisticated runoff computation technique is usually warranted for large drainage areas.

b. Utilizing the Rational Method for determining pre-development discharge conditions and a hydrograph method for determining post-development discharge conditions is prohibited by Hillsborough County because of differences in the methodologies used for determining peak discharge.

c. Uniform rainfall distribution and intensity become less appropriate as the drainage area increases.

C. COMPONENTS

1. Peak runoff rate, (Q)

a. The peak runoff rate occurs when the duration of the precipitation event equals or exceeds the time of concentration of the drainage basin for a uniform rainfall intensity.

2. Rational coefficient, (C)

a. The Rational coefficient, (C), accounts for abstractions (losses) between precipitation (rainfall) and runoff.

b. Losses may result from:

1. interception by vegetation

2. infiltration into permeable soils

3. surface water retention

4. evapotranspiration

c. Additional considerations in determining C include:

1. climatological and seasonal variations

2. antecedent moisture conditions

3. intensity and frequency of the design storm

4. surface slope

d. Rational coefficients should be estimated by using the values listed in Table 4-2 for the 2- to 10-year design frequency storms. For 25- to 100-year frequency storms a correction factor (Table 4-3) is to be applied to the pervious areas unless flood routing computations are appropriate for the basin. Other values may be used, however, the design will be checked using Table 4-2 and Table 4-3.

e. When using these tables one should consider the following conditions:

1. level of development

2. surface types and percentages

3. soil type

4. slope

f. For basins with varying cover, a weighted Rational coefficient can be determined for the basin by the following equation:

Weighted C = C_iA_i (4-5)

A_Total

Where:

C_i = Rational coefficient for area A_i (dimensionless)

A_i = Area of the basin with a relatively uniform land cover, soil type, and slope, in acres

3. Rainfall Intensity, (i)

a. Rainfall intensity, (i), is the average rate of rainfall in inches per hour.

b. Design rainfall intensity is selected according to:

1. design frequency of occurrence

2. critical storm duration

c. Critical storm duration equals the time of concentration of the drainage basin.

d. Rainfall intensity is determined through the utilization of the FDOT Zone 6 rainfall curves (Figure 4-2).

4. Drainage Area, (A)

A specified portion of the hydrologic system which is bounded by drainage divides.

TABLE 4-2

RUNOFF COEFFICIENTS^a FOR A DESIGN FLOW RETURN PERIOD

OF 10 YEARS OR LESS

(from F.D.O.T. Drainage Manual, 1987)

Sandy Soils Clay Soils

Slope Land Use Min. Max. Min. Max.

Flat Woodlands 0.10 0.15 0.15 0.20

(0-2%) Pasture, grass & farmland^b 0.15 0.20 0.20 0.25

Rooftops and pavement 0.95 0.95 0.95 0.95

Pervious Pavements^c 0.75 0.95 0.90 0.95

SFR: 1/2 acre lots & larger 0.30 0.35 0.35 0.45

Smaller lots 0.35 0.45 0.40 0.50

Duplexes 0.35 0.45 0.40 0.50

MFR: Apartments, townhouses

and condominiums 0.45 0.60 0.50 0.70

Commercial and Industrial 0.50 0.95 0.50 0.95

Rolling Woodlands 0.15 0.20 0.20 0.25

(2-7%) Pasture, grass & farmland^b 0.20 0.25 0.25 0.30

Rooftops and pavement 0.95 0.95 0.95 0.95

Pervious pavements^c 0.80 0.95 0.90 0.95

SFR: 1/2 acre lots & larger 0.35 0.50 0.40 0.55

Smaller lots 0.40 0.55 0.45 0.60

Duplexes 0.40 0.55 0.45 0.60

MFR: Apartments, townhouses

and condominiums 0.50 0.70 0.60 0.80

Commercial and Industrial 0.50 0.95 0.60 0.95

Steep Woodlands 0.20 0.25 0.25 0.30

(7%+) Pasture, grass & farmland^b 0.25 0.35 0.30 0.40

Rooftops and pavement 0.95 0.95 0.95 0.95

Pervious pavements^c 0.85 0.95 0.90 0.95

SFR: 1/2 acre lots & larger 0.40 0.55 0.50 0.65

Smaller lots 0.45 0.60 0.55 0.70

Duplexes 0.45 0.60 0.55 0.70

MFR: Apartments, townhouses

and condominiums 0.60 0.75 0.65 0.85

Commercial and Industrial 0.60 0.95 0.65 0.95

^aWeighted coefficient based on percentage of impervious surfaces and green areas must be selected for each site.

^bCoefficients assume good ground cover and conservation treatment.

^cDepends on depth and degree of permeability of underlying strata, and includes unimproved required parking.

NOTE: SFR - Single Family Residential

MFR - Multi-Family Residential

TABLE 4-3

ANTECEDENT PRECIPITATION FACTOR FOR PERVIOUS AREA

RATIONAL COEFFICIENTS IN THE RATIONAL FORMULA

(From F.D.O.T. Drainage Manual, 1987)

Recurrence Adjustment

Interval (Years) Factor C_f

2 to 10 1.0

25 1.1

50 1.2

100 1.25

NOTE: 1. Adjustment factors for pervious areas only. For impervious areas the values in Table 4-2 are to be used for storm events greater than the 10-year.

2. Due to the increase in the time that the peak or near peak discharge rate is released from stormwater management systems, the use of these adjustment factors are not appropriate for flood routing computations.

4.5 MODIFIED RATIONAL METHOD INFLOW HYDROGRAPH APPROACH

For small drainage areas (less than 10 acres) an inflow hydrograph can be developed by utilizing the Modified Rational Method. Using the project drainage area (A), the project runoff coefficient (C), and the rainfall intensities (i) taken from the FDOT Zone 6 intensity-duration-frequency curves, an inflow hydrograph can be developed. The following table can be utilized to develop the inflow hydrograph.

TABLE 4-4

COMPUTATION FORMAT FOR THE MODIFIED

RATIONAL METHOD INFLOW HYDROGRAPH

(1) (2) (3) (4) (5)

Rainfall Rainfall Rainfall Accumulated Inflow

Duration Intensity Duration Rainfall

(Min) (In/Hr) (Hrs) (In) (Ac-ft)

1. Column (1) represents a chosen Time Interval from zero to the total time of the design storm expressed in minutes.

2. Column (2) is the rainfall intensity (i) for each rainfall duration increment for the given design storm frequency. The rainfall intensity values are obtained from the FDOT Zone 6 IDF curves.

3. Column (3) represents the chosen Time Interval from zero to the total time of the design storm expressed in hours.

4. Column (4) is accumulated rainfall for the respective rainfall duration. Column (4) is obtained by multiplying Column (2) by Column (3).

5. Column (5) is the inflow at each rainfall duration increment and is expressed in Ac-ft. Column (5) is obtained by first multiplying the drainage area (A) times the runoff coefficient (C). This value is then multiplied by Column (4) and divided by 12 to convert inches to feet.

4.6 SCS SYNTHETIC UNIT HYDROGRAPH METHOD

A. GENERAL

The unit hydrograph of a drainage basin (watershed) is defined as the runoff hydrograph which represents the time response to one (1) inch of rainfall excess (runoff) distributed uniformly over the basin during a specified period of time (time step).

1. Rainfall excess is the portion of the rainfall remaining after all losses or abstractions have been subtracted.

2. The specified period of time (time step) is the duration of the rainfall excess which, when chosen for design purposes should be a fraction of the basin time of concentration.

B. DETERMINATION OF RAINFALL EXCESS (RUNOFF)

Rainfall excess (runoff) by the SCS method can be expressed mathematically as:

R = (P - 0.2S)² (4-6)

P + 0.8S

S = 1000 - 10 (4-7)

CN

Where:

R = total rainfall excess for design storm event, in inches

P = total precipitation for design storm, in inches

S = soil storage parameter, in inches

CN = SCS Curve Number

C. SCS CURVE NUMBERS

The curve number is a dimensionless parameter that reflects vegetative cover condition, hydrologic soil group, land use, and antecedent moisture condition.

The recommended procedure for determining the SCS curve numbers for project areas within Hillsborough County is as follows:

1. Identify soil types within the project boundaries using the SCS Hillsborough County Soil Survey.

2. Assign a hydrologic soil group classification to each soil type using the appropriate table in Technical Release No. 55 (SCS 1987).

3. Identify areas with uniform soil type and land use conditions.

4. Use Table 4-5 (a-c) to select SCS curve number values for each uniform area based upon land use, vegetative cover, and hydrologic soil group.

5. If the project area is composed of variable land uses a composite CN can be developed as follows, if appropriate:

CN = ^Ai ^(CNi⁾ + ^Ai+1 ^(CNi+1⁾ + .....+ ^An ^(CNn⁾ (4-8)

A_t

Where:

CN = composite curve number for the watershed

CN_i = curve number for each sub-area

A_i = land area for each sub-area

A_t = total land area for the watershed

TABLE 4-5a

RUNOFF CURVE NUMBER FOR URBAN AREAS¹

(210-VI-TR-55, Second Edition, June 1986)

Average Curve Numbers

Percent for Hydrologic

Cover Description: Impervious Soil Group

Cover Type and Hydrologic Condition Area² A B C D

Fully Developed Urban Areas (Vegetation established)

Open Space (lawns, parks, golf courses, cemeteries, etc)³

Poor Condition (Grass cover <50%) 68 79 86 89

Fair Condition (Grass cover 50% to 75%) 49 68 79 84

Good Condition (Grass cover >75%) 39 61 74 80

Impervious Areas

Paved parking lots, roofs, driveways, etc.

(excluding right-of-way) (includes unimproved required parking) 98 98 98 98

Streets and Roads

Paved; curbs and storm sewers (excluding right-of-way) 98 98 98 98

Paved; open ditches (including right-of-way) 83 89 92 93

Gravel (including right-of-way) 76 85 89 91

Dirt (including right-of-way) 72 82 87 89

Urban Districts

Commercial and Business 85 89 92 94 95

Industrial 72 81 88 91 93

Residential Districts by Average Lot Size

1/8 acre or less (Townhouses) 65 77 85 90 92

1/4 acre 38 61 75 83 87

1/3 acre 30 57 72 81 86

1/2 acre 25 54 70 80 85

1 acre 20 51 68 79 84

2 acres 12 46 65 77 82

Developing Urban Areas

Newly Graded Areas (Pervious Areas Only, No Vegetation)⁴ 77 86 91 94

Idle Lands (CN's are determined using cover types similar to

those listed as other agricultural lands)

¹ Average runoff condition and I_a = 0.2S.

² The average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system. Impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition.

³ CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type.

⁴ Composite CN's to use for the design of temporary measures during grading and construction should be computed based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas.

TABLE 4-5b

RUNOFF CURVE NUMBERS FOR CULTIVATED LANDS¹

(210-VI-TR-55, Second Edition, June 1986)

Cover Description: Curve Numbers

for Hydrologic

Hydrologic Soil Group

Cover Type Treatment⁵ Condition⁶ A B C D

Fallow Bare soil -- 77 86 91 94

Crop residue (CR) Poor 76 85 90 93

Good 74 83 88 90

Row Crops Straight row (SR) Poor 72 81 88 91

Good 67 78 85 89

SR + CR Poor 71 80 87 90

Good 64 75 82 85

Contoured (C) Poor 70 79 84 88

Good 65 75 82 86

C + CR Poor 69 78 83 87

Good 64 74 81 85

Contoured & Terraced (C & T) Poor 66 74 80 82

Good 62 71 78 81

C & T + CR Poor 65 73 79 81

Good 61 70 77 80

Small Grain SR Poor 65 76 84 88

Good 63 75 83 87

SR + CR Poor 64 75 83 86

Good 60 72 80 84

C Poor 63 74 82 85

Good 61 73 81 84

C + CR Poor 62 73 81 84

Good 60 72 80 83

C & T Poor 61 72 79 82

Good 59 70 78 81

C & T + CR Poor 60 71 78 81

Good 58 69 77 80

Close-seeded or SR Poor 66 77 85 89

broadcast legumes or Good 58 72 81 85

rotation meadow C Poor 64 75 83 85

Good 55 69 78 83

C & T Poor 63 78 80 83

Good 51 67 76 80

⁵ Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.

⁶ Hydrologic condition is based on combination of factors that affect infiltration and runoff, included (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or close-seeded legumes in rotation, (d) percent of residue cover on the land surface (good >20%), and (e) degree of surface roughness.

Poor: Factors impair infiltration and tend to increase runoff.

Good: Factors encourage average/better than average infiltration and tend to decrease runoff.

TABLE 4-5c

RUNOFF CURVE NUMBERS FOR OTHER AGRICULTURAL LANDS¹

(210-VI-TR-55, Second Edition, June 1986)

Cover Description: Curve Numbers

for Hydrologic

Hydrologic Soil Group

Cover Type Condition A B C D

Pasture, grassland, or range-continuous forage for grazing⁷ Poor 68 79 86 89

Fair 49 69 79 84

Good 39 61 74 80

Meadow-continuous grass, protected from grazing and generally

mowed for hay -- 30 58 71 78

Brush--brush-weed-grass with brush the major element⁸ Poor 48 67 77 83

Fair 35 56 70 77

Good 30⁹ 48 65 73

Woods--grass combination (orchard or tree farm)¹⁰ Poor 57 73 82 86

Fair 43 65 76 82

Good 32 58 72 79

Woods¹¹ Poor 45 66 77 83

Fair 36 60 73 79

Good 30⁹ 55 70 77

Farmsteads--buildings, lanes, driveways, & surrounding lots -- 59 74 82 86

⁷ Poor: <50% ground cover or heavily grazed with no mulch.

Fair: 50 to 75% ground cover and not heavily grazed.

Good: >75% ground cover and lightly or only occasionally grazed.

⁸ Poor: <50% ground cover.

Fair: 50 to 70% ground cover.

Good: >75% ground cover.

⁹ Actual curve number is less than 30; use CN = 30 for runoff computations.

¹⁰ CN's shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture.

¹¹ Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.

Fair: Woods are grazed but not burned, and some forest litter covers the soil.

Good: Woods are protected from grazing, and litter and brush adequately cover the soil.

TABLE 4-6a

SCS TYPE III - RAINFALL DISTRIBUTION (24-HOURS)

NONDIMENSIONAL NONDIMENSIONAL

TIME RAINFALL TIME RAINFALL

0.000 0.000 0.521 0.702

0.010 0.002 0.531 0.729

0.021 0.005 0.542 0.751

0.031 0.007 0.552 0.769

0.042 0.010 0.563 0.785

0.052 0.012 0.573 0.799

0.063 0.015 0.583 0.811

0.073 0.017 0.594 0.823

0.083 0.020 0.604 0.834

0.094 0.023 0.615 0.844

0.104 0.026 0.625 0.853

0.115 0.028 0.635 0.862

0.125 0.031 0.646 0.870

0.135 0.034 0.656 0.878

0.146 0.037 0.667 0.886

0.156 0.040 0.677 0.893

0.167 0.043 0.688 0.900

0.177 0.047 0.698 0.907

0.188 0.050 0.708 0.911

0.198 0.053 0.719 0.916

0.208 0.057 0.729 0.920

0.219 0.060 0.740 0.925

0.229 0.064 0.750 0.929

0.240 0.068 0.760 0.933

0.250 0.072 0.771 0.936

0.260 0.076 0.781 0.940

0.271 0.080 0.792 0.944

0.281 0.085 0.802 0.947

0.292 0.089 0.813 0.951

0.302 0.094 0.823 0.954

0.313 0.100 0.833 0.957

0.323 0.107 0.844 0.960

0.333 0.115 0.854 0.963

0.344 0.122 0.865 0.966

0.354 0.130 0.875 0.969

0.365 0.139 0.885 0.972

0.375 0.148 0.896 0.975

0.385 0.157 0.906 0.978

0.396 0.167 0.917 0.981

0.406 0.178 0.927 0.983

0.417 0.189 0.938 0.986

0.427 0.202 0.948 0.988

0.438 0.216 0.958 0.991

0.448 0.232 0.969 0.993

0.458 0.250 0.979 0.996

0.469 0.271 0.990 0.998

0.479 0.298 1.000 1.000

0.490 0.339

0.500 0.500

0.510 0.662

DELANEY CREEK - RAINFALL DISTRIBUTION (24-HOURS)

NONDIMENSIONAL NONDIMENSIONAL

TIME RAINFALL TIME RAINFALL

0.000 0.0000 0.521 0.5445

0.010 0.0001 0.531 0.5972

0.021 0.0002 0.542 0.6499

0.031 0.0003 0.552 0.6737

0.042 0.0005 0.563 0.6975

0.052 0.0007 0.573 0.7213

0.063 0.0010 0.583 0.7451

0.073 0.0014 0.594 0.7638

0.083 0.0018 0.604 0.7825

0.094 0.0023 0.615 0.8012

0.104 0.0029 0.625 0.8200

0.115 0.0036 0.635 0.8342

0.125 0.0045 0.646 0.8484

0.135 0.0055 0.656 0.8626

0.146 0.0065 0.667 0.8769

0.156 0.0076 0.677 0.8875

0.167 0.0089 0.688 0.8981

0.177 0.0115 0.698 0.9087

0.188 0.0143 0.708 0.9192

0.198 0.0172 0.719 0.9273

0.208 0.0203 0.729 0.9354

0.219 0.0239 0.740 0.9435

0.229 0.0277 0.750 0.9516

0.240 0.0316 0.760 0.9570

0.250 0.0357 0.771 0.9625

0.260 0.0420 0.781 0.9680

0.271 0.0485 0.792 0.9734

0.281 0.0550 0.802 0.9768

0.292 0.0616 0.813 0.9800

0.302 0.0701 0.823 0.9831

0.313 0.0787 0.833 0.9862

0.323 0.0873 0.844 0.9882

0.333 0.0960 0.854 0.9902

0.344 0.1086 0.865 0.9922

0.354 0.1212 0.875 0.9941

0.365 0.1339 0.885 0.9950

0.375 0.1466 0.896 0.9958

0.385 0.1626 0.906 0.9965

0.396 0.1786 0.917 0.9972

0.406 0.1946 0.927 0.9977

0.417 0.2106 0.938 0.9982

0.427 0.2315 0.948 0.9986

0.438 0.2524 0.958 0.9990

0.448 0.2734 0.969 0.9993

0.458 0.2945 0.979 0.9996

0.469 0.3306 0.990 0.9998

0.479 0.3668 1.000 1.0000

0.490 0.4030

0.500 0.4392

0.510 0.4918

D. SCS SYNTHETIC UNIT HYDROGRAPH PROCEDURE

1. The SCS has derived a general dimensionless unit hydrograph from a large number of observed unit hydrographs for watersheds of various sizes and geographic locations. The unit hydrograph is adequate when the shape factor is 484. For areas such as Florida where the unit hydrograph shape is generally flatter and wider a different dimensionless unit hydrograph is required. Such is the case in Hillsborough County where a 256 shape factor is the acceptable value. Once the time to peak and peak flow for a particular unit hydrograph have been defined, the entire shape of the unit hydrograph can be estimated using the appropriate dimensionless unit hydrograph. The recommended distribution for the 256 shape factor for use in Hillsborough County is found in Table 4-7.

