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10/24/94 <br />• CITY of <br />LINO LAKES <br />Standards for Facility Design <br />Size <br />Detention ponds will be designed with standing water or dead <br />storage for pollutant removal. The phosphorous removal efficiency <br />for each pond or pond network must fall in the 65 -70 percent <br />range. Storm water treatment can be provided via a single pond <br />which meets the design and treatment criteria or an on -site <br />network of interconnected ponds. If an on -site pond network is <br />used, the overall pollutant removal efficiency for the network must <br />meet the criteria. The recommended pond design criteria in order <br />of importance are as follows: <br />(1) The permanent pool is important because it provides <br />storage and treatment of runoff during and between storm <br />events. Permanent pool volume should b greater than or <br />equal to the volume of runoff from a 2.5 -inch rainstorm <br />under full projected watershed development. This value <br />has been derived from design criteria developed in Nation- <br />wide Urban Runoff Program (NURP), with a 25% increase <br />in volume to allow for roughly 245 years of sediment <br />accumulation. In the summer, St. Paul climate, this sizing <br />rule provides a mean hydraulic residence time of about 15 <br />days. <br />(2) To promote settling and provide space for sediment <br />accumulation, the mean depth of the permanent pool <br />(volume /surface area) should be greater than or equal to 4 <br />feet. This constraint may be infeasible for small ponds (< <br />approx. 3 acre -feet in volume, see below), where mean <br />depths of 3-4 feet may be used. <br />To prevent development of thermal stratification, loss of <br />oxygen and nutrient recycling from bottom sediments, the <br />maximum depth of permanent pool should be less than or <br />equal to 10 feet. <br />(3) <br />(4) To promote plug flow behavior, the ratio of maximum <br />length to maximum width (Lc /WW) should be greater than <br />or equal to 3. Expected performance is less sensitive to the <br />length /width ratio than to volume or depth. This constraint <br />may be infeasible for some site plans or for small ponds. <br />Local Water Management Plan SEH NO: L1NOL9402.00 <br />Users Manual Page 18 <br />