1515 112TH ST SE BASE FILE 2018-01-02 MF Import

PUBLIC WORKS E ETT MEMORANDUM TO: Alan Locke FROM: Jane Zimmerman DATE: February 13. 1997 SUBJECT: Drainage Calculation Review, Dennis Petroleum, 16th and 112th St. SE Wetoond/D to tion: The design depth of dead storage in the wetvault and treatment cell must be given in the calculation package. The wetvault is required to have a dead storage depth of 3-6 feet, the wetland treatment cell may have a maximum average depth of 1.33 feet. A maximum of 70% of the wetpond design volume may be associated with the open water cell/wet vault. A minimum volume equal to a 15 minute detention time for the peak Flow from the design storm must be associated with the open water cell or a wetvault. Wetpond calculations must be provided. The wetpond design volume is equal to the volume of the 2 year, 24-hour developed storm hydrograph. The standards require a minimum of 50% of the wetpond design volume to be provided as permanent dead storage and for the entire wetpond design volume to be reached during the wetpond design storm. Routing of the wetpond design storm through the pond is required to verify that the wetpond design volume is reached for the wetpond design storm. The size of the primary orifice can be decreased as necessary, to a minimum allowable orifice size of I inch, to achieve enough live storage. If' 1 inch orifice doesn't result in the required storage in the pond, then the percentage of the wetpond design volume provided as dead storage must be increased until the combination of dead storage and peak live storage (during the wetpond design storm) equals the wetpond design volume. Miscellaneous This project is utilizing the biofiltration swale located on the adjacent property to provide part of this project's required runoff treatment. However, the existing Swale does not meet current City standards, particularly with respect to vegetation in the swale. For the requirements of this project to be satisfied, wetland vegetation must be planted in the swale in accordance with current City standards. In addition, to decrease the effective slope of the swale, three additional check dams must be constructed in the swale. One of these must be located approximately 2-3 feet from the swale outlet; the other two should be located midway between the two existing check dams. The existing check dams appear to be constructed of 4-8 inch riprap and are about 6-12 inches high and a foot or so wide. This design appears adequate for this Swale and can be utilized for the new check dams.