The County shall only accept a 256 shape factor unless the site designer demonstrates that another shape factor is more appropriate. Shape factors other than 256 must be approved by the County Administrator.

A more detailed analysis of the unit hydrograph procedure can be found in "SCS National Engineering Handbook, Section 4, Hydrology, Revised 1969" and other publications.

2. The U.S.D.A. Soil Conservation Service developed the Synthetic Unit Hydrograph Method to calculate the peak discharge, (q_p), according to the following equation:

q_p = P_fAR (4-9)

T_p

Where:

q_p = the peak unit hydrograph discharge, in cubic feet per second

A = the area of the drainage basin, in square miles

R = the total unit hydrograph runoff = 1.0"

P_f = unit hydrograph peak-rate or shape factor (256-flat areas, to 484-moderately steep areas, to 600"-very steep areas). In Hillsborough County, the 256 value shall be used unless an alternative value is approved by the County Administrator

T_p = the time of rise or time to peak from the inception of runoff, in hours.

In the SCS procedure, the time to peak, T_p, in hours is computed by the following equation:

T_p = D + L (4-10)

2

Where:

D = the duration of the rainfall excess, in hours (typically in the range of 0.1 T_c to 0.2 T_c for small urban basins)

T_c = the time of concentration for the drainage basin, in hours

L = the lag time, in hours (time between the centroid of the rainfall excess and the peak of the unit hydrograph)

For purposes of computing T_p, the lag time (L) is assumed equal to 0.6T_c. Substituting for L in the equation, one obtains:

T_p = D + 0.6T_c (4-11)

2

Substituting in the peak flow equation, one can compute the peak of the SCS unit hydrograph using the following equation:

q_p = P_fAR (4-12)

D/2 + 0.6T_c

In this procedure, the time of concentration of the drainage basin is computed as defined previously in Section 4.2.

E. GENERATION OF SCS RUNOFF HYDROGRAPH

1. See the literature for procedures on generating runoff hydrographs from the unit hydrographs.

2. The following conditions apply to the use of SCS runoff hydrograph generation in Hillsborough County:

a) For allowable rainfall depths and distributions, refer to Section 4.2.B.

b) In Hillsborough County a shape factor of 256 shall be used unless another value is approved by the County Administrator.

c) The computational time step shall be in the range of 0.1T_c to 0.2T_c.

d) The initial abstraction shall be 0.2S.

TABLE 4-7

DIMENSIONLESS UNIT HYDROGRAPH RATIOS

(256 Shape Factor, Advanced Engineering Technologies)

Time Ratios Mass Curve Ratios

(t/T_p) (q/q_p)

0.00 0.000

0.20 0.150

0.40 0.320

0.60 0.600

0.80 0.930

1.00 1.000

1.20 0.960

1.40 0.880

1.60 0.780

1.80 0.690

2.00 0.590

2.20 0.520

2.40 0.480

2.60 0.430

2.80 0.390

3.00 0.350

3.20 0.320

3.40 0.290

3.60 0.260

3.80 0.230

4.00 0.210

4.20 0.200

4.40 0.190

4.60 0.180

4.80 0.170

5.00 0.160

5.20 0.150

5.40 0.140

5.60 0.130

5.80 0.120

6.00 0.110

6.20 0.100

6.40 0.090

6.60 0.080

6.80 0.070

7.00 0.060

7.20 0.050

7.40 0.045

7.60 0.040

7.80 0.035

8.00 0.030

8.20 0.025

8.40 0.020

8.60 0.015

8.80 0.010

9.00 0.000

4.7 THE SANTA BARBARA URBAN HYDROGRAPH METHOD

A. DEFINITION

The Santa Barbara Urban Hydrograph Method (SBUH Method) was developed by Stubchaer (1975) for the Santa Barbara County (California) Flood Control and Water Conservation District. In many respects, the SBUH method is similar to some of the time-area-concentration curve procedures for hydrograph computation in which an instantaneous hydrograph in a basin was developed and then routed through an element of linear storage to determine basin response. However, in the SBUH method the final design (outflow) hydrograph is obtained by routing the instantaneous hydrograph for each time period (obtained by multiplying the various incremental rainfall excesses by the watershed area, in acres) through an imaginary linear reservoir with a routing constant which is related to the time of concentration of the drainage basin. As a result, the intermediate process of preparing a time-area-concentration curve for the basin is eliminated.

B. GENERATION OF SANTA BARBARA URBAN HYDROGRAPH

1. See the literature for procedures on generating runoff hydrographs, or refer to Table 4-8.

2. The following conditions apply to the use of SBUH for design in Hillsborough County:

a) For allowable rainfall depths and distributions refer to Section 4.2.B.

b) The modified version which considers curve numbers is the acceptable method.

c) The computational time step shall be in the range of 0.5T_c < Î t < T_c.

C. MODIFIED SANTA BARBARA METHOD COMPUTATION FORMAT

(Orlando Urban Stormwater Management Manual)

The following format is provided as an aide to assist in the Modified Santa Barbara Method computations.

TABLE 4-8

COMPUTATION FORMAT FOR THE MODIFIED SANTA BARBARA METHOD

(1) (2) (3) (4) (5) (6)

Time Rainfall Rainfall Runoff Instant Runoff

(Hours) Distribution (Inches) (Inches) Runoff Hydrograph

Ratio Cumulative (cfs) (cfs)

P/P (Design) P R I Q

1. Column (1) represents the cumulative storm duration which progresses in t increments, from zero to the total time of the design storm and is expressed in hours (t = chosen calculation interval within the range of 0.5T_c < t # T_c).

2. Column (2) is the distribution of rainfall versus time in terms of the cumulative rainfall ratios, P/P (Design), corresponding to the Column (1) time values. (P = Cumulative rainfall at time t, P(Design) = Total design rainfall). Refer to Section 4.2.B.2 for the allowable rainfall distribution.

3. Column (3) lists cumulative values of rainfall, P, for the corresponding cumulative time values in Column (1). Column (3) is obtained by multiplying Column (2) by the total design rainfall depth P(Design). Refer to Section 4.2.B.1 for the allowable rainfall depths.

4. Column (4) represents the cumulative runoff depths, R, corresponding to the cumulative time values in Column 1. Runoff depths shall be determined by applying the following SCS runoff equation:

R = (P - 0.2S)² (4-13)

(P + 0.8S)

Where:

S = soil storage parameter in inches

S values are related to curve numbers by the following equation:

S = 1000 - 10 (4-14)

CN

For CN values, refer to Table 4-5.

5. Column (5) represents the instantaneous runoff rates, I, corresponding to the Column (1) time values. These I values do not consider the effect of the linear reservoir routing on the attenuation of peaks. The conversion of the runoff, R, in inches to the instantaneous rate, I, in cfs is based on the following approximation:

I₂ = (R₂-R₁)A (Approximate since (4-15)

t 1 acre-inch 1cfs)

Where:

I₂ = instantaneous runoff rate at time, t, in cfs

6. Column (6) represents points on the runoff hydrograph which are calculated by utilizing the routing equation defined below. The maximum value of Q corresponds to the peak runoff rate or flow.

In the SBUH Method:

Q₂ = Q₁ + K (I₁ + I₂-2Q₁) (4-16)

and K - t (4-17)

2T_c+t

Where:

I₁ = instantaneous runoff rate at time (t-1)

I₂ = instantaneous runoff rate at time (t)

Q₁ = hydrograph rate at time (t-1), in cfs

Q₂ = hydrograph rate at time (t), in cfs

K = routing coefficient, dimensionless

t = routing intervals, in hours

T_c = time of concentration, in hours

4.8 DESIGN FREQUENCY

Structures must be designed to withstand the storm frequencies specified by the County. Required design frequencies are listed in Table 4-9 for various types of structures.

TABLE 4-9

DESIGN FREQUENCY AND FREEBOARD REQUIREMENTS FOR VARIOUS STRUCTURES*

FREQUENCY FREEBOARD

STRUCTURE TYPE (YEARS) REQUIREMENT

Bridges 50 (See Section 5.5.B.3)

Structures for Conveyance

of Off-Site Stormwater

Pipe Systems 25 **

Canals/Ditches 25 (See Section 9.1.K)

Culverts 25 (See Section 5.2.J)

Internal Stormwater

Collection Systems

Storm Sewer Systems

(See Table 10-1) 3 (See Section 6.2.G.3)

Ditches/Swales 3 (See Section 9.1.K)

Culverts 3 (See Section 5.2.J)

Detention Ponds

Commercial 25 (See Section 7.1.B.7)

Subdivision 25 (See Section 7.1.B.16)

Retention Ponds

Commercial 100 (See Section 7.1.B.7)

Subdivision 100 (See Section 7.1.B.16)

Floor Elevations 100 (See Section 7.1.B.6)

* All designs shall be analyzed to verify that potential off-site flood elevations due to the post-development 100 year/24 hour storm event are less than a cumulative 0.1 foot above pre-development flood elevations.

** A minimum freeboard of one (1) foot shall be maintained between the design.

** The hydraulic grade line is permissible up to the edge of pavement, gutter line, top of grate, or top of manhole, whichever is lower.

FIGURE 4-1

FIGURE 4-2

FIGURE 4-3

FIGURE 4-4

FIGURE 4-5

FIGURE 4-6

FIGURE 4-7

FIGURE 4-8

FIGURE 4-9

FIGURE 4-10

CHAPTER 5

CULVERT AND BRIDGE DESIGN

5.1 GENERAL

Culverts are required to convey surface water flows through roadway crossings and other similar obstructions to open channels. Culvert designs involve the consideration of the same design factors considered in storm sewer system design, including the allowance for entrance, exit and manhole/junction losses which can drastically affect the upstream (headwater) elevations and/or culvert size required. Additionally, particular attention should be given to the downstream water surface elevation (i.e. tailwater condition) in selecting the appropriate culvert.

Bridges are required when clearance requirements for navigation, hydraulic efficiency, constructibility, environmental concerns, costs or aesthetics preclude the use of culverts. When evaluating bridges the basic design criteria includes design frequency, high water elevations, vertical and horizontal clearance, and scour protection within the channel and at the embankments.

5.2 CULVERT DESIGN CRITERIA

A. DESIGN FREQUENCY

Refer to Table 4-9.

B. DESIGN FLOWS

The determination of design flows for a culvert installation shall be in accordance with the methods and procedures in Chapter 4.

C. CULVERT ROUGHNESS COEFFICIENTS

The roughness coefficients indicated in Table 5-1 shall be used for culvert design.

D. MINIMUM CULVERT SIZE

Criteria for minimum culvert size is in Table 5-2.

TABLE 5-1

MANNING'S n VALUES FOR CULVERT AND STORM SEWER DESIGN

Pipe n

Concrete Pipe 0.012

Concrete Box Culvert Precast or Cast-in-Place 0.012

Spiral-Ribbed Corrugated Metal Pipe - 18" to 96" 0.012

Corrugated Metal Pipe (Helical - 2-2/3 x 1/2" Corrugation)

Diameter or Span n

12" 0.011

15" 0.012

18" 0.013

24" 0.015

36" 0.018

42" 0.019

48" 0.020

60" and larger 0.021

Corrugated Metal Pipe (Helical - 3" x 1" Corrugation)

Diameter or Span n

48" 0.023

54" 0.023

60"+ 0.024

66" 0.025

72" 0.026

78" and larger 0.027

Corrugated Structural Plate Pipe and Pipe Arch

All Sizes:

Corrugation Size n

6" x 1" 0.030

6" x 2" 0.033

9" x 2-1/2" 0.034

Refer to Section 5.4 A and B for acceptable materials to be used for pipes to be maintained by Hillsborough County.

TABLE 5-2

MINIMUM CULVERT SIZE CRITERIA

(From FDOT Drainage Manual, 1987)

Type of Culvert Minimum Size

Cross Drains 18 inches or Equivalent

Storm Sewer 15 inches or Equivalent

Side Drains 18 inches or Equivalent*

Box Culvert 3 feet x 3 feet

* Restricted to areas where minimal stormwater discharge is expected.

E. PIPE SIZE INCREMENT

Pipe sizes above eighteen (18) inches or equivalent shall be based upon a six (6) inch increment or equivalent.

F. LENGTH

The maximum length of culvert to be used without an access structure is specified in Table 5-3.

TABLE 5-3

MAXIMUM CULVERT LENGTH CRITERIA

(From FDOT Drainage Manual, 1987)

The maximum culvert length without an access structure shall be as follows:

Culvert Size (Inches) Maximum Length (Feet)

15" Pipe or Equivalent 200

18" Pipe or Equivalent 300

24" to 36" Pipe or Equivalent 400

42" and Larger Pipe or Equivalent 500

Box Culvert 500

G. MINIMUM PHYSICAL SLOPE

The minimum physical slope for culverts shall be that which will produce a velocity of 2.5 feet per second when the culvert is flowing full with the hydraulic grade line at the crown of the pipe.

H. MAXIMUM VELOCITY

The maximum allowable internal velocity for culverts shall be governed by Table 6-1. The maximum allowable outlet velocity for culverts shall be governed by Tables 9-1 and 9-2 unless acceptable energy dissipation and erosion protection measures are provided.

I. DESIGN TAILWATER

1. All culvert installations shall be designed taking into consideration the tailwater of the receiving facility or body of water (inlet or outlet control). Generally, the tailwater must be determined by calculations based upon the design criteria and frequencies contained in Section 4.8.

a. When the tailwater elevation is higher than the proposed culvert crown elevation, the hydraulic gradient shall begin at the tailwater elevation.

b. If a tailwater elevation cannot be determined, the hydraulic gradient shall begin at or above the crown of the proposed culvert.

2. For design purposes, Tampa Bay and all adjoining bays shall be assumed to be at elevation 2.0 feet NGVD.

J. ALLOWABLE HEADWATER

1. The allowable headwater of a culvert installation should be set by the Site Designer for an economical installation; however, it should never be set at an elevation that would violate the minimum freeboard requirements of Section 9.1.K., for the design storm specified in Table 4.9.

2. When endwalls are used, the headwater shall not exceed the top of the endwall at the entrance. If the top of the endwall is inundated, then special protection of the roadway embankment and/or ditch slope may be necessary for erosion protection.

K. ENDWALLS

1. Endwalls shall be installed on all culverts (except side drains which shall have mitered end sections) unless other provisions are made for erosion protection.

2. Endwalls shall conform to the Standard Indexes and the Florida DOT Standard Specifications for Road and Bridge Construction, (latest edition).

3. Entrance loss coefficients (K_e) for standard inlet configurations are shown in Table 5-4. Entrance head loss is to be computed by the following equation:

H_e = K_eV² (5-1)

2g

4. Endwalls shall not be used inside detention/

retention ponds, unless otherwise approved by the County Administrator.

