Agenda - 08-23-1993 - C 2 and C 3 attch

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137 Figure 2: Cross Sections of a Wet Detention Pond. A Proven BMP Studies have shown that wet detention ponds can be very effective in removing and treating pollutants washed off impervious surfaces. Research has found that most pollutants are removed by the first half inch to one inch of runoff, the so called "first flush of runoff The degree of pollutant removal is a function of the size and shape of the permanent pool, the characteristics of individual urban pollutants, and the intensity of development (area covered by impervious surfaces and the type of land use) in the watershed. When properly sized and designed wet ponds have a high removal rate of sediment, nutrients, and trace metals. Biological processes in the pond remove soluble nutrients that cause eutrophication of streams below the pond even though this may result in periodic water quality problems in the pond itself. Multiple Use Ponds Properly designed detention ponds can serve several purposes: removal of pollutants; mitigating flooding by controlling the rate of stormwater runoff from the project; prevent downstream channel erosion; provide wildlife habitat, landscaping, and recreation; and serve as a component of open space. In some situations it can be used as a temporary sediment pond during construction and then be converted (sediment and debris removed, outlet structure converted) into a permanent detention pond. This can make pond construction more economical. Designing the pond for multiple uses can make it an attractive, integrated part of a development instead of just an eyesore and maintenance problem in a remote corner. Minimum Drainage Area As a general rule, 10 acres of drainage area per acre of surface area in a pond is the recommended minimum to provide sufficient runoff and provide a reliable source of water so that the permanent pool is maintained, desirable biological activity is sustained, and fish habitat created. This is a conservative recommendation; the necessary watershed will vary depending upon the runoff coefficients of the watershed: wooded land will produce very little (but very clean) runoff because the soil, leaf litter, and other debris act like a sponge and absorbs water; most of the rainfall on pavement will run off (but is not of very desirable quality) because the soil is effectively sealed. Leaks, evaporation, and infiltration losses can result in pond drawdowns where there is insufficient inflow to replace these losses. These losses vary depending on location. Springs can help maintain volume, but few locations have adequate flow to actually fill a pond that has significant volume. Runoff from rainfall is needed to fill the pond after construction and replenish it after a drawdown. However, drawdown is more of a nuisance and aesthetic problem that may adversely affect fish population but does not necessarily affect its ability to remove pollutants. A partially filled or dry pond may increase holding time, allowing more sedimentation, and removing more pollutants. The Purpose of a Wet Pond The goal is to establish a permanent impoundment of water above which stormwater runoff is stored and released at a much lower rate than which it enters the pond. The purpose of the permanent pool is to create a viable pond environment between rainfall events and to create a mechanism to capture pollutants and either retain them or treat them. A possible additional benefit is to more closely approximate the runoff conditions that existed below the pond prior DRAFT 3 June 1993