11-19-2018 Planning Commission Packet

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CMP Part 3A. Environmental Protection Plan These natural ponding areas serve an integral function within the natural ecological system of Lake Minnetonka. The "Harza Study" includes quantitative data identifying how the wetland soils easily retain phosphorus,and how marsh plants take up and incorporate large amounts of nitrogen and phosphorus into their growing tissue. Thus,the marshlands are the primary pollution filters for Lake Minnetonka's surface water runoff. Since marshes and small lakes in the watershed aggregate nearly as much area as the main body of Lake Minnetonka, their potential for supplying water to the subsurface formations is appreciable. In addition, marshes provide diversity in the kinds of open space available within the region. They provide a habitat which is uniquely suited for certain birds and mammals. They can be utilized to provide storage of storm runoff with minimal disruption to their function as a habitat for wildlife and their ability to reduce the phosphorus input to the lake. Development Impacts on Wetlands. When urban development occurs, the function of the natural drainageways and filtration system is adversely affected in three ways: The speed and quantity of the storm water runoff are increased as the absorption ability of the land is eliminated with hardcover(houses, driveways, roads, etc.). This increase in the speed and quantity of storm water runoff causes the third adverse effect, which is the increase of the amount of nutrients which are contained in the storm water runoff which now has less time in the natural marsh filtration system to be purified before entering the lake.Nutrients contained in dying vegetation,lawn fertilizers,and from other urban sources are discharged into the lake faster and in greater quantities as the level of urbanization increases. (Harza Study) Orono does not rely solely on the ability of the natural filtration system of the marshes and drainageways to purify storm water so necessary for the preservation of Lake Minnetonka. Supplementary Stormwater Ponding & Alternatives. While Orono's Surface Water Management Plan adopted in 2002 places a significant emphasis on preservation of wetlands, it also provides for the establishment of supplementary stormwater management ponds to help counteract the impacts of development within the watershed. NURP (National Urban Runoff Program) pond design criteria are primarily based on the removal of suspended particles such as sand, silt, etc. Secondary 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 mainly 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 efficiencies of 47 to 68 percent according to Walker.Possibilities for improving this include(1)increasing mean pond depth; (2)promoting infiltration; (3)promoting plug flow conditions(i.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. Constructed wetlands have potential benefits as well as drawbacks. Additionally,the cost of constructing stormwater ponds and storm sewer systems is enormous compared to the cost of wetland preservation. City of Orono Community Management Plan 2020-2040 Part 3A, Page 6