TABLE 5-4

CULVERT ENTRANCE LOSS COEFFICIENTS

OUTLET CONTROL, FULL OR PARTIALLY FULL

(From USDOT, FHWA, HEC-5, 1965)

TYPE OF STRUCTURE AND DESIGN OF ENTRANCE COEFFICIENT K_e

Pipe, Reinforced Concrete

Projecting from fill, socket end (groove end) 0.2

Projecting from fill, square cut end 0.5

Straight headwall

Socket end of pipe (groove-end) 0.2

Square-edge 0.5

Rounded (radius = 1/12D) 0.2

Mitered to conform to fill slope 0.7

End section conforming to fill slope 0.5

Beveled edges, 33.7B or 45B bevels 0.2

Side- or Slope-tapered inlet 0.2

Straight sand-cement 0.3

U-type with grate 0.7

U-type 0.5

Winged concrete 0.3

U-type sand-cement 0.5

Flared end concrete 0.5

Side drain, mitered with grate 1.0

Pipe or Pipe-Arch, Corrugated Metal

Straight endwall-rounded (radius = 1/12D) 0.2

Projecting from fill (no headwall) 0.9

Headwall or headwall and wingwalls, square-edge 0.5

Mitered to conform to fill slope 0.7

End section conforming to fill slope, paved or unpaved 0.5

Beveled edges, 33.7B or 45B bevels 0.2

Side- or slope-tapered inlet 0.2

TYPE OF STRUCTURE AND DESIGN OF ENTRANCE COEFFICIENT K_e

Box, Reinforced Concrete

Headwall parallel to embankment (no wingwalls)

Square-edged on three edges 0.5

Rounded on three edges to radius of 1/12

barrel dimension or beveled edges on three

sides

Wingwalls at 30B to 75B to barrel

Square-edged at crown 0.4

Crown edge rounded to radius of 1/12 barrel

dimension, or beveled top edge 0.2

Wingwalls at 10B to 25B to barrel, square-edged

at crown 0.5

Wingwalls parallel (extension of sides)

Square-edged at crown 0.7

Side- or slope-tapered inlet

5. End sections conforming to fill slope, made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests, they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections incorporating a closed taper in their design have a superior hydraulic performance.

L. MINIMUM CLEARANCE

The minimum clearance for all pipe culverts are specified in Table 5-5.

TABLE 5-5

MINIMUM CLEARANCE CRITERIA FOR PIPE CULVERTS

From To Clearance

Outside crown of pipe Low edge of 1.0 foot

pavement

Shell of pipe Shell of utility 0.5 foot

crossing

5.3 CULVERT DESIGN PROCEDURES

A. CULVERT HYDRAULICS

For flow in culverts, a wide range of flow regimes can occur depending on flow rate, bed slope and cross-sectional geometry as well as other factors. For a given flow rate, there are two different depths of flow that have the same energy; a high velocity with low depth (supercritical flow) and a low velocity with a high depth (subcritical flow). There is one depth for a given flow, the critical depth, which corresponds to the minimum energy of flow and depends on the shape and size of the culvert. For any given discharge and cross-section, there is a unique slope (critical slope) that will produce and maintain flow at critical depth. Generally, for most culverts, the culvert slope will not be critical, and flow will be supercritical or subcritical.

A culvert can operate under either inlet control (the barrel has a greater hydraulic capacity than the inlet) or outlet control (the inlet has a greater hydraulic capacity than the barrel). During a given storm event a culvert may operate under inlet control, outlet control, or may transition from one control to the other as the storm progresses.

1. Culvert Design Criteria

Culverts shall be designed according to the "inlet and outlet control" methodology. Refer to references for procedures or follow procedures specified in Sections 5.3.C, 5.3.D, and 5.3.E.

2. Inlet Control

Inlet control occurs when the capacity of the culvert is controlled at the culvert entrance by the depth of headwater and the entrance geometry of the culvert, including barrel shape, cross-sectional area, type of inlet edge, and shape of headwall. With inlet control, the entrance acts similar to an orifice or a weir, depending on the headwater depth. Barrel friction in the culvert is not a factor during inlet control. Three different configurations of inlet control are shown in Figure 5-1. When the depth of water at the culvert entrance (headwater) is less than the culvert height, the flow rate is governed by critical depth or, in general, by broad-crested weir control. As the depth of headwater increases, the entrance of the culvert will be considered submerged when the ratio of the depth of the headwater (HW) to the height of the culvert (D) exceed 1.2. When the entrance is submerged and the control is at the inlet, the flow will be governed, in general, by orifice flow.

Culvert nomographs for conventional culverts operating under inlet control have been developed by the USDOT and FHWA. Some are included in Figures 5-2, 5-3, 5-4, while others can be located in USDOT, FHWA, HEC-5, 1965.

3. Outlet Control

When a culvert is designed to operate under outlet control, the barrel typically flows full. In addition to the influences on the culvert capacity described for inlet control, outlet control involves the additional consideration of the elevation of the tailwater in the outlet channel, and the slope, roughness, and length of the culvert barrel. Culverts under outlet control can flow with the culvert barrel full or partially full for all or part of the barrel length. Full flow outlet control conditions are shown in Figure 5-5 (Parts A and B), while partially full outlet control conditions are shown in Figure 5-5 (Parts C and D).

In addition to full and partially full flow conditions, submerged and unsubmerged tailwater conditions may exist. Typical unsubmerged flow conditions are shown in Figure 5-5 (Parts B, C and D). When the discharge is sufficient to give a critical depth equal to the crown of the culvert barrel, full flow exists at the outlet even for unsubmerged tailwater conditions, as shown in 5-5 (Part B).

Outlet control nomographs have been developed by the USDOT and FHWA. Some are shown in Figures 5-2, 5-3, 5-4, while others can be located in USDOT, FHWA, HEC-5, 1965.

B. DESIGN COMPUTATION FORMS

The use of the design computation form (Figure 5-6) is a convenient method to use to obtain consistent cost-effective designs.

C. SUGGESTED DESIGN METHODOLOGY (from Orlando Urban Stormwater Management Technical Manual)

The determination of the required size of a culvert installation can be accomplished by the use of mathematical equations or through the utilization of design nomographs. The mathematical solution will give a precise result, but is time consuming and somewhat non-productive when considering the inaccuracies of estimating design flows and flood water elevations. Representative culvert nomographs are included in Figures 5-2, 5-3, and 5-4. The following procedure is suggested for culvert size selection:

1. Determine and analyze site data

a. Site characteristics include the generalized shape of the conveyance facilities leading into and away from the culvert installation, the geometry of the roadway or embankments, site elevations, approximate length of the installation, allowable maximum velocities, approximate slope of the culvert and the allowable headwater depth.

b. In determining the allowable headwater depth (AHW EL.), roadway elevations and the elevations of upstream properties must be considered.

2. Perform hydrologic analysis - by hydrologic methods specified in Chapter 4.

3. Select Trial Culvert Size - the design of a culvert is usually a trial and error process. Consequently, the initial size of a culvert can be selected by dividing the design flow by an average velocity of five (5) feet per second since outlet control and full flow conditions will probably control the design.

4. Determine headwater for inlet control trial culvert size

a. Using the trial size from Step 3, find the headwater depth (HW) through the utilization of the appropriate inlet control nomograph.

b. Tailwater (TW) conditions are to be neglected in this determination.

c. If HW is greater or less than allowable, try another culvert size until HW is acceptable for inlet control.

d. After an acceptable culvert size has been determined for inlet control, then compute HW for outlet control.

5. Determine controlling headwater depth for trial culvert size

a. Approximate the depth of tailwater (TW) in feet, above the outlet invert for the design flood condition in the outlet channel.

b. For tailwater (TW) elevation equal to or greater than the crown of the culvert at the outlet, set H₀ equal to TW and find HW by the following equation:

HW = H + H₀ - LS₀ (5-2)

Where:

HW = vertical distance, in feet, from culvert invert (flow line) at entrance to the pool surface

H = head loss, in feet, as determined from the appropriate nomograph (Figures 5-2, 5-3 and 5-4)

H₀ = vertical distance in feet from culvert invert at outlet to the hydraulic grade line (in this case H₀ equals TW, measured in feet above the culvert invert)

S₀ = slope of barrel (dimensionless)

L = culvert length, in feet

c. For TW elevations less than the top of the culvert at the outlet, find headwater by:

HW = H + H₀ - LS₀ (5-3)

Except that:

H₀ = (d_c + D)/2 or TW (5-4)

(whichever is the greater)

Where:

d_c = critical depth in feet (Figures 5-7 and 5-8) NOTE: d_c cannot exceed D

D = height, in feet, of culvert opening

6. Determine controlling headwater

a. Compute the headwaters found in Step 4 and Step 5 (Inlet Control and Outlet Control). The higher headwater governs and indicates the flow control existing under the given conditions for the trial culvert size selected.

b. If outlet control governs and the HW is higher than is acceptable, select a larger trial culvert size and find HW as instructed in Step 5.

7. Alternative culvert types and sizes

a. Try a culvert of another type material or shape and determine size and HW by the above procedure.

b. Always try several types, shapes, and sizes to ensure the most economical design.

8. Determine Channel Protection Requirements

a. Compute outlet velocities (for size and types) at culverts selected to be considered and determine need for channel protection.

b. If outlet control governs, outlet velocity equals Q/A_o, where A_o is the cross-sectional area of flow in the culvert barrel at the outlet.

1. If d_c or TW is less than the height of the culvert barrel, use either A_o corresponding to d_c or TW depth, whichever gives the greater area of flow.

2. A_o should never exceed the total cross-sectional area, A, of the culvert barrel.

c. If inlet control governs, outlet velocity can be assumed to equal mean velocity in open-channel flow in the barrel, as computed by Manning's equation for the rate of flow, barrel size, roughness and slope of culvert selected.

9. Design summary of selected culvert size. Record final selection of culvert with size, type, required headwater, outlet velocity, economical justification, and channel protection on the design computation form.

D. INLET-CONTROL NOMOGRAPHS - INSTRUCTIONS FOR USE (from Orlando Urban Stormwater Management Technical Manual)

1. To determine headwater (HW), given Q, and size and type of culvert.

a. Connect with a straight edge the given culvert diameter or height (D) and the discharge (Q), or Q/width (B) for box culvert; mark intersection of straight edge on HW/D Scale marked (1).

b. If HW/D Scale marked (1) represents entrance type used, read HW/D on Scale (1). If another of the three entrance types listed on the nomograph is used, extend the point of intersection, determined in (a), horizontally to Scale (2) or (3) and read HW/D by culvert height (D).

c. Compute HW by multiplying HW/D by culvert height (D).

2. To determine discharge (Q) per barrel, given HW, and size and type of culvert.

a. Compute HW/D for given conditions.

b. Locate HW/D on scale for appropriate entrance type. If Scale (2) or (3) is used, extend HW/D point horizontally to Scale (1).

c. Connect point on HW/D Scale (1) as found in (b) above and the size of culvert on the left scale. Read Q or Q/B on the discharge scale.

d. If Q/B is read in (c) multiply by B (span of box culvert) to find Q.

3. To determine culvert size, given Q, allowable HW and type of culvert.

a. Using a trial size, compute HW/D.

b. Locate HW/D on scale for appropriate entrance type. If Scale (2) or (3) is used, extend HW/D point horizontally to Scale (1).

c. Connect point on HW/D Scale (1) as found in (b) above to given discharge and read diameter, height or size of culvert required for HW/D value.

d. If D is not that originally assumed, repeat procedure with a new D.

E. OUTLET CONTROL NOMOGRAPHS - INSTRUCTIONS FOR USE (from Orlando Urban Stormwater Management Technical Manual)

Outlet control nomographs may be used to solve for head (H) when the culvert barrel flows full for its entire length. They are also used to determine head (H) for some partial flow conditions with outlet control. These nomographs do not give a complete solution for finding headwater (HW), since they only give H in the equation:

HW = H + H₀ - LS₀ (5-2)

1. To determine head (H) for a given culvert and discharge (Q).

a. Locate the appropriate nomograph for type of culvert selected and find K_e for the desired entrance type.

b. Begin nomograph solution by locating the starting point on the length scale, according to the instructions listed below:

1. If the n value of the nomograph corresponds to that of the culvert being used, select the length curve for the proper K_e and locate the starting point at the given culvert length.

a. If a K_e curve is not shown for the selected K_e, see (2) below.

b. If the n value for the culvert selected differs from that of the nomograph, see (3) below.

2. For the n value of the nomograph and a K_e intermediate between the scales given, connect the given length on adjacent scales by a straight line and select a point on this line spaced between the two chart scales in proportion to the K_e values.

3. For a different roughness coefficient (n₁) than that of the chart (n), use the length scales shown with an adjusted length L₁, calculated by the formula:

L₁ = L(n₁)² (5-5)

c. Using a straight edge, connect the point on the length scale to the size of the culvert barrel and mark the point of crossing on the "turning line".

d. Pivot the straight edge on this point of the turning line and connect with the discharge rate. Read head (feet) on the head (H) scale.

5.4 MATERIAL SPECIFICATIONS FOR ALL SUBDIVISION CULVERTS AND FOR OTHER CULVERTS TO BE MAINTAINED BY HILLSBOROUGH COUNTY

A. CULVERT MATERIALS

1. Reinforced Concrete Pipe (round and oval)

2. Corrugated Aluminum Alloy Pipe and Pipe Arch

3. Corrugated Aluminized Type 2 Steel Pipe and Pipe Arch

4. Corrugated Bituminous or Polymeric Coated Galvanized Steel Pipe and Pipe Arch

5. Spiral-Ribbed Corrugated Metal Pipe and Pipe Arch (Alum., Aluminized Type 2, and Bituminous or Polymeric Coated Galvanized Steel)

6. Cast-in-Place and Precast Reinforced Concrete Box Culvert

7. Structural Plate Aluminum Alloy Pipe, Pipe Arch, Arch and Box Culvert

8. Structural Plate Steel Pipe, Pipe Arch, Arch and Box Culvert

B. CULVERT DESIGN SERVICE LIFE

1. Design Service Life (DSL) is the projected maintenance-free time period for the pipe material, and is the standard used to qualify the durability of a culvert material.

2. Recommended minimum requirement for pipe durability is a 50-year DSL.

C. DURABILITY INVESTIGATION

1. The soil and water at each project should be tested for pH, minimum resistivity, sulfates and chlorides; and results submitted to the Planning & Development Management Department or Engineering Department, whatever is applicable, with plans.

2. Testing requirements:

Soil: One series of tests per soil type per 200 lineal feet of stormwater pipe.

Water: Minimum of one series of tests per project.

Unless impractical, the field testing should be conducted in approximate area and depth of proposed pipe.

D. DESIGN SERVICE LIFE OF MATERIALS

1. Aluminum

A 50-year DSL can be achieved if used in areas where the pH is between 4 and 9 and the resistivity is greater than 500 ohm-cm.

Aluminum can be used in salt and brackish environments when backfilled with granular free draining material.

Bituminous Coating should be considered if the method of muck removal cannot ensure total muck elimination, or if significant muck sediment build-up in the pipe is anticipated.

2. Aluminized Type 2 (ALT2)

Use Figure 5-9 to determine years to first perforation for 16 gauge galvanized. Then multiply the ALT2 gauge factors listed to determine DSL.

Satisfactory performance can be expected where the pH is between 5 and 9, and the resistivity is greater than 1500 ohm-cms.

If additional DSL is required, increase gauge of metal and/or add organic protective coatings.

3. Concrete

Use Figure 5-10 to determine DSL for concrete. If additional DSL is required, use protective features listed to increase DSL.

4. Non-Metallic Coated Galvanized (Bituminous Coated or Polymeric)

Use Figure 5-9 to determine years to first perforation for 16 gauge galvanized. Then multiply by appropriate gauge factor to increase DSL.

Add additional DSL, based on type of coating shown below.

Bituminous Coated adds 10 years to DSL. Polymeric Coated (10 mils inside/10 mils outside) adds 10 years to DSL. Bituminous Coated with Paved Invert adds 25 years to DSL.

E. CULVERT SPECIFICATIONS

1. Workmanship and culvert materials shall conform to the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

2. Corrugated Aluminum Alloy Pipe and Pipe Arch may be used in tidal areas, but shall not be used near dissimilar metals that may cause galvanic action.

3. Concrete and Reinforcing Steel for Cast-in-Place Reinforced Concrete Box Culverts shall conform to the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

4. Precast Reinforced Concrete Box Culvert shall conform to the applicable American Society for Testing and Materials Standard Specifications (C789 or C850).

5. The methods for construction of trench and foundation, and for laying and backfilling shall conform to the requirements specified in the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

F. CULVERT JOINTS

1. Joints and joint material for culverts shall conform to the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

2. Field joints for Precast Concrete Box Culvert shall be made with butyl rubber based preformed plastic gasket material.

