§ 5. Stormwater management requirements.

A.

Methodologies. Computations should consider the duration, frequency, and intensity of rainfall, wet season water table, antecedent moisture conditions, upper soil zone and surface storage, time of concentration, tailwater conditions, changes in land use or land cover, and any changes in topography and hydraulic characteristics. Large systems should be divided into sub-basins according to artificial or natural drainage divides to allow for more accurate hydraulic simulations. Examples of acceptable methodologies for computation of runoff are as follows:

1.

Santa Barbara Urban Hydrograph Method.

2.

Soil Conservation Service Curve Number and Unit Hydrograph (TR-20 and TR-55).

3.

U.S. Army Corps of Engineers HEC-1 Computer Program.

4.

Other hydrograph methods approved by the South Florida Water Management District.

B.

Rainfall intensity. In determining peak discharge rates, intensity of rainfall values shall be obtained through a statistical analysis of historical long term rainfall data or from sources or methods generally accepted as good engineering practice. Examples of acceptable sources include:

1.

USDA Soil Conservation Service, "Rainfall Frequency Atlas of Alabama, Florida, Georgia, and South Carolina for Duration from 30 Minutes to 24 Hours and Return Periods from 1 to 100 Years" January 1978; Gainesville, Florida.

2.

U.S. Weather Bureau Technical Paper No. 49.

3.

U.S. Weather Bureau Technical Paper No. 40.

C.

Water quantity requirements. All stormwater management systems will be evaluated on the ability of the system to prevent flooding of on-site structures, adjacent property, roads, right-of-ways, and to prevent impacts on the watershed basin and natural systems.

1.

Peak discharge.

The post-development peak rate of discharge must not exceed the pre-development peak rate of discharge. The 25-year/24-hour and the five-year/24-hour storm shall be used in determining rate of peak discharge.

For aggregate discharge where multiple off-site discharges are designed to occur, the total post-development peak discharge shall not exceed the pre-development peak discharge for the combined discharges rather than for each individual discharge.

2.

Volume.

The post-development volume of direct runoff must not exceed the pre-development volume of direct runoff. This can be accomplished through retention with percolation or, if the soil conditions are not sufficient for percolation, then through retention for a duration sufficient to mitigate adverse impacts on the receiving waters. In determining the volume of direct runoff, 96-hour duration storm is to be used.

3.

Floodplain stage and conveyance. Floodway and floodplain, and levels of flood flows or velocities of adjacent streams, impoundments or other water courses must not be altered so as to adversely impact the off-site storage or conveyance capabilities of the water resources.

a.

A stormwater management system or development shall not cause a net reduction in flood storage capacity within the 25-year floodplain or floodplain bank.

b.

A stormwater management system or development shall not cause a reduction in the flood conveyance capabilities provided by the floodway.

D.

Water quality requirements. All stormwater management systems will be evaluated based on the ability of the system to prevent degradation of receiving waters, prevent adverse impacts on the site's natural systems, the efficiency of the system to remove pollutants and the ability of the system to conform to State water quality standards, as set forth in chapters 17-3 and 17-4, F.A.C. The following criteria are based on the principle that the first flush of runoff contains the majority of the pollutants and that the pollutant removal efficiencies differ with the type of retained or detained is dependent on the method of stormwater management and land use.

1.

Retention and/or detention requirements. Retention and/or detention in the overall system, including swales, lakes, canals, greenways, etc., shall be provided for the "first flush" removal of pollutants with one of the two following criteria or equivalent combinations thereof:

a.

Detention volume shall be the first inch of runoff generated from the developed project, or the total runoff of 2.5 inches times the percentage of imperviousness, whichever is greater.

b.

Retention volume shall be provided equal to 50 percent of the above amounts computed for detention. Properties designed to accommodate only one single family unit, unless exempt under section 4, provided it is not part of a large common plan of development or sale, shall provide a volume of on-site treatment equivalent to one-half inch of depth over the entire site or lot. Prior to issuance of a building permit, a stormwater permit must be approved by the building official.

E.

Design standards. To comply with the foregoing performance standards, the proposed stormwater management system shall conform to the following design standards:

1.

Detention and retention systems shall be designed in conformance with the Stormwater management manual.

2.

Erosion and sediment control best management practices shall be used as necessary during construction to retain sediment on-site. These management practices shall be designed by an engineer or other competent professional experienced in the fields of soil conservation or sediment control according to specific site conditions and shall be shown or noted on the plans of the stormwater management system. The engineer or designer shall furnish the contractor with information pertaining to the construction, operation and maintenance of the erosion and sediment control practices.

3.

