8.C.1. PRSR 09 08 2004 - Laserfiche WebLink

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ITEM / $.C.1. SHERBURNE C~!~; SOIL ANA WATER CONSERVATION DISTRICT August 31, 2004 To: Bill Maertz, City of Elk River From: Mark Basiletti, Sherburne SWCD 14855 Highway 10 Elk River, MN 55330 Tel. (763) 241-1170 ext. 3 Website: www.sherbu rneswcd.org Re: Lake Orono Park rain garden and shoreline buffer demonstration project. Purpose: Currently storm water runs directly from the park impervious areas to the lake. This untreated storm water is a source of pollutants such as nutrients and fecal coliform bacteria. Much of the storm water can be treated by a system of rain gardens, swales and buffer areas. Rain gardens are designed to infiltrate most of the runoff. Vegetation in the rain gardens will tie up nutrients and filter pollutants from the runoff. For large runoff events, excess runoff will overflow into a buffer area which will provide further treatment. The SWCD's preliminary assessment is that storm water from most of the impervious surface could be treated by rain gardens and buffer areas. A rain garden was not planned for the parking lot adjacent to the beach because of possible future relocation of this parking lot. Three possible garden and buffer locations were identified (see attached map). The estimated areas of the proposed rain gardens would come within the guideline that the size of the rain garden be approximately 8% of the drainage area being treated. Water would enter the rain gardens via curb cuts where concrete gutters currently exist in the parking areas. Overflow from RG 1 would flow through buffer 1. Overflow from location RG 2 would flow via a Swale to location RG 3 and then through Buffer 2 to the lake. 2. Depth to the water table: The depth to the water table was estimated under the assumption that the water table is approximately at the lake level. Information from MPCA's "Protecting Water Quality in Urban Areas" states that the bottom of an infiltration basin should be 2 to 4 feet above the water table. The locations labe]ed RG 1 and RG 2 would most likely meet this criteria. The lowest point of RG 3 is only about 13 feet above the water table. The design for this treatment area should take this into account. An existing ice ridge at this location already provides a shallow depression. It may be desirable to create only a buffer at this location and design the buffer to avoid ponding to the extent possible. Depth to the water table will need further assessment for all locations. Soil borings should be done for each.