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36 <br /> Appendix <br /> Extended Wet Detention Pond Standards <br /> Scope <br /> These standards are intended to meet the State's requirements for wet detention ponds in <br /> protected watersheds where a High Density Development Permit has been approved. <br /> These guidelines and standards are for designing and building ponds on a small scale, less than <br /> 100 acres drainage area, serving individual developments. Other appropriate standards should <br /> be used for regional basins serving larger, developed watersheds. <br /> Details of runoff calculations, pond design guidelines, etc. are not included since proven <br /> references are readily available from other sources. Refer to the references at the end of this <br /> standard. <br /> General Description <br /> Extended wet detention ponds are intended to capture and treat polluted runoff from developed <br /> areas during both infrequent small storms and infrequent larger events. <br /> Why Ponds Are Favored <br /> Because of the low infiltration capacity of soils in Orange County detention ponds are tilt <br /> preferred Best Management Practice (BMP)and extended wet detention pond are preferred over <br /> other types of ponds. Detention ponds that are dry between rainfall events lack the most <br /> desirable conditions for capturing pollutants. In addition, dry ponds cannot maintain the favored <br /> biological conditions between events that are necessary to breakdown pollutants. <br /> A Typical Pond <br /> In Orange County the topography and soil conditions are such that most ponds are created by <br /> constructing an earthen dam between two hills and impounding runoff in the valley above the <br /> dam. Pipes through the dam and an emergency spillway around one end of the dam control the <br /> waterlevel and release overflow. Figures 1 and 2 illustrate a typical wet detention pond. <br /> The site plan must locate the pond so that it intercepts runoff and pollutants from the developed <br /> area. Diversions and storm drains can be used to intercept and channel runoff to the pond. <br /> Other Possible Locations <br /> It may be possible, where suitable soil conditions exist, to construct a basin on a slope where <br /> there is no valley. This approach involves excavating and constructing a dam on three sides and <br /> building diversions to intercept runoff and channel it to the pond. Such sites must be evaluated <br /> carefully so that the pond is positioned where diversions will work and the watershed is <br /> sufficient to establish a viable pond. <br /> Excavated Ponds <br /> Simply digging a hole to create a pond is usually impractical in Orange County because of the <br /> shallow soil in most locations. There may be exceptions where soil borings show that the depth <br /> DRAFT 1 June 1993 <br />