Agenda - 08-23-1993 - C 2 and C 3 attch

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feet deep. A shallow forebay and an "aquatic bench" 10 to 20 feet wide around the perimeter is suggested. (To avoid access problems that shallow edges create for recreational use, 14 ,constructing a pier before the pond is filled can provide access for swimming, boating, and ffishing.) The Pond Environment Many of the characteristics of a pond that enhance pollutant capture and treatment (shallow water, plant and algae growth) have a negative impact on its aesthetic qualities (mosquitos, odor, turbidity, algae). These must be balanced if the pond is to serve multiple uses, such as recreation, by providing shallow areas to encourage vegetation and deeper areas for access and recreation. WARNING ! Designers, developers, and potential residents must be informed that the pond will not necessarily be an ideal water environment because its purpose of capturing and treating pollution almost certainly precludes this. The intent is to protect water quality downstream, not in the pond itself. To improve the quality of the water in the pond the volume should be increased above the minimum required while not exceeding the size that the watershed can keep the pond full. Peak Discharge Control for Streambank Protection An additional function of the pond must be to control the increase in the peak rate of runoff from the site. The post-development discharge below the outlets for the two-year and ten-year storms must not exceed the predevelopment rates. The purpose is to protect the receiving channel from erosion resulting from the increased rate and volume of runoff. Installing a dewatering device to store the one inch runoff volume above the permanent pool may meet or exceed this requirement, but this needs to be confirmed and calculations provided. The optimal level of flood control and streambank protection is when multiple design storms are controlled, both the 2-year and 10-year design storm is sufficient. Because development of the site will surely increase the rate and volume of runoff above the pond, provisions must be made for protecting the channels upstream of the pond so that they do not erode and cause unacceptable levels of sediment to be carried into the pond. Pipes The use of reinforced concrete pipe is encouraged because of its long life even though it may be more difficult and expensive to install initially. Plain corrugated metal pipe (CMP) is not acceptable. It must be coated (preferably aluminized) because of the short life expectancy of plain pipe and the trouble and expense of replacement, in addition to the risk of failure of the structure. Pipe Inlet Protection A trash rack or hood is required to trap debris in the pond and prevent the outlet from clogging. Wire screen or bars across the inlet is not acceptable because trash and debris can accumulate over the inlet and such devices reduces the efficiency of the inlet. DRAFT 8 June 1993