3. Pipe joints for other than reinforced concrete pipe when allowed under the pavement of County maintained roadways, right-of-ways and easements, shall have passed Florida DOT requirements for soil tight joints (2 PSI maintained without leak for 24 hours). Additionally, coupling bands for metal pipe shall be so constructed as to lap on an equal portion of each of the pipe sections to be connected. Each end of each pipe section shall have a minimum of two annular corrugations. The connecting bands shall have a minimum of two annular corrugations and shall fully engage, over the entire periphery, one corrugation on each pipe end.

If gaskets are required, they shall be either sleeve type or O-ring type:

a. Sleeve Type:

Sleeve type gaskets shall be closed-cell neoprene, skin on all four sides. They shall meet the requirements of ASTM D-1056, Grade SCE-43, and shall be of one-piece construction. The thickness shall be 3/8 inch and the width shall be 1/2 inch less than the width of the connection band required.

b. O-ring Type:

O-ring type gaskets shall meet the requirements of ASTM C-361. The gaskets shall have the following cross-sectional diameters for the given sizes of pipe:

CORRUGATION SIZE PIPE SIZE CHORD DIAMETER

INCHES INCHES INCHES

2 2/3 x 1/2 12 thru 36 13/16

2 2/3 x 1/2 and 42 thru 120 7/8

3 x 1 and

125mm x 25mm

5.5 MATERIAL SPECIFICATIONS FOR NON-SUBDIVISION CULVERTS NOT MAINTAINED BY HILLSBOROUGH COUNTY

It is recommended that the materials specified in 5.4 be used, however, other materials such as polyethylene or aluminized steel pipe can be used if approved by the County Administrator.

5.6 BRIDGE DESIGN

A. GENERAL

It is recommended that the design of bridge structures be undertaken by engineering staff and/or subconsultants with competence and experience in understanding and designing all phases of the bridge. Bridges are used when clearance requirements for navigation, hydraulic efficiency, geometrics, constructibility, environmental concerns, costs, or aesthetics preclude the use of multiple culverts. The basic hydraulic design criteria for bridges includes the frequency of the storm event, high water, vertical and horizontal clearance, and channel and abutment protection. Bridges designed to cross water bodies shall include these factors in the hydraulic design. Risk and site-specific factors are different for each bridge so there may be some variation in design requirements. Any variations in meeting the bridge design criteria specified in this Manual must have prior approval in writing from the County Administrator. FDOT design criteria and procedures (Drainage Manual) should be followed unless otherwise stated in this Manual.

B. HYDRAULIC BRIDGE DESIGN CRITERIA

Each bridge requires development of site specific design criteria that will meet the needs of the crossing.

1. Design Frequency

Refer to Table 4-9.

2. High Water

Design high water for freshwater channels is defined as the water elevation just upstream of the bridge for the 50-year frequency, 24-hour duration storm event.

In tidally influenced areas, the design high water is the greater elevation of either the 50-year frequency tidal surge event or the 50-year fresh water event superposed with the one-year tidal surge event.

The 100-year flood should be conveyed through the structure with no greater than a 0.10 foot allowable cumulative rise in the preconstruction upstream, off-site 100-year water surface elevation.

3. Vertical Clearances

For bridges on piers or piles, the minimum required freeboard clearance is measured from design high water to low chord member. For collector and arterial roads, the minimum freeboard clearance shall be 1 foot unless otherwise approved by the County Administrator.

4. Horizontal Clearance

Horizontal clearance should be provided to comply with applicable Coast Guard or District requirements. In addition, environmental or fill constraints may influence the selected horizontal clearance. Bridge embankments shall not encroach into FEMA designated floodways per Section 3.2.A.5.

5. Scour

The effects of scour should be considered in the design of all bridge crossings. Consideration should be given for scour protection if velocities exceed 3.0 feet per second, or if highly erodible soil is encountered (See Tables 9-1 and 9-2).

TO CALCULATE DESIGN SERVICE LIFE - CONCRETE

Individual reduction valves should be totaled and then rounded to the next highest 5-year interval.

Total reduction value is subtracted from 100 years to give a DSL.

If pH is less than 4.5, approval of County Stormwater Engineer is required.

To increase DSL, use the following protective features:

Protective Feature Additional DSL Years

1" Additional Cover 10^a

Type II or C₃A1 8% max 10^b

7 Bag Min w/C₃ A1 5% max 15^b

Penitent Sealers 10

Epoxy Coating 25

^a Additional cover may be achieved by the use of "C" wall pipe, by adjustment in steel placement (increase in steel reinforcement normally would be required), or some combination of methods.

^b Addition is applied against the reduction determined in above chart for excessive chlorides and sulfates only.

CHAPTER 6

STORM SEWER DESIGN

6.1 GENERAL

The purpose of this section is to outline design criteria for all storm sewer systems constructed within Hillsborough County. This criteria is intended to govern the design of new systems as well as the analysis and/or redesign of existing systems. Design information and criteria for evaluating the drainage of street and roadway pavement prior to discharge into the storm sewer system is included in Chapter 10.

6.2 DESIGN CRITERIA

A. DESIGN FREQUENCY

Refer to Table 4-9

B. DESIGN DISCHARGES

The determination of design flows for internal storm sewer systems shall be in accordance with the Rational Method as set forth in Chapter 4 (individual subareas draining to inlets are typically less than 10 acres due to constraints on inlet capacity and spread-of-flow on pavement criteria). For calculated times of concentration of 15 or less, a minimum time of concentration of 15 minutes shall be used in the storm sewer analysis. If the drainage area to an inlet is greater than 10 acres for either internal or external systems, the appropriate method for estimating design flows will be determined by the Site Designer and approved by the County Administrator.

C. ALLOWABLE MATERIALS

Refer to Sections 5.4 and 5.5

D. ROUGHNESS COEFFICIENTS FOR STORM SEWER SYSTEMS

Refer to Table 5-1

E. MINIMUM PIPE SIZE AND PIPE SIZE INCREMENT

Refer to Table 5-2

F. MINIMUM PHYSICAL SLOPE

Refer to Section 5.2.G

G. MAXIMUM HYDRAULIC GRADIENT SLOPE AND ELEVATION

1. The maximum hydraulic gradient slope allowed will be that which will produce a velocity as in Table 6-1.

TABLE 6-1

MAXIMUM VELOCITIES FOR GIVEN TYPES OF PIPE

Type of Pipe Maximum Velocity

O-Ring RCP 20 feet per second

Diaper Joint RCP 12 feet per second

Gasketed CMP 12 feet per second

2. For storm sewer outfalls, the maximum allowable velocity should be consistent with the soil stability requirements at the pipe outlet. If the velocities exceed permissible velocities for the outlet soil conditions (see Section 9.1.G), the installation of staked sod, pavement or structural energy dissipators may be required.

3. The maximum hydraulic gradient elevation shall be no higher than the edge of pavement at the inlet throat for internal stormwater collection systems.

H. MAXIMUM LENGTH OF PIPE

Refer to Table 5-3

I. MISCELLANEOUS MINIMUM PIPE CLEARANCES

The minimum clearances listed in Table 5-5 shall be used when determining pipe elevations. Should it be impossible to maintain these separations, then adequate means (i.e., concrete encasement, etc.) must be utilized to protect both the storm sewer system and the obstructing facility. In addition, utility lines passing through conflict structures shall be a minimum of 1 foot above the bottom of the structure and any energy losses created by the presence of the utility shall be calculated and accounted for in the storm sewer tabulations of the hydraulic grade line.

J. DESIGN TAILWATER

All storm sewer systems shall be designed taking into consideration the potential tailwater of the receiving facility or body of water. Generally, the tailwater must be determined by calculation, considering the same design storm frequency used to estimate the design storm sewer flows. If a tailwater elevation cannot be determined by this method, the hydraulic gradient shall begin at the crown of the discharge pipe at the receiving facility, or at an elevation equal to the average of the design high water and normal pool elevations, the estimated tailwater corresponding to the design frequency of the storm sewer system, whichever elevation is higher.

K. STORM SEWER TABULATIONS

In developments where storm sewers are planned, the Stormwater Design Calculations shall include the Storm Sewer Tabulation Form (Figure 6.1) and a map containing the individual sub-basin delineations.

L. INLETS, MANHOLES AND JUNCTION BOXES

All inlets, manholes and junction boxes shall conform to the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition. All inlets are to have manhole lids for maintenance access. All pipe are to be cut flush with the inside walls of manholes and junction boxes. Conflict manholes shall be included in the design where conflicts occur with other utilities.

M. STORM SEWER ALIGNMENT

All storm sewer layouts shall avoid abrupt changes in direction or slope and shall maintain reasonable consistencies in flow velocity. Where abrupt changes in direction or slope are encountered, provisions shall be made to handle the resulting headlosses. Any abrupt changes to alignment shall take place within manholes or junction boxes. It is preferred to locate pipes outside the edge of pavement. In those limited cases where it is unavoidable to place long sections of pipe under pavement, approval by the County Administrator is required. In addition, the pipe shall be oriented such that maintenance can be accomplished within one (1) lane of the roadway. The width of the maintenance area shall be determined from Section 3.4.B.2.

N. DETERMINATION OF DESIGN HYDRAULIC GRADIENT LINE

Local losses at structures shall be determined for all inlets, manholes, wye branches or bends in the design of closed conduits. When more than one discharge enters a manhole, the pipes shall not be oppositely aligned. If directly opposite discharge pipes are necessary, a deflector shall be placed within the manhole in order to reduce the losses caused by the discharge jets impinging on each other. Deflector installations must have prior approval of the County Administrator.

The energy losses associated with the turbulence in the individual manholes are minor for an open channel or gravity storm sewer system and can typically be overcome by adjusting (increasing) the upstream pipe invert elevations in a manhole by a small amount. However, the energy losses associated with the turbulence in the individual manholes can be significant for a pressure or surcharged storm sewer system and must be accounted for in establishing a reasonable hydraulic gradient line. A table of acceptable head loss coefficients (K) for various types of surcharged manholes/catch basins/junctions is given in Figure 6-2.

O. INLET CAPACITIES

1. Maximum inlet capacities for Hillsborough County Standard inlets are listed in Table 6-2.

TABLE 6-2

MAXIMUM INLET THROAT CAPACITY CRITERIA

Inlet Throat Type Flow Capacity (cfs)

Type 1 5.0

Type 2 (single wing) 10.0

Type 3 (double wing) 15.0

2. If F.D.O.T. standard inlets are to be used, the F.D.O.T. inlet capacity charts shall be utilized to determine inlet capacities.

3. In all cases the effects of an inlet bypass shall be considered and quantified in the drainage calculations.

P. INLET LOCATION

The location and spacing of inlets and manholes shall be based on maximum allowable pipe lengths, inlet capacity and spread-of-flow criteria. Water shall not be carried across intersections or in gutters for distances exceeding 800 feet, unless otherwise approved by the County Administrator.

6.3 DESIGN PROCEDURE

A. GENERAL (From Omaha Metropolitan Stormwater Management Design Manual)

There are several general rules to be observed when designing storm sewer systems to alleviate or eliminate the common mistakes made in storm sewer design. These rules are as follows:

1. Select pipe size and slope so that the velocity of flow will increase progressively, or at least will not appreciably decrease, at inlets, bends, or other changes in geometry or configuration.

2. Do not discharge the contents of a larger pipe into a smaller one.

3. In gravity systems where changes in pipe size occur, the Site Designer should match the crowns of the two pipes at the same level rather than matching flow lines. (When necessary for minimal fall, match the "0.8 x Diameter" point of each pipe.)

4. Conduits should be checked at the time of their design with reference to critical slope. If the slope of a conduit is greater than critical slope, the unit will likely be operating under entrance control instead of the originally assumed normal flow. Conduit slope should be kept below critical slope if at all possible. This also removes the possibility of a hydraulic jump within the system.

B. PRELIMINARY DESIGN PROCEDURE

1. Prepare a drainage map of the entire area to be drained by proposed improvements using topographic maps and field reconnaissance.

2. Make a preliminary layout of the proposed stormwater collection system, locating all inlets, manholes, mains, laterals, ditches, culverts, etc.

3. Delineate the drainage area to each proposed inlet.

4. Label each drainage area and include the size of area, the direction of surface runoff by small arrows, and the coefficient of runoff (rational coefficient) for the area (Chapter 4).

5. Show all existing underground facilities.

6. Establish design rainfall frequency (Table 4-9).

7. Establish minimum inlet time of concentration (Chapter 4).

8. Establish the typical cross section of each street.

9. Establish permissible spread of flow on all streets within the drainage area (See Section 10.10.A.2).

10. Include 1 through 9 on the paving and drainage plans to be submitted for review.

C. INLET SYSTEM

Determining the size and location of inlets is largely a trial and error procedure. The following steps will serve as a guide to the procedure to be used.

1. Beginning at the upstream end of the project drainage basin, outline a trial subarea and calculate the expected flow from it using the Rational Method (Chapter 4).

2. Compare the calculated flow to allowable street capacity.

a. If the calculated flow is greater than the allowable street capacity reduce the size of the trial subarea.

b. If the calculated flow is less than street capacity, increase the size of the trial subarea.

c. Repeat this procedure until the calculated flow equals the allowable street capacity. This is the first point at which a portion of the flow must be removed from the street by an inlet.

3. Record the drainage area, time of concentration, Rational coefficient and calculated flow for the subarea. This information shall be recorded in the storm sewer tabulation form (See Section 6.4).

4. If an inlet is to be used, determine the inlet size, type, amount of intercepted flow, and amount of the flow carried over (bypassing the inlet).

5. Continue the above procedure for other subareas until a complete system of inlets has been established. Remember to account for any carry-over from one inlet to the next.

6. After a complete system of inlets has been established, modification should be made to accommodate special situations such as point sources of large flow, and variation of street alignments and grades.

7. Record information as in (3) and (4) for all proposed inlets.

8. After the inlets have been located and sized the inlet pipes can be designed.

D. STORM SEWER SYSTEM

After the computation of the rate of storm runoff entering each inlet, the storm sewer system required to carry the flow is designed. The rate of flow to be carried by any particular section of the storm sewer system is not the sum of the contributing inlet design flows to that section, but is less than this arithmetic total. The lesser value results from the fact that in actuality, the individual inlet design flows do not enter and instantaneously move through the storm sewer system.

1. Preliminary sizing - Assumption of Gravity System

a. Storm sewer design equations

All storm sewers shall be designed by the application of the continuity equation and Manning's Equation either through the appropriate charts and nomographs or by direct solution of the equations as follows:

(Continuity) Q = AV, and (6-1)

(Manning's) Q = 1.49 AR^2/3 S_f^1/2 (6-2)

n

Where:

Q = Pipe Flow, in cubic feet per second

A = Cross-sectional area of pipe, in square feet

V = Velocity of flow, in feet per second

n = Coefficient of roughness for pipe (Table 5.1)

R = Hydraulic radius of pipe = A/W_p in feet

S_f = Friction or energy slope for flow in pipe, in foot per foot

W_p = Wetted perimeter within pipe, in feet

b. Storm sewer pipe

1. The ground line profile is now used in conjunction with the previous runoff calculations determined for each inlet.

2. The elevation of the hydraulic gradient is initially established approximately at the ground surface or at the inlet throat elevation, whichever is applicable. The final elevation of the hydraulic grade line must conform to Section 6.2.G.3.

3. When this gradient is set and the design discharge is determined, a Manning's flow chart may be used to determine the pipe size and velocity.

4. Velocities can be read directly from a Manning's flow chart based on given discharge, pipe size and gradient slope.

2. Final Design - Determination of Expected Hydraulic Gradient Line

The designer is referred to standard texts for theory and equations describing energy grade line (EGL) and hydraulic grade line. It is recommended that the EGL be computed first and then used to determine the hydraulic grade line. The effects of tailwater must be accounted for in the calculations. The Site Designer is referred to Figure 6-2 for criteria on calculating local (minor) losses.

6.4 STORM SEWER TABULATION FORM (From F.D.O.T. Drainage Manual, 1987)

A. GENERAL

The form for tabulating the results of hydrologic and hydraulic calculations for storm sewer systems is presented in Figure 6.1. This form is for representative purposes only. A large scale form is available from the County Administrator. The items to be recorded on this form have been identified by number on the figure and are briefly described below:

NOTE: All elevations should be give to the nearest 0.1 foot.

1. Selection of Rational Coefficients (C)

Various contributing areas for each inlet should be broken down into high and low rational coefficient areas. Guidance for selecting rational coefficients is presented in Section 4.4.

2. Notes

This space should contain information such as the design storm frequency, roughness coefficients, minimum cover for culverts, and other data pertinent to the entire system.

3. Station

This column should show the survey station number for the structure in question.

4. Distance (Dist)

This column should give the distance of the centerline of the structure from the reference station.

5. Side (S)

This column should give the side, right (Rt) or left (Lt), of the reference station.

