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ITE FACTORS 'rainage Area AC, iv�Ll'10 AC possible II hym logy capable of ¢uopor' '19 .vn1r I':Vols Max Site Slope h Min. Depth o Bedrock ., fl In 1111;H meas Min Depth to Seasonally High Water Table 'prnld hollom m Ist be above bedlork c.Q tin re+r a NRCS Soil Type 'A and H sods require liners Freeze/ Thaw Suitability Pogr' 'unless adapled Vnlh additional storage for snowmen Description: Stormwater ponds are constructed basins placed in the landscape to capture stormwater runoff. The pond is graded and outlet structures are designed in such a way that specified volumes of water (part or all of the VWQ, VCP, VP10, and VP100) are either held until displaced by future runoff or detained for a specified period of time. While the runoff is being held in the pond, sediment and associated pollutants settle to the bottom. Pollutants can also be removed from the stormwater through microbial, plant and algal biological uptake. Additional stormwater pond storage provided above the VWQ allocation is used to control flows of particular frequencies at pre -development or other specified levels to prevent downstream channel degradation and flood damage. Storage in a stormwater pond can be either permanent pool or extended detention storage. As the name implies, water in the permanent pool is meant to remain in the basin, allowing settling and biological uptake to occur between storms and protecting against sediment resuspension. To maintain a permanent pool over a period of time, the pond must be designed with a sufficiently large drainage area. Extended detention storage refers to the volume above the permanent pool which is controlled by an outlet structure, sized so that runoff from larger storms can be captured and released over a period of time, allowing some settling to occur and keeping flow rates in check. Ponding requirements for new impervious area treatment are mandated in the state Construction General Permit (CGP). It is possible to design a pond with no permanent pool (dry pond, ponded only for a maximum of several days following a storm event), but stormwater ponds used for water quality treatment should be designed with a permanent pool. Dry ponds are highly susceptible to sediment resuspension and generally do not meet water quality performance criteria. However, dry ponds can be employed in some circumstances, such as for supplemental storage when the VWp has already been treated with upstream BMPs. Sediment and pollutant removal in stormwater ponds can be enhanced through the use of multiple cells in succession. In particular, a sediment forebay (pre- treatment basin) sized at 10% of the pond area should be considered upstream of the pond. This allows for settling of heavier materials in a designated area, enhancing the main pond performance and reducing the maintenance burden by concentrating the majority of sediments in one location. It is also possible to design a pond with no extended detention storage (wet pond), in which the only outlet is the spillway and where 100% of the treated volume is within the permanent pool. When a storm occurs, the treated runoff in the pond is displaced by new (untreated) runoff. Wet ponds are able to fulfill water quality treatment requirements, but do not mitigate the channel degradation or flooding effects of increased flow from larger storms. Also, wet ponds are not generally recommended for cold climates because during frozen conditions, runoff can flow over the ice of the permanent pool, effectively negating the benefits of the intended treatment volume. Generally the most desirable pond configuration for Minnesota is a large enough permanent pool to meet water quality needs, and sufficient extended detention to ensure adequate treatment during frozen conditions. Chapter.12-POND Volume 2 2005 Minnesota Stormwater Manual, Version 1.0