2002-06-26 CC Packet - Laserfiche WebLink

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... . Retentioft-Svstems -werponas Description (continued) although some infiltration can occur (depending on groundwater depth and soil type), as well as evaporation in summer months. Wet ponds work best when the water already in the pond is moved out en masse by incoming flows, a phenomena called "plug flow." Because treatment works on this displacement principle, the permanent pool of wet ponds may be provided below the groundwater level without interfering unduly with treatment effectiveness. Local authori- ties' regulations concerning groundwater should be consulted before constructing such a wet pond, however. Removal efficiency is primarily dependent on the length of time that runoff remains in the pond, which is known as the pond's Hydraulic Residence Time (HRT). As discussed above, wet ponds can remove pollutants not only through sedimentation but also through biological uptake processes, whose removal of pollutants is proportional to the length oftime runoff remains in the pond. Studies have shown that more than 90 percent of the pollutant removal occurs during the quiescent period (the period between the rainfall events) (MD DEQ, 1986). However, some removal occurs during the dynamic period (when the runoff enters the pond). Modeling results have indicated that two-thirds ofthe sediment, nutrients and trace metal loads are removed by sedimentation within 24 hours. These projections are supported by the results of the EPA's 1993 Nationwide Urban Runoff Program (NURP) studies. However, other studies indicate that an HRT of two weeks is required to achieve significant phosphorus removal (MD DEQ, 1986). Researchers have devel- oped design guidelines (discussed below) that are based in part on the influence ofthe HRT on a wet pond's treatment efficiency. When properly designed and maintained, a wet pond can add recreation, open space, fire protection and aesthetic values to a project area. Enhancement Options Following the recommendations of this BMP section, by providing deep inlet and outlet zones or multiple pools, will usually result in designs that are robust enough to handle the winter and spring conditions without special consider- ations. However, runoff volume from spring snowmelt events can be very large, often the largest-volu,me event of the year. Ponds designed to function effectively in summer are often disrupted by winter and spring events. Inspection and maintenance during spring runoff should be a consistent feature of stormwater treatment systems in cold climates. There are several common modifications that can be made to a wet pond to increase its pollutant removal effec- tiveness. These options are described below. Varied Depths Throughout the Permanent Pool Intermittent benches around the perimeter of the pond are recommended for safety and to promote vegetation. The safety bench should be designed to be at least 10 feet wide and located above normal pool elevations. The aquatic bench should be a minimum often feet wide and depths of6-18 inches should be maintained at normal elevations to support aquatic vegetation. Deeper depths near the outlet will yield cooler bottom water discharges that may mitigate downstream thermal effects during the summer. Figure 2 shows a wet pond with both safety and aquatic benches. 3-252 Minnesota Urban Small Sites BMP Manual l_ __