6. Structure No.

This column gives the sequential numbers of the drainage structures in the system (S-1, S-2, etc.).

7. Type of Structure

The type of structure is usually indicated by abbreviations such as Type 1, 2, or 3 for inlets; ditch bottom inlets (DBI); manhole (MH). For standard F.D.O.T. structures, the inlet or manhole type should be specified as indicated in the F.D.O.T. Standard Specifications for Road and Bridge Construction, latest edition.

8. Type of Line

The type of line is shown as (M) for mainline and (S) for stub-line.

9. Length (Feet)

This is the length, in feet, from the centerline of the structure in question to the centerline of the next structure.

10. Increment

Increment refers to the incremental area (in acres) corresponding to the rational coefficient being used. It is normally an area of overland flow contributing to the particular inlet. However, a contribution through an existing pipe should also be noted in this column.

Manholes usually do not have an associated incremental area as they are handling areas which have already been tabulated. Bypass caused by insufficient inlet capacity would be adjusted for in the total runoff column (Item 17).

11. Subtotal

This is the subtotal of the total area, for each Rational coefficient value used, contributing flow to or through the structure in question.

12. Subtotal (CA)

To arrive at this figure, the individual subtotal areas are multiplied by their corresponding Rational coefficients.

13. Time of Concentration (Min)

The time of concentration is the time required for runoff to travel from the most representative hydraulically distant point of the total area drained, to the point of the storm sewer system under consideration. This time consists of overland flow, gutter flow, and pipe flow time within the system.

See Section 4.3 for procedures to estimate time of concentration.

14. Time of Flow in Section (min)

This is the time it takes the flow to pass through the section of pipe in question; it depends on the computed velocity in the pipe segment.

15. Intensity (Rainfall)

Rainfall intensity values are determined from the intensity duration-frequency (IDF) curves (Zone 6) developed by the F.D.O.T. (Figures 5-2). Design Rainfall Intensity at any point in the system depends on the design frequency and the time of concentration to that point in the system.

16. Total (CA)

The total (CA) is the sum of the subtotal CAs.

17. Total Runoff (cfs)

Total runoff is the product of the intensity and the total CA, plus or minus any anticipated inlet bypass flows.

18. Inlet Elevation (feet)

This column lists the elevation of the edge of pavement at the inlet throat if the structure is a curb inlet. In the case of manholes and ditch bottom inlets, either the top or grate elevation, and any slot elevation are shown.

19. Upper End (Hydraulic Gradient)

This column shows the elevation of the hydraulic gradient at the upper end of the pipe section.

20. Lower End (Hydraulic Gradient)

The elevation of the hydraulic gradient at the lower end of the pipe section is shown.

21. Upper End (Crown Elevation)

This elevation is the inside crown elevation at the upper end of the pipe section being designed.

22. Lower End (Crown Elevation)

This is the inside crown elevation at the lower end of the pipe section being designed.

23. Upper End (Flow Line)

This elevation is the flow line of the pipe section at the upper end.

24. Lower End (Flow Line)

This elevation is the flow line of the pipe section at the lower end.

25. Fall (Feet)

The hydraulic gradient fall is shown (to the closest one-tenth of a foot).

26. Fall (Feet)

The total physical fall of the pipe section is shown (to the nearest one-tenth of a foot).

27. Diameter (inches)

This column shows the diameter of the pipe used for the section, (in inches) or, if a box culvert is used, the width and height (in feet).

28. Slope (%)

The hydraulic gradient slope is shown above the physical slope of the pipe.

29. Velocity (fps)

The velocity produced by the gradient slope is shown above the velocity produced by the physical slope of the pipe.

30. Capacity (cfs)

The capacity of the pipe on the hydraulic gradient slope is shown above the physical slope.

31. Remarks

This space is available to record specific remarks on the storm sewer system design. The anticipated design flow rate for the typical drainage area should be provided and the intercepted flow used to establish the pipe size. The by-pass flow should be recorded and accounted for. Energy Grade Line elevations and structure loss coefficient/values should also be noted.

CHAPTER 7

DETENTION AND RETENTION PONDS

7.1 GENERAL CRITERIA

A. PURPOSE

The purpose of detention and retention ponds is to serve as a buffer to attenuate peak flows and/or excess runoff volume from urbanized areas.

B. MINIMUM CRITERIA FOR DETENTION/RETENTION PONDS (Commercial and Subdivision)

1. A detention or retention pond shall be provided in accordance with Section 3.1.B.

2. For the design of most detention ponds, the instantaneous peak discharge expected for the undeveloped site due to a 10-year/24-hour rainfall shall not be exceeded by the instantaneous peak discharge from the developed site due to a 25-year/24-hour rainfall. However, if the receiving waters to which the site discharges is considered to have "peak sensitive", or "volume sensitive", or "more than adequate" capacity, other criteria will apply (See Section 3.1.B).

3. Calculation of the instantaneous peak discharge from the undeveloped site shall consider the affect of existing storage in attenuating this peak. Pre- and post-development initial elevations for estimating storage shall be the seasonal high water elevation (as determined by biological indicators or other suitable methods), and controlled seasonal high water elevation, respectively.

4. Retention/detention ponds shall not be constructed above natural ground without a detailed geotechnical report certifying that potential seepage will not affect adjacent properties.

5. Off-site runoff must be routed around or through the project without combining with on-site runoff unless the pond and discharge structure are designed to accept this off-site runoff.

6. The runoff from the 25 year/24 hour rainfall event is to be stored entirely in the pond.

The runoff associated with the 100-year/24-hour event shall be routed through a detention/retention pond to establish the minimum residential floor slab elevation and floodproofing elevation (commercial sites only). In no case should Residential floor slab elevations and floodproofing elevations lower than any flood elevations established by FEMA. The Site Designer is referred to the Hillsborough County Flood Damage Control Ordinance (#87-5) for minimum floor slab and floodproofing elevation requirements. In the special case of retention ponds without an out-of-bank overflow outlet, the residential floor elevations shall be no lower than 2.5 feet above the retention pond Design High Water elevation.

7. For non-residential commercial sites, a minimum of six inches of freeboard is to be provided for sites less than or equal to five acres provided that the product of the rational coefficient times the area (CA) is less than or equal to two (2). If the area is greater than five acres or CA is greater than two, or if the site is residential commercial, then a minimum of one foot of freeboard is to be provided.

8. For non-residential commercial sites only, no more than 50% of the required freeboard may be provided outside of the designated pond area. However, if porous concrete is used to temporarily store runoff in excess of the runoff from the 25-year event, up to 100% of the freeboard requirement may be waived.

9. For commercial, non-residential sites with an area less than (1) one acre, detention storage may be allowed on top of paved parking areas provided that:

a. The design high water elevation is at or below the lowest graded elevation prior to berming or curbing.

b. Maximum depth of parking area storage is six inches.

c. Water quality volume is not to be stored on paved areas.

d. Site freeboard may be accomplished by berming or curbing.

10. For non-residential commercial sites of any size, some storage may be allowed on top of paved parking areas when designing to the volume sensitive criteria, provided:

a. The difference in the pre-developed and post-developed volumes due to the 25-year/24-hour volume is stored entirely in the pond.

b. The design high water elevation is at or below the lowest graded elevation prior to berming or curbing.

c. Depth of storage on the pavement does not exceed six inches.

d. Adequate freeboard is provided, and can be accomplished by berming or curbing.

11. Hydrologic and hydraulic calculations shall be provided for each facility and shall include:

a. Stormwater Management System design calculations and support information such as: stage-storage data, stage-discharge data (if applicable), inflow hydrographs, outflow hydrographs, etc.

b. Input data, when computerized flood routing techniques are utilized, including, but not limited to: basin areas, curve numbers, Rational coefficients, inflow hydrographs, SCS peak rate factor, time of concentration values, rainfall distribution data, stage/storage information, etc.

12. For commercial sites, the maximum sideslope for ponds shall be no steeper than 4:1. If, however, a request for steeper slopes is made for a non-residential commercial site and is granted by the County Administrator, the entire area of the pond shall be fenced with a minimum five (5) foot high chain link fence with a twenty (20) foot double swing gate. In no case are sideslopes for ponds in residential commercial development to be steeper than 4:1 to a depth of 6 feet below normal water level.

13. For subdivisions, the maximum sideslope for ponds shall be no steeper than 4:1 (horizontal:vertical). A steeper sideslope below the minimum depth stated in 7.1.B.14 (detention ponds) or 7.1.B.15 (retention ponds) of up to 2:1 may be utilized, providing that there is proper documentation to ensure that the sideslopes will be stable.

14. For subdivisions, wet detention ponds shall include a minimum depth of 6 feet below normal water level, and shall have side slopes no steeper than 4:1 (horizontal:vertical) to this minimum depth. The minimum depth requirement will not apply to littoral zones, however, the side slope requirement will still apply in this case.

15. Wet retention ponds shall have a minimum depth of 6 feet below normal groundwater elevation and shall have sideslopes no steeper than 4:1 above, and 2:1 below, this minimum depth, providing that sideslope stability can be justified below the minimum depth. The minimum depth requirement will not apply to littoral zones, however, the sideslope requirements will still apply in this case.

16. For subdivisions, the minimum freeboard for ponds shall be one (1.0) foot between design high water and top of bank. When the adjacent property slopes upward from the outer edge of the maintenance area, credit will be given for freeboard to the external limit of the maintenance area. If the outside edge of the maintenance area slopes downward (maintenance area is on an embankment), one (1) foot of freeboard above design high water at the inside edge of the maintenance area shall be included in the design. All points within the adjacent maintenance area shall be at or above the elevation of the top of bank. The cross slope of the maintenance area shall be no steeper than 20:1 (horizontal:vertical). The maintenance area shall be at least 20 feet wide unless otherwise approved by the County Administrator. If the maintenance area is on an embankment, the external slope of the embankment shall be no steeper than 4:1 (horizontal:vertical) and the toe of the external slope shall not extend beyond the boundary of the subdivision. The external slope of the embankment shall be stabilized in accordance with Section 7.1.C.

17. For subdivisions or other County maintained detention/retention ponds, inflow into the pond shall occur by a pipe conveyance system. Mitered end and flared end sections (no endwalls) shall be used inside detention/retention ponds. Other designs, as approved by the County Administrator, may be used which account for the structural stability of the end treatment for pipe entrances and exists within ponds.

18. Recovery of flood storage and pollution abatement volume for detention/retention ponds is considered to be the storage available 72 hours after the end of the storm event. There is an exception to this criteria for detention ponds in volume sensitive areas. (See Section 3.1.B.4.a.2.b.).

19. Vertical walled ponds are not permitted in residential commercial, subdivision, or other County-maintained ponds.

20. Vertical walled ponds are permitted for non-residential commercial sites only, provided that proper berm/embankment width exists at the top of the wall (See Section 7.1.B.21). Back slopes shall not exceed 2:1 (Horizontal:Vertical). Freestanding walls are not permitted.

21. For commercial sites, a minimum berm width of 5.0 feet is to be provided for detention/retention ponds, unless otherwise approved by the County Administrator.

C. GRASSING AND MULCHING

1. The pond maintenance area shall be grassed and mulched in accordance with the Florida DOT Standard Specifications for Road and Bridge Construction (latest edition).

2. All retention and detention ponds shall be stabilized to the normal water line with suitable vegetation. Sideslopes steeper than 5:1 (horizontal:vertical) shall be sodded.

3. Plans and specifications submitted to the County Administrator shall include provisions for establishing vegetation on:

a. Berms

b. Sideslopes

c. Other locations to be stabilized with suitable vegetation as necessary to prevent erosion, silting and maintenance problems.

4. When pond side slopes or soil conditions warrant, sod should be staked to ensure stabilization.

7.2 DETENTION PONDS

A. SEASONAL HIGH GROUNDWATER ELEVATION

1. The seasonal high groundwater elevation shall be determined by a qualified soils engineer or scientist from existing soil conditions and profiles, existing water levels, etc. for the location(s) proposed to be utilized as detention ponds.

2. No storage credit will be given below the controlled seasonal high groundwater table elevation.

B. WATER LEVEL CONTROL STRUCTURES

1. The outlets of detention ponds shall have water level control structures that enable the ponds to function as indicated in the hydraulic calculations.

a. A water level control structure shall not be a pipe riser and shall not be adjustable.

b. Acceptable water level control structures include:

1. A ditch bottom inlet constructed in accordance with the Standard Indexes and the Florida DOT Standard Specifications for Road and Bridge Construction (latest editions).

2. In the event a ditch bottom inlet will not enable the pond to function as indicated in the hydraulic calculations, the water level control structure used shall meet the approval of the County Administrator.

2. All control structures shall be designed to prohibit the entrance of floating debris into the structure. This shall be achieved by attaching a skimming device to the outfall structure. The bottom of the skimming device should be at least 6 inches below the normal water level and the top no lower than the design highwater elevation. An appropriate hydraulic design of the device will be required to insure that the skimming device will not control pond discharge.

3. The control structure shall have a slot or orifice design of no less than three (3) inches unless otherwise approved by the County Administrator and the design low water elevation of the detention pond shall be at the slot or orifice invert elevation. The top of the control structure should be at the elevation of the design high water.

Since weir coefficients have been developed only for flow conditions where the length of the weir (L) is much greater than the head on the weir H (depth of water above weir crest), an orifice rather than a slot should be used if H/L>1 at any time during the design storm event.

4. The water quality storage volume may also be considered for peak flow attenuation, only:

a. If the pond is for retention only and meets the requirements for percolation, or if the pond is designed in accordance with Section 3.1.B.4.a.2.b.

b. For that portion of the pollution abatement volume which is bled down within 72 hours after the end of the storm through a positive discharge structure such as an orifice and:

1. The orifice opening is at least three (3) inches in diameter.

2. If rectangular, the smallest dimension is at least three (3) inches.

3. If a Cipoletti weir is used, the bottom weir length shall be no less than three (3) inches.

5. Tailwater conditions downstream of the water level control structure shall be accounted for in the design of the water level control structure.

6. Underdrains are not considered to be a positive means of low water control, therefore, the storage volume designed to be dissipated by underdrains below the slot or orifice elevation of the outlet structure, can not be considered for peak flow attenuation.

7. Control structures are not to be placed within County road right-of-ways.

C. DETENTION POND OUTFALL CONTROL DESIGN

1. Direct discharge by means of control structures into storm drains or through culverts will be permitted if the receiving systems have the capacity for such discharges. Such systems include:

a. Storm sewer systems

b. Manmade ditches

c. Natural waterways

d. Lakes

2. When direct point discharge is expected to degrade waters of natural streams, marshes, environmentally sensitive areas, and lands naturally receiving sheetflow, the discharge structure shall direct the flow to an intermediate spreader swale system.

3. In designing detention ponds where direct discharge is allowed, discharge may be controlled by the use of a weir or orifice structure. The designer should refer to standard hydraulic references for the theory and equations which govern weir and orifice flow.

4. The designer must also check the capacity of the outfall or discharge pipe to determine whether this pipe controls the discharge, rather than the weir or orifice, at any time during the design runoff event. The Site Designer shall consider local losses in the evaluation of detention pond outfall pipe systems regardless of flow velocities in the pipe(s).

5. Where environmentally feasible, the discharge from detention ponds to the Hillsborough, Alafia, and Little Manatee Rivers shall flow through vegetated swales.

D. NATURAL DEPRESSED AREAS

1. Natural depressed areas located entirely within the project boundaries may be used for detention purposes when not adversely affecting off-site water levels.

2. Depressed areas which may be considered for detention include, but are not limited to:

a. Viable wetlands

b. Habitat diversity systems

c. Centralized preservation areas

d. Environmentally sensitive areas

3. In all cases, the County Administrator shall make the final determination of suitability of natural areas for stormwater detention.

4. The storage above the seasonal high water level in natural depressed areas shall be considered in the determination of pre-development flows.

7.3 RETENTION PONDS

A. SEASONAL HIGH GROUNDWATER ELEVATION

1. The seasonal high groundwater table shall be determined using acceptable engineering practices, by a qualified soils engineer or scientist, based upon SCS methodologies and the following factors:

a. Existing soil conditions (spodic stain lines, where applicable)

NOTE: The seasonal high groundwater level is typically 1 to 2 feet above the spodic stainline (per SCS).

b. Soil profiles

c. Measured groundwater levels

d. Measured water levels surrounding water bodies

2. This elevation shall be included in the Stormwater Management System design plans.

B. DESIGN CRITERIA FOR RETENTION PONDS

The following criteria shall be used to design retention ponds:

1. A suitable overflow outlet (man-made or natural) shall be provided for retention ponds where practical. Such an overflow is to be designed so that the discharge during a post-development 100-year/24-hour storm, will not exceed the 100-year/24-hour pre-development flow rate. The character of the pre-development flow pattern shall be maintained by the use of spreader swales or other features.

