2002-06-26 CC Packet - Laserfiche WebLink

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<, " Retention-Svstems - "Werponcts Description A wet pond, also known as a wetpool, a retention basin, or a "NURP" pond (iftbe pond incorporates specific design parameters), is a constructed stormwater pond that retains a permanent pool of water. Wet ponds are generally on-line, end-of-pipe BMPs. The primary pollutant removal mechanism in a wet pond is sedimentation. Significant loads of suspended pollutants, such as metals, nutrients, sediments, and organics, can be removed by sedimentation. Dissolved contaminants are removed by a combination of processes: physical adsorption to bottom sediments and suspended fine sediments, natural chemical flocculation, bacterial decomposition, and uptake by aquatic plants and algae. Wet ponds have a moderate to high capacity for removing most urban pollutants, depending on how large the volume of the permanent pool is in relation to the runoff from the surrounding watershed. Figure 1 shows a schematic of a typical wet pond. Generally, large contributing watersheds are required to maintain pool elevations. Minimum contributing watersheds should be at least 10 acres, but not more than one square mile. Sites with less than 10 acres of contributing watershed may be suitable if sufficient ground- water flow is available. Wet ponds can be used at residential, commercial and industrial sites. Since wet ponds have the capability of removing soluble pollutants, they are snitable for sites where nutrient loadings are expected to be high. Wet ponds may be single-purpose facilities, providing only runoff treatment, or they may be incorporated into an extended storage or a detention pond design to also provide flow control. Wet ponds themselves are generally ineffective in decreasing runoff volumes, Metropolitan Council! Barr Engineering Co. Purpose Water Quantity Flow attenuation . Runoffvolumereduction 0 Water Quality Pollution prevention Soil erosion N/A Sediment control N/A Nutrient loading N/A Pollution removal Total suspended sediment (TSS) . Total phosphorus (P) IJ Nitrogen(N) I] Heavy metals I] Floatables' . Oil and grease I] Other Fecal colifono I] Biochemical oxygen demand IJ (BOD) . Primary design benefit IJ Secondary design benefit o Little or no design benefit · Only if a skimmer is incorporated 3-251