§ 50-107. Performance standards; runoff computations.

(a)

Performance standards.

(1)

Generally. Performance standards to be followed in the design of lesser or standard development projects are as follows:

a.

Stormwater runoff shall be subjected to best management practice prior to discharge into natural or artificial drainage systems. For purposes of this section, the term "best management practice" shall mean a practice, or combination of practices, determined by the local government to be the most effective, practical means of preventing or reducing the amount of pollution generated by the project to a level compatible with state water quality standards found in chapter 17-3, Florida Administrative Code.

b.

Site alteration activities.

1.

No site alteration shall cause siltation of wetlands or pollution of downstream wetlands, or reduce the natural retention or filtering capabilities of wetlands.

2.

No site alteration shall allow water to become a health hazard or contribute to the breeding of mosquitoes.

3.

All site alteration activities shall provide for such water retention and settling structures and flow attenuation devices as may be necessary to ensure that the standards and requirements of this subsection are met.

c.

Design of water retention or detention structures and flow attenuation devices shall be subject to the approval of the local government pursuant to the standards of this division. Detention structures should be designed to release runoff to the downstream drainage system over a period of time so as not to exceed the capacity of the existing downstream system. The local government shall establish minimum side slope standards for open retention and detention areas (ponds, etc.) in accordance with the objectives of this division.

d.

A positive drainage system shall be provided which will not adversely impact downstream owners or adjacent lands.

e.

Where possible, natural vegetation shall be used as a component of drainage design. The water table should not be manipulated so as to endanger natural vegetation beneficial to water quality unless natural vegetation can be replanted and survive with a lowered water table condition.

f.

Runoff from higher adjacent lands shall be considered, and provisions for conveyance of such runoff shall be included in the drainage plan.

g.

Runoff shall be treated to remove oil and floatable solids before discharge from the site in a manner approved by the local government.

h.

Erosion by wind or water shall be prevented throughout the construction process.

i.

Direct discharge to class II waters is prohibited. A workable filter system must be provided prior to any discharge to class II waters.

j.

For the purpose of this division, it is presumed that the lowering of the water table for the purpose of constructing detention/retention basins and for the purpose of permanently protecting road construction does not conflict with the stated objectives of this division if all of the following are met:

1.

The development site is not in an area known to the local government, based on data collected and interpreted by the U.S. Geological Survey, the St. Johns River Water Management District, the county or municipal engineers, and other professional investigators, as important to recharge or to prevention of discharge of the Floridan Aquifer.

2.

The proposed lowering of the water table shall be no more than 15 percent of the site to a depth of five feet below the surface of the existing undisturbed ground, or an equivalent volume, provided that there be a maximum depth of five feet, said area to be measured at the overflow elevation of the retention area.

3.

If ditches, underdrains or similar devices are used to lower the water table, the lateral volumetric effect will be calculated, and the volume will be deducted from that allowed for retention areas.

4.

The high-water table may be lowered up to two feet below the undisturbed ground in the vicinity of roads for the purpose of protecting the subbase and base of the roadway and/or for the purpose of preventing mosquito breeding in the roadside swales.

5.

The lowering of the water table has no adverse effect on wetlands.

6.

The lowering of the water table does not increase flows to the detriment of neighboring lands.

(2)

The following additional performance standards are to be followed in the design of the project: The minimum volume of retention to be provided shall be equivalent to one-half inch of depth over the entire project area. For certain soil conditions or groundwater table conditions which do not permit the percolation of this volume within the five days following a storm event, the local government may approve detention with the filtration systems in lieu of retention.

(3)

The discharge hydrograph produced for the developed or redeveloped site shall not exceed, in terms of peak flow and total volume, the hydrograph produced by conditions existing before development or redevelopment for a 24-hour, 25-year frequency storm unless the intent of this recharge provision will be met through detention of the difference between said volumes, in which case said volume difference may be released over not less than a 24-hour and not greater than a 72-hour period of time. However, the design standards for wet retention areas, when approved by the local government, shall prevail. This requirement may be waived by the local government for sites consisting predominately of poorly drained soils having permanently and naturally impaired recharge potentials. However, the runoff from the first one inch of rainfall for each storm falling on all areas of the project shall be retained on the site except in cases where the local government concurs that soil and/or groundwater table conditions are not conducive to such practice, in which case said first inch shall be detained and released over a period of 24 to 72 hours, in a manner acceptable to the local government. However, in the case of wet detention, standards approved by the local government shall prevail. In addition, the cumulative impact of the outflow hydrograph on downstream flow shall be considered. Runoff rates and volumes resulting from the project, in excess of existing amounts, shall be accommodated on the site. Off-site retention may be permitted if, in the opinion of the local government, the recharge requirements of this division are met.

(b)

Runoff computations. Runoff computations shall be based on the most critical situation (rainfall duration, distribution and antecedent soil moisture condition) and conform to acceptable engineering practices using rainfall data and other local information applicable to the affected area.

(Ord. No. 88-15, §§ 204.00, 204.01, 5-19-88; Ord. No. 97-5, § IV, 2-6-97)