2. A subsoil investigation shall be conducted and shall include one boring for each one-third (1/3) acre of pond bottom, if percolation will primarily be through the pond bottom. One boring will be needed for each 500 feet of pond perimeter, if percolation will primarily be through the pond sideslopes.

a. There shall be a minimum of two (2) borings per retention pond.

b. The borings shall extend twenty (20) feet below the pond bottom and shall be uniformly distributed, unless otherwise approved by the County Administrator.

c. The soil profile and existing ground water elevation and estimated seasonal high groundwater elevation shall be determined for each boring.

d. The soils shall be sampled and classified in accordance with the American Society for Testing and Materials (ASTM) Standard Method D2487.

e. The seasonal high groundwater table elevation shall be determined by a qualified soils engineer or scientist and the results shall be included in a subsoil investigation.

f. Existing ground surface elevations at each boring location shall be provided.

g. A subsoil investigation report shall be included with the Stormwater Management System Design Calculations.

3. Retention ponds shall have an infiltration rate test performed for each one-half (1/2) acre of pond bottom, if percolation will primarily be through the pond bottom. One infiltration rate test will be needed for each 500 feet of pond perimeter, if percolation will primarily be through the pond sideslopes.

a. There shall be a minimum of one infiltration rate test for each retention pond.

b. The infiltration rate test (Double Ring Infiltrometer Test) shall be ASTM Standard Method D3385-75.

1. The representative field infiltration rate shall be the lowest rate measured.

2. Test results shall be included in the Stormwater Management System Design Calculations.

c. The infiltration rate test shall be performed at the depth and location which will provide representative test results for use in design of the retention pond.

d. The subsoil investigation report shall provide detailed information on all test procedures, test depths and locations and data measurements and results.

4. The design infiltration rate shall be determined by a qualified soils engineer or scientist. The retention pond design shall be based on an infiltration rate that is no greater than three-fourths (3/4) of the lowest infiltration rate obtained from the tests, with the maximum allowable rate not to exceed 18 inches per hour.

5. The retention pond bottom shall be no less than fifteen (15) feet above SM, SC, ML, CL, OL, MH, CH, OH and PT soils as defined by ASTM Standard Method D2487 and shall be no less than twenty (20) feet above bedrock. The seasonal high groundwater shall be at least five (5.0) feet below the retention pond bottom for drainage CA (C = coefficient of runoff and A = area in acres) of 2.0 or less. This clearance shall increase linearly to 10.0 feet between CA = 2.00 and CA = 4.0. For a CA of 4.0 or more, the clearance to seasonal high groundwater shall be at least 10.0 feet. If the above specified clearances cannot be met, the Site Designer must demonstrate, by detailed calculations, that the retention pond will function according to County criteria and the intended design. These calculations must take into consideration the effects of groundwater mounding on percolation both during the rainfall event and during the recovery of the design storage volume.

Where discontinuous restrictive layers exist or where a definable combination of restrictive and non-restrictive layers exist, the Site Designer shall consult with the County Administrator to determine the applicability of either of the above criteria and the need for additional soils data.

6. The retention pond bottom, for dry retention ponds, shall be uniformly graded to provide a low point twelve (12) inches below the bottom perimeter elevation. The final grading of the pond bottom shall remove the final six (6) inches and shall be the last work in the construction of the road, bridge and stormwater management facilities. Also, for dry retention ponds, a minimum of two (2) feet of clearance is required between the seasonal high groundwater table and the proposed pond bottom.

7. All pertinent information and calculations described above shall be included with the Stormwater Management System Design plan.

7.4 DETENTION/RETENTION POND ANALYSIS

A. GENERAL

A routing analysis is required for the design of all detention ponds. A routing analysis is also required for retention ponds where percolation is considered during the runoff event. The Storage Indication Method (Modified Plus Method) and other hydrodynamic methods are the only routing method recognized by Hillsborough County. Tailwater conditions must be considered in the routing calculations.

B. STRAIGHT LINE (CONSTANT) DISCHARGE

Straight line or constant, non-varying discharge is not acceptable.

7.5 MAINTENANCE OF COMMERCIAL RETENTION/DETENTION PONDS

A statement concerning the proper maintenance of commercial retention/detention ponds shall be shown on the side Stormwater Management System Design plans. The statement shall include, but not be limited to, provisions for minimizing erosion, maintaining vegetation, and maintaining siltation, and shall indicate the expected frequency of maintenance.

CHAPTER 8

ROADWAY UNDERDRAIN DESIGN

8.1 ROADWAY UNDERDRAIN CRITERIA

A. GENERAL

1. Underdrains may be required to facilitate groundwater control for roadways which are to be maintained by Hillsborough County and also for private subdivision roadways.

2. When the use of underdrains is required, the site construction plans shall include all details necessary to indicate the underdrain locations and design/construction parameters.

3. Underdrains are considered to be permanent controls of the groundwater table. The design of the roadway may be based on long term groundwater level control through the use of underdrains. However, the underdrain system must be designed by a qualified professional with experience in groundwater analysis.

4. The use of roadway underdrains is dependent on soil types, seasonal high groundwater (SHG) elevation, landslope, and elevation of the roadway base. Roadway underdrains are required as indicated below:

a. SHG within 2-3 feet of low edge of pavement - underdrain on one side of road.

b. SHG within 1-2 feet of low edge of pavement - underdrain on both sides of road.

c. SHG less than 1 foot below low edge of pavement - elevate roadway.

5. The underdrains shall be 18 inches outside of the curb and a minimum of 24 inches below the bottom of the curb, and shall have a positive slope to a positive outfall.

6. All underdrain designs shall utilize filter fabric, underdrain pipe, and filter aggregate.

7. When an underdrain is required to control high groundwater adjacent to proposed roadways, the roadway base shall not be limerock.

B. FILTER FABRIC

1. A filter fabric envelope shall be used with underdrains and shall be an appropriate strong, porous nylon, polyester, polypropylene or other fabric approved by the County Administrator which completely covers the underdrain surface in such a way as to prevent infiltration of surrounding material.

2. The filter envelope shall weigh a minimum of 2.5 ounces per square yard, shall retain soil particles larger than two hundred twelve (212) microns (No. 70 sieve) and shall pass particles finer than twenty-five (25) microns.

3. When tested in accordance with ASTM D1682, the grab strength (wet) of the filter fabric shall not be less than one hundred (100) pounds and the grab elongation shall not be less than sixty (60) percent.

4. Storage and handling of the filter fabric shall be in accordance with the manufacturer's recommendation.

5. Torn or punctured filter fabric shall not be used.

6. The filter fabric shall not be exposed to sunlight for periods exceeding the manufacturer's recommendation, or six (6) weeks, whichever is shorter.

7. The filter fabric is to be placed around the underdrain pipe and, when appropriate, around the aggregate.

C. UNDERDRAIN PIPE

1. Underdrain pipe shall be of sufficient size (6-inch minimum) to effectively control and convey the anticipated flow. Calculations are to be submitted to justify proposed underdrain size.

2. The length is not to exceed 500 feet without increasing pipe size to the next larger diameter. Cleanouts are to be spaced no greater than every 250 feet and at the ends of the underdrain. Cleanouts are to extend to the ground surface in accordance with the Florida DOT Standard for Road and Bridge Construction, latest edition.

3. Underdrain pipe slopes should be sufficient to maintain velocities at or above 2 feet per second for design flow conditions. Underdrain shall typically be constructed on a grade parallel with the edge of pavement profile, but in no case shall the minimum underdrain slope be less than one-tenth percent (0.10%).

4. Underdrain pipe shall be concrete, corrugated aluminum, polyvinyl-chloride, corrugated polyethylene or other material approved by the County Administrator.

a. Concrete, corrugated aluminum and polyvinyl-chloride underdrain materials shall be in accordance with the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

b. Corrugated aluminum underdrain pipe may be used in tidal areas but shall not be used near dissimilar metals that may cause galvanic action.

c. Corrugated polyethylene tubing underdrain installations shall conform to the following:

1. corrugated polyethylene tubing and fittings shall meet the requirements of AASHTO M252, latest edition.

2. the minimum wall thickness of the crown, sidewalls or valley shall be 0.025 inches.

3. coiling of tubing is not permitted.

d. The tubing shall not be exposed to sunlight for periods exceeding the manufacture's recommendation, or six (6) weeks, whichever is shorter.

e. Tubing shall be placed and maintained true to line and grade until secured with compacted backfill.

f. Perforated tubing shall not be placed under street pavement. When tubing must be extended beneath the crossroad, a non-perforated section shall be used.

g. Underdrain sections which deflect or collapse greater then five (5) percent shall be rejected.

D. AGGREGATE

1. Fine aggregate for cement concrete, in accordance with the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition, shall be used to backfill the trench, except that the County Administrator may approve other backfill material provided tests are submitted indicating the material will adequately serve as a filter.

2. The minimum density of the backfill shall be ninety-five (95) percent of the standard laboratory density determined in accordance with AASHTO T99 (Method A).

3. The underdrain cross-section is to be in accordance with Florida DOT Standard Specifications for Road and Bridge Construction, latest edition, unless otherwise approved FDOT Type II will be the only underdrain design allowed for arterial and collector roadways within Hillsborough County.

CHAPTER 9

NONROADWAY DITCH DESIGN

9.1 GENERAL DESIGN CRITERIA

A. DESIGN FREQUENCY

Refer to Table 4-9

B. DESIGN DISCHARGE

The determination of design flows for an open channel system shall be in accordance with the methods and procedures set forth in Chapter 4.

C. DESIGN METHODOLOGY

1. A ditch shall be initially sized using Manning's formula. The initial ditch size shall then be evaluated relative to additional potential energy losses (i.e. bends, expansions, constrictions, etc.) and the impacts of tailwater (backwater). If required, the initial ditch section shall be increased or otherwise modified to properly accommodate the design flow. In all cases, data including drainage area, velocity and depth of flow shall be provided in the Stormwater Management System Design Calculations along with typical sections. The Site Designer is referred to standard hydraulics texts for the definitions and application of Manning's Equation.

2. No credit for stormwater will be given for ditches. A ditch is considered to be a means of conveyance only.

3. No ditch blocks will be permitted in road right-of-ways for internal subdivision drainage.

D. MAXIMUM SIDESLOPE

The maximum sideslope shall be 4:1 for all nonroadway ditches except in cases of overriding public interest where necessity deems it appropriate for steeper sideslopes to be constructed.

E. MINIMUM BOTTOM WIDTH

The minimum bottom width for ditches shall be 3.0 feet.

F. DESIGN TAILWATER

1. All open channel systems shall be designed taking into consideration the tailwater of the receiving facility or body of water.

2. The appropriate tailwater elevations must be determined by calculations based upon the design criteria and frequencies contained in Table 4-9.

G. MAXIMUM ALLOWABLE VELOCITIES FOR UNLINED AND LINED OPEN CHANNELS

1. The maximum allowable velocities for unlined open channels (bare soil condition) are listed in Table 9-1.

TABLE 9-1

MAXIMUM ALLOWABLE VELOCITIES FOR UNLINED OPEN CHANNELS

(From FDOT Drainage Manual, 1987)

Allowable Velocity

For a Flow Depth of

Soil Type About 3 Ft. (f.p.s.)

Silt or Fine Sand 1.50

Sandy Loam 1.75

Silt Loam 2.00

Firm Loam 2.50

Stiff Clay 2.75

Hardpans 6.00

2. The maximum allowable velocities for lined open channels are listed in Table 9-2.

TABLE 9-2

MAXIMUM ALLOWABLE VELOCITIES FOR LINED OPEN CHANNELS

(From FDOT, Drainage Manual, 1987)

Type Allowable Velocity (fps)

Grassing & Mulching Same as Unlined Channels

(Table 9-1)

Standard Sod 4.0

Lapped Sod (25% overlap) 5.5

Asphaltic Concrete 8.0

Concrete Ditch Paving 10.0

H. MINIMUM LONGITUDINAL GRADE

For open channels that are intended to remain dry except during runoff conditions, the minimum grade allowable shall be 0.10%.

I. DITCH ALIGNMENT

The alignment of existing ditches should be preserved whenever practical. For skewed ditch crossings, the culvert shall be skewed to maintain the existing ditch alignment.

J. CHANNEL CURVATURE

A minimum centerline radius of fifty (50) feet or ten (10) times the bottom width, whichever is larger, shall be utilized. Channel protection shall be provided when channel curvature produces erosive velocities in excess of those shown in Section 9.1.G.

K. MINIMUM FREEBOARD

A minimum freeboard of one (1) foot shall be maintained between design high water surface elevation and the adjacent top of bank, except in cases of overriding public interest where a lesser freeboard may be appropriate.

L. DITCH EROSION PROTECTION

1. Ditches shall be provided with permanent erosion protection. Erosion protection may be turf, using an approved type grass, or an approved type of liner.

2. When turf protection is used, ditches shall be sodded, sprigged or seeded for a lateral distance extending from within one (1) foot of the road pavement to the top of the swale ditch backslope.

3. Ditches shall be grassed and mulched in accordance with the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

4. Ditch pavement shall be in accordance with the Standard Indexes and the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

5. Sideslopes above the paved section shall be shaped and sodded on a slope of four horizontal to one vertical or flatter, except in cases of overriding public interest where steeper slopes may be appropriate.

M. GRADING ADJACENT TO DITCHES

Areas adjacent to ditches shall be graded to preclude the entrance of excessive stormwater runoff except at locations provided for such purpose.

9.2 UTILITIES CROSSING DITCHES

Where it is necessary for a utility to cross a ditch, the following minimum requirements shall be adhered to:

A. AERIAL CROSSING

Utilities crossing ditches shall have a minimum of one (1) foot clearance above design high water with the area underneath the crossing to be concrete lined to prevent vegetative growth.

B. UNDERGROUND CLEARANCE

A utility crossing a ditch shall have a minimum of a two (2) foot clearance below the invert of the ditch.

C. UTILITY IDENTIFICATION

Utilities shall be adequately marked to protect against accidental damage during maintenance operations.

D. AERIAL SUPPORTS

No aerial supports shall be allowed in the confines of the ditch cut unless authorized by the County Administrator.

E. UNDERGROUND CROSSINGS

Underground utility crossings of all floodways, open channels and ditches shall be clearly labeled on-site with suitable markers or permanent signs.

CHAPTER 10

ROADWAY (PAVEMENT) DRAINAGE DESIGN

10.1 GENERAL

Good pavement drainage design consists of the proper selection of materials (i.e. grades, cross slopes, curb types, inlet locations, etc.) to remove the design storm rainfall from the pavement in a cost effective manner. In addition, the design should provide for the preservation of safety, traffic capacity and integrity of the highway and street system.

10.2 ELEVATION OF LOW EDGE OF PAVEMENT

Design criteria for the location of the low edge of street pavement elevation are specified in Table 10-1 below. Also refer to Chapter 8 for additional criteria related to controlling the seasonal high groundwater level.

TABLE 10-1

DESIGN CRITERIA FOR THE LOCATION OF THE

LOW EDGE OF STREET PAVEMENT

Water Body/Stormwater Freeboard from DHW to Design High

Management Structure Low Pavement Elevation Water

Type of Roadway Elevation

Near Hillsborough Bay $ 0 foot 6.0 ft. NGVD

Old Tampa Bay & Tampa Bay

Near retention ponds $ 1.0 foot 100 year

Near detention ponds

Streets with Soil Cement

Roadway Based $ .5 foot 25 year

Streets with All Other

Roadway Bases > 1.0 foot 25 year

10.3 MINIMUM ROADWAY GRADES

The Site Designer is referred to Hillsborough County roadway design criteria for acceptable minimum roadway grades.

10.4 MINIMUM ROADWAY CROSS-SLOPE

For drainage purposes, a cross slope of 1/4 inch/foot shall be used for roadways with a central crown line. Refer to Hillsborough County Highway and Bridge Technical Manual for acceptable cross slopes for super-elevated roadway sections.

10.5 DESIGN FREQUENCY

Refer to Table 4-9 for the design frequency to be utilized for the design of pavement drainage.

10.6 RUNOFF DETERMINATION

The peak rates of runoff for the design of the pavement drainage system shall be determined by the Rational Method.

10.7 CONCRETE CURB, GUTTER AND SIDEWALKS

A. DESIGN DETAILS

Details of concrete curb, gutter and sidewalks shall conform to the Standard Indexes available from Hillsborough County or the Florida Department of Transportation, if applicable.

B. MATERIALS AND INSTALLATION

Materials and installation shall conform to the Florida DOT Standard Specification for Road and Bridge Construction, latest edition. Compaction densities under curbs and gutters shall be a minimum of ninety-eight (98) percent Modified Proctor for a six (6) inch depth.

10.8 GRASSING, MULCHING AND SODDING

In residential subdivisions where home construction is not imminent, areas located within thirteen (13) feet of the back of curb that are disturbed by construction and have slopes of 6:1 or greater shall be grassed and mulched or sodded in accordance with Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

10.9 ROADWAY DITCHES

A. DESIGN

1. Roadway ditches shall be sized using the criteria in Section 9.1.

2. In all cases, data giving drainage area, velocity, and depth of flow shall be included in the Stormwater Management System Design Calculations.

