01-24-2000 Council Work Session - Laserfiche WebLink

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Man-made Stormwater Ponding While Qrono's newlv-adopted Surface Water Manauement Plan places a significant emphasis on preservation of wetlands, it also provides for the establishment of su pplementary stomiwater management ponds to help counteract the impacts of development within the watershed. The City of Orono in October 1993 amended the 1980 Comprehensive Plan bv adoption of the use of NURP fNational Urban Runoff Program) standards for the design of man-made stormwater ponds. This requirement has not vet been adopted into the zoning code, but the Citv has been requiring NURP ponds for stormwater management within new developments in accordance with standards of the Minnehaha Creek Watershed District. William W. Walker. Jr., an environmental engineer from Concord. Massachusens. is credited with the 'model* which evaluated the design criteria for NURP ponds. He used phosphorus retention data from 60 reser\ oirs and compared it to phosphorus retention data for a number of ponds and lakes, including manv in the Twin Cities area. He concluded that his 'model' could be used to predict the phosphorus retention capability of urban lakes and wet detention basins based on a number of design parameters. Conversely, it is then possible to design ponds which should retain a given amount of phosphorus. Phosphorus inflow from a watershed to a basin is reasonably predictable given the characteristics of the watershed. NURP pond design criteria is primarily based on the removal of suspended particles such as sand, silt, etc. Secondar\ design criteria enhance the capability of NURP ponds to remove other pollutants such as trace metals, hydrocarbons, nutrients (such as phosphorus) and pesticides. Phosphorus is rnainlv removed through biological uptake in algae and aquatic plants. When the algae die, the nutrients fall to the bottom of the pond and become part of the sediments. Ponds in the Twin Cities area designed according to NURP standards have total phosphorus removal eftlcicncies of 47 to 68 percent according to Walker. Possibilities for improving this include (1) increasing mean pond depth: (2) promoting infiltration: (3) promoting plug flowconditionsti.e. multiple cell ponds): (4) applying chemicals to precipitate out the orthophosphorus (alum treatments): (5) encouraging growth of certain aquatic plants: and (6) design of outlet structure to provide extended detention of large runoff events. Whether each of these enhancements can or should be incorporated is dependent on the desired level of control and other site specific conditions. CMP3A-9 I I I 1 1 1 t < ► < - I