Runoff from roads, parking lots, roofs, and other impervious areas shall be directed to retention areas, detention devices, filtering and cleansing devices, and are subject to some type of best management practice prior to discharge from the project site.

4.

Configurations which create stagnant water conditions such as hydraulically dead end canals are to be avoided, regardless of the type of development.

5.

The proposed stormwater management system shall be designed to accommodate the stormwater that originates within the development and stormwater that flows onto or across the development from adjacent lands.

6.

The proposed stormwater management system shall be designed to function properly for as long as the system generating stormwater is functioning.

7.

The design and construction of the proposed stormwater management system shall be certified as meeting the requirements of this ordinance by a professional civil engineer, registered in the State of Florida.

8.

No surface water may be channeled or directed into a sanitary sewer.

9.

The proposed stormwater management system shall be compatible with the drainage systems or drainage ways on surrounding properties or streets, taking into account the possibility that substandard systems may be improved in the future. The following mechanisms should be investigated: master drainage plan for watershed sub-basins, coordinated planned improvements with appropriate agencies and land owners, or cooperative retrofitting programs.

10.

The banks of retention and retention areas should be sloped 4:1 (horizontal: vertical) to two feet below the controlled water level to accommodate, and should be planted with, appropriate native vegetation.

11.

Dredging, clearing of vegetation, deepening, widening, straightening, stabilizing or otherwise altering natural surface waters should be the minimized in the overall drainage plan.

12.

Natural surface waters shall not be used as sediment traps during or after development.

13.

Water reuse and conservation shall, to the maximum extent practicable, be achieved by incorporating the stormwater management system into irrigation systems serving the development.

14.

Native vegetation buffers of sufficient width to prevent erosion shall be retained or created along the shores, banks or edges of all natural or manmade surface waters.

15.

Construction methods and materials used shall meet the minimum criteria as established in the town's building code.

F.

Stormwater management information to be submitted.

1.

General requirements. A development plan or application for stormwater permit shall contain sufficient information to allow the building official to determine whether the development plan meets the requirements of this ordinance. The information may be in the form of construction plans, maps, graphs, charts, tables, photographs, and narrative descriptions, explanations and citations to supporting references, as appropriate.

2.

Specific requirements. The following information shall be submitted as part of the development plan or application for stormwater permit, as the case may be:

a.

A recent (taken not more than three years before the day of application) aerial photograph encompassing the project area and total land areas. The scale shall be no more than one inch equals 2,000 feet.

b.

A topographic map of the site clearly showing the location, identification, and elevation of bench marks, including at least one benchmark for each major water control structure. Unless otherwise approved by the building official, the minimum contour interval of the topography map shall be two feet.

c.

A detailed overall project area map showing existing hydrography and runoff patterns, and the size, location, topography, and land use of any off-site areas that drain onto, through, or from the project area.

d.

A current land use map.

e.

A soils map of the site (existing U.S. Soil Conservation Service maps are acceptable).

f.

Specify the seasonal high water-table and low-water table elevations.

g.

A map of vegetative cover.

h.

A map showing the location of all soil borings or percolation tests. Percolation tests representative of design conditions shall be performed if the stormwater management system will use swales, percolation (retention), or exfiltration (detention with filtration) designs.

i.

Grading plans specifically including perimeter grading.

j.

Paving, road and building plan showing the location, dimensions, and specifications of roads and buildings (including elevations).

k.

An erosion and sedimentation control plan that describes the type and location of control measures, the stage of development at which they will be put into place or used, and maintenance provisions.

l.

The description, assumptions, and calculations of the proposed stormwater management system, including:

(1)

Channel, direction, flow rate, and volume of stormwater inputs, outputs and routing through and from the site, with a comparison to natural or existing conditions.

(2)

Detention and retention areas, including plans for the discharge of contained waters, maintenance plans, and predictions of surface water quality changes.

(3)

Areas of the site to be used or reserved for percolation including an assessment of the impact on groundwater quality.

(4)

Location of all water bodies to be included in the surface water management system (natural and artificial) with details of hydrography, side slopes, depths, and water surface elevations and hydrographs.

(5)

Any off-site easements required for the proper functioning of the system.

(6)

Drainage basin or watershed boundaries and show where off-site waters are routed onto, through, or around the project.

(7)

Right-of-ways and easements for the system including locations and a statement of the nature of the reservation of all areas to be reserved as part of the stormwater management system.

(8)

The location of off-site water resource facilities such as works, surface water management systems, wells, or well fields, that might be affected by the proposed project, showing the names and addresses of the owners of the facilities.

(9)

The entity or agency responsible for the operation and maintenance of the stormwater management system.

(Ord. No. 90-3, § 1(5), 6-6-1990)