3. The minimum bottom width shall be three (3.0) feet unless otherwise approved by the County Administrator.

4. The maximum sideslope shall be 4:1, unless otherwise approved by the County Administrator.

5. Standard ditch sections shall be provided along external roadways.

B. MAXIMUM ALLOWABLE VELOCITY

The maximum allowable velocities for unlined and lined open channels are listed in Tables 9-1 and 9-2, respectively.

C. ROADWAY DITCH GRADES

1. Minimum

A minimum of 0.10% or the minimum required to provide for the design flow, whichever is greater, shall be the minimum allowable grade.

2. Maximum

Maximum allowable ditch grades will be governed by maximum allowable velocities as listed in Tables 9-1 and 9-2.

3. Ditch Protection

Ditch protection is required when the design velocities exceed allowable velocities for unlined channels. Refer to Tables 9-1 and 9-2.

D. DEPTH OF ROADWAY DITCHES

1. Minimum Depth: 2.0 feet from the low edge of shoulder unless otherwise approved by the County Administrator.

2. Maximum Depth: 3.5 feet from the low edge of shoulder unless otherwise approved by the County Administrator.

E. GRASSING AND MULCHING, SODDING

All roadway ditches shall be grassed and mulched or sodded in accordance with the Florida DOT Standard Specifications for Road and Bridge Construction, latest edition.

10.10STREET DRAINAGE

A. DESIGN CRITERIA

1. The maximum length of gutter prior to the need for an inlet shall be controlled by the allowable spread of flow on the pavement.

2. The spread of flow shall be limited to the crown of the road, or to a point on the road corresponding to a maximum depth of 5 inches at the edge of pavement, whichever is more restrictive, during the design storm which has a frequency equal to that for which the storm sewer system will be designed. The time of concentration for spread of flow calculations shall be equal to the appropriate overland flow travel time.

3. Water shall not be carried across street intersections, and the gutter run shall not exceed eight hundred (800) feet, unless otherwise approved by the County Administrator.

4. Streets that exceed a 1% longitudinal slope shall have inlets constructed at intersections to intercept upstream flow.

B. GUTTER FLOW CALCULATIONS

The designer is referred to standard hydraulic texts for the proper use of Manning's Equation in determining the spread of flow in gutters. Maximum flows for standard roadway sections which will not require spread of flow calculations are listed in Table 10-2.

TABLE 10-2

MAXIMUM FLOWS FOR STANDARD 2 LANE ROADWAY SECTION WHICH WILL NOT REQUIRE

SPREAD OF FLOW CALCULATIONS

Maximum Flow (CFS)

Longitudinal Slope (%) Miami Gutter Standard Gutter

.24 1.9 2.2

.30 2.1 2.4

.40 2.4 2.8

.50 2.7 3.1

.60 3.0 3.4

.70 3.2 3.7

.80 3.5 3.9

.90 3.7 4.2

1.0 3.9 4.4

1.5 4.7 5.4

2.0 5.5 6.2

NOTE: Allowable Spread = 12 foot; Cross-slope = 1/4" foot; n = 0.015

CHAPTER 11

PHOSPHATE MINING DRAINAGE

11.1 GENERAL

Phosphate mining in Florida is regulated by federal, state, and local regulatory agencies. Hillsborough County has adopted Phosphate Mining Regulations to control phosphate mining within Hillsborough County. The following section highlights phosphate mining specifications which are applicable to drainage criteria. See LDC, Div. 3.7 for specific criteria.

11.2 PHOSPHATE MINING DRAINAGE REQUIREMENTS

Within the County Phosphate Mining Ordinance, requirements relating to Phosphate Mining Drainage include:

A. MINING AND RECLAMATION PLANS

1. Mining and reclamation plans that are submitted to the County shall include:

a. Sufficient topographic maps to insure adequate definition of all drainage characteristics of the applicant's lands and their affects upon neighboring lands,

b. The seasonal high groundwater elevation,

c. Drainage runoff calculations,

d. A description of all points of discharge from the applicants property,

e. An estimate of the rate of discharge:

1. during normal operations,

2. during mean annual operations,

3. during 25- and 100-year floods, and

4. as an assessment of the effect that the proposed mining and reclamation operations can have upon the background drainage regime.

2. Plans shall include drainage and flood control features that are to be accomplished during and after the conclusion of mining and reclamation operations.

B. FLOW DIVERSIONS

1. No water will be diverted from premining natural and artificial stream channels or lakes unless specifically approved by the Board of County Commissioners.

2. Diversions shall be permitted only after a thorough analysis of stream flow conditions, and shall be limited to quantities that are not detrimental to

upstream or downstream property owners or the environment.

C. REGULATION OF PHOSPHATE MINING DRAINAGE

1. Clearing of land, mining, placement of earthen fill, construction of buildings, access roads, utilities or other facilities will not be permitted:

a. between the 25-year flood elevation and the 100-year flood elevation, unless reviewed by the appropriate regulatory agencies and specifically approved by the County Administrator.

b. below the 25-year flood elevation, unless reviewed by the appropriate regulatory agencies and specifically approved by the Board of County Commissioners at a noticed public hearing.

2. The approvals listed above may be granted only if the permittee can show that his operations:

a. will not adversely affect the flooding characteristics of the stream or adjoining properties.

b. will comply with the provisions of the Hillsborough County Flood Damage Control Ordinance.

c. will comply with Environmental Protection Commission Rules and the rules of any state regulatory agency if the proposed activities are to occur in wetlands.

3. Flood elevations will be those established by the following agencies:

a. Hillsborough County

b. Southwest Florida Water Management District

c. U. S. Geological Survey

d. Federal Emergency Management Agency

4. Where flood elevations have not yet been determined, the County Administrator will make a decision based upon a review of the best available information, including, but not limited to:

a. a proposal from the permittee.

b. calculations submitted by the permittee's Site Designer.

D. REGULATION OF SETTLING PONDS

1. All dams containing settling and thickening ponds will be located, designed, constructed and maintained in compliance with the rules and regulations of the Department of Environmental Regulation of the State of Florida, and in accordance with sound engineering practices.

2. Outlet structures from settling ponds will be designed and constructed according to accepted, sound engineering practices.

3. Settling ponds will be designed with the capability of either storing or releasing twelve (12) inches of rainfall over the affected watershed in a period of twenty-four (24) hours (but not less than six (6) inches in three (3) hours) without encroaching on the minimum five (5) feet of freeboard required by the Florida Department of Environmental Regulation.

APPENDIX A

GLOSSARY

ADEQUATE OUTFALL

Outfall which has no known inadequacies or flooding conditions but would experience increased water elevations if discharge areas were increased.

ADVERSELY IMPACT

To destroy or damage or to contribute to the destruction or damage of something or created harmful effects.

ANGLE OR REPOSE

Maximum angle in which soil slopes will not fail.

ANNUAL FLOOD

The highest peak discharge in a twelve month period.

ANTECEDENT MOISTURE CONDITIONS

The degree of moisture within a drainage basin and/or watershed at the beginning of a storm.

AREA OF SPECIAL FLOOD HAZARD

Land in the floodplain within a community subject to a one percent or greater chance of flood in any given year.

ARTERIAL STREET

A road or thoroughfare that has been or may be designated for the movement of large volumes of traffic between points in the County, which said road will ordinarily have controlled or limited right of access.

AVERAGE ANNUAL FLOOD

A flood which has a recurrence interval of 2.33 years and equals maximum annual floods during the period of record.

BASE FLOOD

A flood having a one percent chance of being equaled or exceeded in any given year.

BASEFLOW

Surface water recharge which originates from ground water seepage during low flow conditions.

BASIN (DRAINAGE BASIN)

Surface drainage area which is self contained.

BRIDGE

Bridge structure, including supports, erected over a depression or an obstruction, such as water, a highway or a railway. It has a deck for carrying traffic or other moving loads.

CAPACITY

The limiting amount that the conveyance channel, pond, pipe or other hydraulic structure can manage in accordance with the criteria specified in this Manual.

CAPITAL IMPROVEMENT

The acquisition of land or property and/or the construction of, or improvements to any, but not limited to, the following: Building or structure, utility, road, park, open area or other public place requiring the expenditure of public monies.

CATCH BASIN

Structure that is usually built at the curb line of a street which permits surface water runoff to flow into a storm sewer while retain grit and debris below the point of overflow.

CATCHMENT

Area of surface drainage which is bounded by opposing drainage divides.

CHANNEL

A passageway to convey water which is defined by bed, sides and banks.

CHANNELIZATION

Alteration of a stream channel's width, depth, length, and/or geometry in order to improve drainage characteristics.

COLLECTOR STREET

A street designed or designated to connect a number of local streets with arterial streets.

COMPENSATING STORAGE

Storage volume required to compensate for storage volume lost due to filling within the 100-year floodplain.

CONFINED AQUIFER

Aquifer which is confined between two layers of impermeable material.

CONTROLLED SEASONAL HIGH GROUNDWATER ELEVATION

The elevation which the groundwater can be expected to rise to due to a normal wet season after modifications to the surface and groundwater regime have occurred in the area.

CONVEYANCE

A path for water to move from one place to another in a continuous flow.

COUNTY ADMINISTRATOR

The Chief Executive Office of Hillsborough County, or his designee.

CRITICAL DEPTH

Depth at which the specific energy (sum of depth and velocity head) is minimum.

CRITICAL SLOPE

Slope required to maintain uniform flow at critical depth.

CUT AND FILL

Alteration of land surface by excavating part of an area and using the excavated material for adjacent embankments or fill areas.

CONSTRUCTION PERMIT

Surface water management permit issued by the Southwest Florida Water Management District authorizing construction, alteration or abandonment of a surface water management system in accordance with the terms and conditions of the permit.

Also, a permit obtained from the Building Department to construct in County right-of-way or easements.

CULVERT

Structure which conveys stormwater discharge contained in open ditches, swales, and lakes under roadways and other obstructions.

DAM

A barrier which either confines or transports water for storage, diversion, or detention or one of the following purposes: (1) create a hydraulic gradient, (2) prevent erosion downstream, or (3) to retain sediment or debris.

DEPRESSION STORAGE

Capacity of a watershed, catchment or drainage basin to retain water in puddles, depressions and/or foilage.

DESIGN HIGHWATER

Peak elevation of a surface water body which is determined according to the flow conditions of specified design floods.

DESIGN LIFE

Period of time for which a facility is intended to perform its designated function.

DESIGN LOW WATER

Elevation in a surface water body below which no credit is allowed for meeting Hillsborough County storage requirements.

DESIGN STORM

Selected precipitation trend which is used to provide a characteristic amount, intensity, duration, and frequency for the basis of design criteria and specifications.

DETENTION POND

A storage area used to delay stormwater runoff prior to discharge into a receiving system/pond.

DETENTION TIME

Theoretical time required to displace a unit volume of water at a given rate of discharge.

DETENTION VOLUME

Volume of water equal to the change in hydraulic head between the overflow elevation and the control elevation of the discharge structure multiplied by the average area of the open surface area behind the discharge structure.

DEVELOPMENT

Any man-made change to real property, including but not limited to dredging, filling, grading, paving, excavating, clearing, timbering, ditching or draining.

DIKE

An embankment or structure which confines or controls water.

DRAIN

An open or closed conduit which transports excess surface water or groundwater.

DRAINAGE

Interception and removal of surface water or ground water by artificial or natural means.

DRAINAGE BASIN

(See definition for "Basin").

DRAINAGE DIVIDE

Imaginary boundary across which there is no flow, and which is separated by two or more catchment basins.

DRAWDOWN

Lowering the level of surface water, groundwater or the potentiometric surface as a result of changes in outflow in the system.

EASEMENT

An interest in land owned by another that entitles its holder to a specific limited use or enjoyment.

EMBANKMENT

Man-made impoundment constructed of soil, rock or other material.

ENCROACHMENT

Infringement into the floodplain/floodway by development causing a reduction in volume and/or conveyance.

ENERGY GRADE LINE

Line showing the total energy at any point in a pipe.

ENVIRONMENTALLY SENSITIVE AREAS

Conservation Areas and Preservation Areas pursuant to the Conservation Element of the Hillsborough County Comprehensive Plan. Conservation Areas include the following types of wetlands (w), natural water bodies (nwb), and uplands (u): freshwater marshes (w), shallow grassy ponds (w), hardwood swamps (w), cypress swamps (w), natural shorelines other than natural beaches and dunes (w), Class III Waters (w, nwb), and sand pine-scrub communities (u). Preservation Areas include the following types of wetlands, natural water bodies and uplands: coastal marches (w), mangrove swamps (w), marine grassbeds (w, nwb), natural beaches and dunes (w, u), Class I and II Waters (w, nwb), aquatic preserves (w, nwb), critical habitat for endangered, threatened or rare species (w, nwb, u), and State wilderness (w, nwb, u).

EROSION

Process which results in the physical movement of sediment from the bed or banks of a channel, river, canal or stream caused by flowing water. Movement of sediment at outfall of pipe to a channel's bed and banks or pond bottom and embankment.

EVAPORTRANSPIRATION

Loss of water which results from evaporation of soil, water, vegetation and other surfaces in combination with transpiration from plants.

FILTER BLANKET

Layer of sand and/or gravel which prevents the movement of fine-grained soils.

FILTER FABRIC

Water-permeable material which prevents the clogging and bridging of aggregates by fine soil particles.

FIRST FLUSH

First portion of stormwater runoff which is generated by a rainfall event and contains the bulk concentration of contaminants which are washed into the drainage network by the storm.

FLASH BOARD

Temporary barrier placed along the crest of the spillway which allows the surface water elevation in a reservoir to be raised above the outflow level to increase the storage capacity.

FLOOD

A general and temporary condition of partial or complete inundation of normally dry land areas from: (1) the overflow of inland or tidal waters; or (2) the unusual and rapid accumulation or runoff of surface waters from any source.

FLOOD HAZARD BOUNDARY MAP (FHBM)

An official map of a community, issued by the Federal Insurance Administration, where the boundaries of the area of special flood hazards have been designated as Zone A.

FLOOD INSURANCE RATE MAP (FIRM)

An official map of a community, on which the Administrator had delineated both the special hazard areas and the risk premium zone applicable to the community.

FLOOD PEAK

Highest value of discharge or stage which a flood attains for a given frequency event.

FLOOD ROUTING

Determination of changes in flood water elevation throughout the course of a stream, channel, lake or reservoir.

FLOOD STAGE

Stage at which surface water begins to overflow from the natural banks of a stream, lake or pond.

FLOOD ZONE

An area of land inundated by the overflow of a watercourse or water body, the accumulation of runoff, the rise of groundwater, or the rise of the tidal waters.

FLOODPLAIN OR FLOODPRONE

Any land area susceptible to being inundated by water from any source.

FLOODPROOFING

The modification of individual structures and facilities, their sites and their contents to protect against structural failure, to keep water out or to reduce effects of water entry.

FLOODWAY

The channel of a stream and any adjacent floodplain areas that must be reserved in order to discharge the 100-year flood without increasing flood heights by a specified amount.

FLUME

An inclined channel which permits recovery of hydraulic head volumes on level grounds.

FREEBOARD

A vertical distance between the elevation of the design highwater and the inside top of the bank, control structure, dam, ground level, pavement or levee.

GRADING

Leveling or planing land to a smooth horizontal or sloping surface by the use of mechanical leveling or grading equipment or, in the case of stockpile soil, other mechanical equipment.

GROUNDWATER RECHARGE

Addition of water to the saturated zone by subsurface inflow or seepage as a result of natural and/or artificial means.

HEAD LOSS

Loss of energy which results from friction, eddies, changes in velocity or direction of flow.

HEADWATER

Source of a stream or the water upstream from a control structure.

HYDRAULICS

Study of the mechanics of water movement.

HYDRAULIC CONDUCTIVITY (COEFFICIENT OF PERMEABILITY)

The amount of water that will flow through a unit cross sectional area of aquifer per unit hydraulic gradient.

HYDRAULIC GRADE LINE

The line showing the pressure head, or piezometric head as an elevation above a datum at any point in a pipe.

HYDROGRAPH

Graph of the stage or discharge of a water body versus time.

HYDROLOGIC CYCLE

The continuous circulation of a particle of water from the ocean to the atmosphere, to the land, and ultimately discharging back into the ocean.

HYDROLOGY

Study of the occurrence and movement of water on the surface at and beneath the surface of the earth.

IMPERVIOUS SURFACE

Land surface which prohibits the transportation of water through the ground surface and into the soil zone.

INFILTRATION

Movement of water through the ground surface and into the soil zone.

INSIDE TOP OF BANK

The "waterward" or internal top of bank at the highest point on the sideslope of a pond.

INITIAL ABSTRACTION

Maximum amount of precipitation that is infiltrated into the ground, intercepted by plant cover, and stored in depressions under specific conditions prior to the production of runoff.

