§ 34-249. Retention, detention facilities.

(a)

Design criteria for pollution abatement utilizing retention or detention with filtration (dry bottom ponds). The design of ponds for the required retention or detention with filtration may consider separate facilities. Pollution abatement may be combined into the design of the detention pond required to reduce the peak rate of flow from the developed site to that peak rate of flow prior to the development of the site. There will be a minimum of two (2) feet of filter media required for filtering the pollution abatement volume. A positive, nonfiltering bleed-down device with an operable gate or valve shall be installed as a backup system in the event that the filtration system fails. The gate or valve shall normally be set and locked in the closed position.

The bottom of a required retention or detention with filtration pond shall be a minimum of three (3) feet above the estimated wet-season water table. Where this is not possible due to a high water table, underdrains will be installed with a minimum invert elevation of one (1) foot below the pond bottom, along the entire perimeter of the pond unless a geotechnical engineer can show to the satisfaction of the county engineer that a lesser amount of underdrain can adequately control the estimated wet seasonal high water table.

Underdrains used to filter and discharge the pollution abatement volume must be located in the "sideslope" position. It is also recommended that underdrains used solely to control a high groundwater table be located in the sideslope position; however, the "pond bottom" location for this type of underdrain will be considered on a case-by-case basis. The use of a "mound-type" underdrain is strictly prohibited.

Final design seepage and underdrain collection rates will be determined by a geotechnical engineer. All necessary calculations to support the above shall be submitted to the county engineer.

(b)

Design criteria for pollution abatement utilizing wet retention or detention with filtration. Wet bottom ponds will be allowed, provided that a minimum of six (6) feet of water depth below the control water level is provided and that a maximum length to width ratio of two (2) to one (1) is maintained. The depth is required to minimize the growth of undesirable vegetation. The length to width ratio is to minimize the number of lots affected by the pond and to minimize maintenance costs.

(c)

Design criteria for use of positive bleed-down devices for wet bottom retention ponds. The county engineer may approve the use of a positive bleed-down device (orifice) as an alternative to the use of underdrains provided certain criteria are met. Retention of the pollution volume must be provided for one (1) inch of rainfall runoff over the developed project area or the total runoff of two and one-half (2.5) inches of rainfall times the percentage of imperviousness, whichever is greater. Pool depths should be a minimum of six (6) feet below the design "normal" water level. Oil skimmers must also be included on the discharge/outlet structures.

A shelf with no additional plantings must be provided around the entire pond by extending and sloping the facility (5:1 or flatter) from a point one (1) foot above the control elevation to a point two (2) feet below the seasonal low water elevation. All necessary calculations to support the above shall be submitted to the county engineer.

(d)

Pollution abatement volume recovery rate. The pollution abatement volume recovery rate shall be as required by the appropriate water management district.

A maximum of one-half (½) of the pollution abatement volume may be used in the routing calculations to reduce the peak rate of discharge from the developed site to the peak rate of flow prior to development occurring.

(e)

Design criteria for detention facilities to reduce peak rate of discharge for 25-year frequency/24-hour duration storm. The detention pond will be sized to limit the peak rate of discharge from the developed site to that discharge generated prior to development. Supporting calculations shall be submitted and will contain, as a minimum, runoff hydrographs for the predeveloped site and the postdeveloped site, and a discharge hydrograph after routing through the proposed detention facility. All routing calculations to be submitted must consider the tailwater of the receiving facility. If the receiving facility is an existing storm sewer, the Hydraulic Gradient Line (HGL) elevation (HGL) of this receiving facility can be assumed at one-half (½) foot below its crown of pipe elevation unless a detailed study of the existing system indicates otherwise.

(1)

Credit for seepage to further reduce the peak rate of discharge will not be allowed unless accompanied by supporting documentation prepared by a geotechnical engineer.

(2)

A minimum of fifty (50) percent of the total volume of water required to attenuate the peak discharge of the facility in excess of the pollution abatement volume must be evacuated within twenty-four (24) hours. The remaining fifty (50) percent must be evacuated within an additional seventy-two (72) hours. All stormwater evacuation from detention facilities in excess of the pollution abatement volume will be accomplished by a positive, nonfiltering discharge structure only. The use of underdrains to accomplish this required evacuation is prohibited.

(3)

The outflow structure shall be designed to skim floating debris, oil, and grease from an elevation six (6) inches below elevation of inflow into the structure to an elevation six (6) inches above the design high water level of the pond and shall cover all directions of inflow to the outfall structure. The design of this control system will make adequate provision to minimize erosion.

(f)

Design criteria for recharge areas. Recharge facilities may be required in developments where Hydrologic Soil Group Type "A" soils, as indicated on the soils survey map for the county prepared by the U.S.D.A. Soil Conservation Service, are predominant. Type "A" soils have a high infiltration rate when thoroughly wet and experience a low rate of runoff, thus providing the potential for prime aquifer recharge.

(1)

If a site has Type "A" soils, a detailed soils report prepared by a geotechnical engineer shall be submitted to the county engineer for review prior to the development of final detailed drainage plans for the site. The report shall contain a recommendation as to whether or not recharge is feasible on the proposed site. The county engineer, after review of the report and recommendation of the geotechnical engineer, shall make a final written decision regarding whether or not recharge design criteria will be required.

(2)

If recharge design criteria are required by the county engineer, retention of the total runoff generated by a 25-year frequency, 24-hour duration storm event from the developed site will be required. Where a positive outfall is not available, the site shall be required to retain the 100-year frequency/24-hour duration storm.

(3)

Final design seepage rates utilized in the design of the recharge facilities will be determined by a geotechnical engineer. All necessary calculations to support the above shall be submitted to the county engineer.

(g)

Design criteria where a positive outfall is not available. When a positive outfall is not available or discharge into a lake without a positive outfall is proposed, the pond design shall retain the 100-year frequency/24-hour duration storm event. The pond shall be designed to completely evacuate a 100-year frequency/24-hour duration storm by natural seepage or positive bleed down within fourteen (14) days. Drainwells will not be considered as a positive outfall for stormwater systems. Final design seepage rates will be determined by a geotechnical engineer. All necessary calculations to support the above shall be submitted to the county engineer.

(h)

Design criteria for off-site drainage. Off-site areas which drain to or across a site proposed for development must be accommodated in the stormwater management plans for the development. The stormwater management system for the development must be capable of conveying existing off-site flows through or around the development. The estimation of the off-site flows must be done separately from the estimation of on-site postdevelopment flows (i.e., separate off-site and on-site hydrographs must be computed due to the typically significant differences in land use characteristics). It is strongly recommended that the project engineer meet with the appropriate county engineering staff prior to generating final detailed design calculations in order to establish off-site design requirements for a particular project.

(i)

Mitigation areas. Mitigation areas within retention/detention areas shall be prohibited.

(Ord. No. 91-29, § 2(Exh. A), 12-10-91; Ord. No. 94-4, § 1(Exh. A), 2-8-94; Ord. No. 2000-14, § 1, 6-27-00)