LAG TIME

The time from the center of mass of rainfall excess to the peak of a unit hydrograph.

LAND ALTERATION

Any activity which changes the physical features of the land, including vegetation and soil.

LANDFILL

Land used for the disposal of waste, excluding hazardous waste.

LANDLOCKED AREA

An area in which runoff does not have a surface outfall up to and including the 100-year flood elevation.

LOCAL STREET

A street of limited continuity used primarily for access to abutting property and the local needs of the neighborhood.

NON-POINT SOURCE POLLUTION

Pollution that enters a water body from diffused origins in the watershed and/or drainage basin and does not result from discernible, confined or discrete conveyances.

NORMAL WATER LEVEL

Normal (not seasonal high) water elevation of a surface water body or wetland. For definition purposes this elevation is at or below the design low water elevation and the seasonal high water elevation.

100-YEAR STORM EVENT

A storm event which has a 1/100 (1 percent) chance of being exceeded in any given year.

ORIFICE

An opening with a closed perimeter through which a fluid flows.

OUTFALL

The point, location or structure where stormwater runoff discharges from a sewer to a receiving body of water.

OUTLET

Point at which stormwater runoff discharges from a stream, river, lake or drain.

OUTSIDE TOP OF BANK

The "landward" or external top of bank which is typically the highest point at the external limit of the maintenance area.

OPERATION PERMIT

Surface Water Management permits issued by the Southwest Florida Water Management District authorizing the operation and maintenance of a surface water management system in accordance with the terms and conditions of the permit.

OVERFLOW

Structure which transports excess stormwater into receiving water after the maximum capacity of a limited discharge device has been exceeded.

PEAK DISCHARGE

Maximum instantaneous flow from a given storm event for a specific location.

PEAK SENSITIVE

Areas sensitive to changes in timing and/or magnitude of peak flows.

PERCOLATION

Movement of water through the soil.

PERCOLATION TEST

Determination of a soil's percolation rate calculated as the amount of time it takes for water of known head to drain a one inch unit volume in a test hole.

PERMISSIBLE VELOCITY

Maximum velocity at which water may be transported through a structure, canal, or storm drainage system without excessive drainage or erosion.

PLAN

This term refers to the Hillsborough County Stormwater Management Master Plan.

POINT SOURCE

Geometry of a contaminant plume whereby pollution is produced from a single location.

POPOFF

Elevation at which discharge occurs from a water body.

POSITIVE OUTFALL

A general, uncontrolled gravity discharge of drainage waters into a watercourse that does not significantly alter the quantity characteristics for the watercourse.

POROSITY

Ratio of the volume of pore space to the total unit volume of a soil or rock.

RATIONAL METHOD

Method for computing storm discharge flow rates according to the formula:

Q - CIA

Where:

C = Coefficient describing the physical drainage area.

I = The rainfall intensity expressed in inches per hour.

A = The area expressed in acres.

REACH

Specified portion of a watercourse which consists of a generally homogeneous, uniform length of open channel or underground conduit.

RECHARGE

Addition of water to the groundwater system by infiltration, percolation and surface water seepage.

RECHARGE BASIN

Basin which is underlain by an unconfined aquifer composed of deep sands, gravels and cobbles that allow replenishment of groundwater supplies.

RETENTION POND

A pond in which recovery of the available storage volume occurs only by percolation and evapotranspiration.

RIGHT-OF-WAY

Land dedicated, deeded, used or to be used for a street, walkway, boulevard, drain for ingress and for egress.

RUNOFF

That portion of water leaving a specific drainage area through a surface watercourse and not being transpirated or infiltrated to the water table or deep aquifer.

SANTA BARBARA UNIT HYDROGRAPH

Method used to estimate stormwater runoff from effective rainfall and unit hydrograph analysis.

SATURATION POINT

Point at which a soil or aquifer can no longer absorb any additional water without losing an equal amount.

SCOUR

Abrasive action of flowing water on sediments in pipes, channels or ponds causing sediments to move from their existing location.

SEASONAL HIGH GROUNDWATER ELEVATION

The elevation to which the groundwater can be expected to rise due to normal wet season.

SETTLING POND

Areas of a stream, channel or pond which has been physically altered and enlarged to permit suspended sediment and debris to settle out from discharging surface water.

SHEETFLOW

Uniform overland flow of water in a layer over a sloping surface.

SITE DESIGNER

The professional with legal authority and responsibility to sign and seal the required documents.

SPILLWAY

A conduit, channel or passageway for surplus water to be transmitted over and around a structure or dam.

SPODIC STAINLINE

Soil stainline indicating presence of groundwater at some point in time.

STORM FREQUENCY

Time interval between storm events which produce similar predetermined intensity and runoff volumes which are used in designing surface and stormwater management systems.

STORM SEWER

Sewer that conveys surface water discharge and stormwater runoff.

SURCHARGE

Flow condition which occurs in a closed conduit when the hydraulic grade line is above the crown of the sewer.

SURFACE WATER MANAGEMENT SYSTEM

Collection of facilities, improvements, or natural systems whereby surface waters are collected, controlled, conveyed, impounded or obstructed. The term includes dams, impoundments, reservoirs, appurtenant works and works as defined in Subsections 373.403(1)-(5), Florida Statutes.

SURFACE WATER MANAGEMENT PERMIT

Letter of conceptual approval, construction permit or operation permit issued by the Southwest Florida Water Management District.

SURFICIAL (UNCONFINED) AQUIFER

A saturated bed, formation or group of formations with a free (upper) surface open to atmosphere which yields water in sufficient quantity to be of consequence as a source of supply.

SWALE

A grassed waterway which has a cross-sectional ratio for top width to depth of at least 6 to 1 or a maximum sideslope ratio at 3:1, and has been designed to resist soil erodibility, slumpage and contamination which may result from stormwater runoff.

TAILWATER DEPTH

Depth of water at the point of discharge, immediately downstream from the discharge structure.

TIME OF CONCENTRATION

Time required for the surface runoff from the most hydraulically distant point of the drainage basin to reach the outfall or gage point.

25 YEAR STORM EVENT

A storm event which has a 1/25 (4 percent) chance of occurring in any given year.

UNIFIED SOIL CLASSIFICATION SYSTEM

Identification system for classifying soils according to their physical properties, including particle size, pore space, plasticity index, and gradation.

UNIT HYDROGRAPH

Time distribution of runoff from a basin caused by one inch of direct runoff distributed uniformly over the basin in space and time.

URBAN RUNOFF

Surface water runoff from an urban drainage area which discharges into a stream, storm drainage system or other watercourse.

VARIANCE

A relaxation by the Board of Adjustment of the dimensional regulations of the Code where such action will not be contrary to the public interest and where, owing to conditions peculiar to the property and not the result of actions or the situation of the applicant, a literal enforcement of this Code would result in unnecessary and undue hardship.

VOLUME SENSITIVE

An area, lake or depression where water does not deplete from the area except of percolating, evaporating or overflow by sheetflow.

WATERCOURSE

Any natural or artificial channel, ditch, canal, stream, river, creek waterway or wetland through which water flows in a definitive channel, bed, bank or other discernible boundary.

WATERSHED

The region drained by or contributing to a stream, lake or other body of water.

WATER SURFACE ELEVATION

Surface water elevation expressed in terms of feet above mean sea level according to the National Geodetic Vertical Datum (NGVD).

WATER TABLE

Level in the saturated zone at which fluid pressure of the pores of a porous medium is exactly atmospheric.

WEIR

Instrument for measuring or regulating surface water discharge.

WEIR NOTCH

Opening in a weir through which water flows.

WETLANDS

Land that is inundated or saturated by surface or ground water in years of normal water conditions at a frequency and duration sufficient to support and that, under normal circumstances, does support a dominance of vegetation typically adapted for life in saturated soil conditions. Wetland also includes non-vegetated beaches, mudflats and salt barriers.

ZONING

The subdivision of municipality or other local community into districts, and the regulation of land according to its nature and uses.

APPENDIX B

REFERRAL INDEX

CSX Transportation

5656 Adamo Drive

Tampa, FL 33619-3240

City of Tampa Planning Division

306 East Jackson Street

PH: 223-8401

City of Tampa Water Resources and Public Works

306 East Jackson Street

PH: 223-8071

City of Temple Terrace Building and Zoning

City Hall

1250 North 56th Street

PH: 989-7132

City of Temple Terrace Public Works

6009 Whiteway Drive

PH: 989-7170

Federal Emergency Management Agency

Flood Map Distribution Center

6930 (A-F) San Tomas Road

Baltimore, MD 21227

PH: (301) 335-6565

Florida Department of Environmental Regulation

4520 Oakfair Boulevard

Tampa, FL 33610

PH: 623-5561

Florida Department of Natural Resources

3900 Commonwealth Boulevard

Tallahassee, FL 30303

Division of Beaches and Shores

PH: (904) 488-3180

Bureau of State Land Management

PH: (904) 488-2291

Florida Department of Transportation

Tampa Division Office

4950 Kennedy Boulevard West

PH: 874-3368

Hillsborough County Board of County Commissioners

Courthouse - Room 214B

PH: 272-5660

Hillsborough County Code Enforcement Board

800 East Twiggs Street Room 305

PH: 272-5360

Hillsborough County Planning & Development Management Department

800 East Twiggs Street Room 204

PH: 272-5920

Hillsborough County Engineering Services Department

1000 Ashley Street

PH: 272-5912

Hillsborough County Environmental Protection Commission

1900 Ninth Avenue

PH: 272-5960

Hillsborough County Parks and Recreation

1101 River Cove Drive

PH: 272-5840

Hillsborough County Print Shop

800 East Twiggs Street Room 104

PH: 272-5236

Hillsborough County Public Information

Courthouse - Room 223

PH: 272-5314

Hillsborough County Solid Waste

410 Ware Street Suite 800

PH: 272-6674

Hillsborough County Public Utilities Department

925 East Twiggs Street

PH: 272-6664

Metropolitan Planning Organization

403 Morgan Street

PH: 272-5940

National Climatic Center

Federal Building

Ashville, NC 28801

PH: (704) 259-0682

National Weather Service

1408 24th Street, SE

Ruskin, FL 33570

PH: 645-2181

Plant City Planning Division and Department of Public Works

301 North Wheeler Street

Plant City, FL 34289-9003

PH: (813) 752-3125

Port Authority

811 Wynkoop Road

Tampa, FL 33605

PH: 248-1924

Soil Conservation Service

5339 County Road 579

PH: 621-8824

Southwest Florida Water Management District

2379 Broad Street

Brooksville, FL 33512-9712

PH: (904) 796-7211

United States Army Corps of Engineers

Department of the Army

P.O. Box 4970

Jacksonville, FL 32232

PH: (904) 791-3423

United States Coast Guard

Chief Aid to Navigation Branch

7th Coast Guard District

51 SW First Avenue

Miami, FL 33130

PH: (305) 350-4103]

United States Geological Survey

Water Resources Division

4710 Eisenhower Boulevard

PH: 228-2124

APPENDIX C

BIBLIOGRAPHY

A. HILLSBOROUGH COUNTY

1. Hillsborough County Land Development Code; 1992

2. Hillsborough County Flood Damage Control Ordinance; January, 1978

B. SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

1. Management and storage of surface waters, Permit Information Manual, Volume 1, March 1988, and Revisions.

2. *, Rules of the *, Chapter 400-4; Revised: October 1, 1986.

3. Wanielista, M.P. and R.L. Machowicz, Frequency Analysis for Southwest Florida, (Unpublished).

C. FLORIDA DEPARTMENT OF ENVIRONMENTAL REGULATION

1. Florida Department of Environmental Regulation, REG Files, Chapter 17-25, Tallahassee, Florida; May, 1986.

2. Wanielista, M.P.; et al., Stormwater Management Manual, Orlando, Florida; October, 1981.

D. FLORIDA DEPARTMENT OF TRANSPORTATION

1. Florida Department of Transportation, Drainage Manual - Policy, Volume 1, Drainage Design Office, Tallahassee, Florida, 1987.

2. Florida Department of Transportation, Drainage Manual - Procedures; Drainage Design Office, Tallahassee, Florida.

3. Florida Department of Transportation, Drainage Manual - Theory, Drainage Design Office, Tallahassee, Florida; 1987.

4. Florida Department of Transportation, First District, Driveway and Drainage Permit Procedures Manual; July 1, 1986.

5. Florida Department of Transportation, Manual of Uniform Minimum Standards for Design, Construction and Maintenance for Streets and Highways, Tallahassee, Florida; June 1, 1976.

E. CITY OF TAMPA

1. Stormwater Management Division, City of Tampa, General Design Information for Building Permit Applications, Tampa, Florida; Revised: October 18, 1983.

F. FEMA

1. Federal Emergency Management Agency, Conditions and Criteria for Floodway Revisions; August 27, 1984.

2. Federal Emergency Management Agency, Conditions and Criteria for Map Revisions; December 30, 1985.

3. Federal Emergency Management Agency, Natural Hazard Mitigation, Floodplain Management, Ways of Estimating Wave Heights in Coastal High Hazard Areas in the Atlantic and Gulf Coast Regions, Volume TD-3, Atlanta, Georgia; April, 1981.

4. Federal Emergency Management Agency, Users Manual for Waive Height Analysis, Washington, D.C.; Revised: 1981.

5. National Academy of Sciences, Methodology for Calculating Wave Action Effects Associated with Storm Surges, Washington, D.C.; 1977.

G. OTHER COUNTIES OF FLORIDA

1. Lake County Pollution Control Board, Rules of the Lake County Pollution Control Board, Chapter 1-6, Revised: July 9, 1984.

2. Board of County Commissioners of Orange County, Site Development Ordinance, Ordinance No. 86-20; August 25, 1986.

3. Board of County Commissioners of Orange County, Subdivision Regulations; Revised: May 13, 1985.

4. Board of County Commissioners of Pasco County, Florida, Floodplain Ordinance Manual; Revised: Ordinance No. 85-07, 1985.

5. Board of County Commissioners of Pasco County, Florida, Subdivision Regulations; Revised: Ordinance No. 83-15, 1983.

6. Department of Public Works and Utilities, Pinellas County Subdivision Regulations, Clearwater, Florida; December 20, 1983.

7. Board of County Commissioners of Polk County, Florida, Polk County Flood Protection and Surface Water Management Code, Ordinance 85-07.

8. Board of County Commissioners of Polk County, Florida, Polk County Subdivision Regulations; Revised: November 22, 1983.

9. County of Sarasota Planning Department, Land Development Regulations; Revised: Ordinance No. 81-12, 1981.

10. County of Sarasota Planning Department, Zoning Regulations; Revised: Ordinance No. 86-80, August 26, 1986.

APPENDIX D
FUTURE OF HILLSBOROUGH COMPREHENSIVE PLAN
Listed below are Elements and Policies of the Future of Hillsborough Comprehensive Plan for Unincorporated Hillsborough County, Florida, relating to the Stormwater Management Technical Manual.

FUTURE LAND USE ELEMENT
Policy A-1.1 Chapter 3, Section 3.1

Development of land shall utilize required methods as adopted in the land development regulations to control erosion and sedimentation to help minimize the destruction of soil resources.

Policy A-1.2 Chapter 4

Soil capability analysis for flood hazards, stability, permeability and other relevant soil characteristics shall be considered when planning for new development.

Policy A-1.4 Chapter 3, Section 3.1.B.3

Development within areas defined by Hillsborough County as "Volume or peak sensitive" shall require higher performance standards to mitigate stormwater runoff.

Policy A-1.5 Chapter 3, Section 3.2

All development within the 100-year floodplain shall be in strict conformance with all development regulations that have jurisdiction.

Policy A-8.7 Chapter 3, Section 3.1

Require that stormwater management systems be designed to reduce pollution through compliance with regional and local filtration, retention, and detention standards.

Policy A-8.8 Chapter 7, Section 7.1.B

The County, through the land planning and development review processes, shall continue to prohibit unmitigated encroachment into the 100-year floodplain of riverine systems.

Policy 6.10 Chapter 7

The County shall, through the land development review process, restrict the substantial lowering of the water table to meet stormwater treatment or storage requirements.

Policy 11.1 Chapter 4

During the land development review process, the County shall recommend the use of spoils in accordance with the soil suitabilities by the U.S.D.A. Soil Conservation Service. Prior to the commitment of resources to development and land use activities, the County shall continue to require site-specific analysis when the proposed use and the identified soil suitabilities appear to be incompatible.

Policy 11.2 Chapter 4

The County, in cooperation with the U.S.D.A. Soil Conservation Service, shall require that top soil best management practices be observed during all land alteration activities. To the greatest degree practable, erosional soil loss due to construction and agricultural activities shall be minimized.

Policy 11.3 Chapter 4

During the land development review process, the County shall continue to evaluate and utilize, where appropriate, soil capability analysis for flood hazard, stability, permeability, and other relevant soil characteristics when permitting new development.