1125 80TH ST SW 2016-01-01 MF Import

Wetpond/Detention• a The wetpond volume calculation appears to be incorrect - the amount of impervious area was given as 0.00 acres in the basin summary. The wetpond volume is equal to ' the volume of the 6 month, 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 itte_entiresvetpond.design volume to be reached for the wetpond design storm (6 month, 24-hour storm). Routing of the wetpond design storm developed hydrograph through the pond is required to verify that the wetpond design volume is reached for the design storm. The size of the primary orifice can be decreased as necessary, to a minimum allowable orifice size of I inch, to back up enough live storage. If a 1 inch orifice doesn't result in the required live storage in the pond (during the wetpond design storm), 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 equals the wetpond design volume. The control structure must ensure that detention release rates do not exceed allowable release rates tied that the detained volume in the pond reaches or exceeds the wetpond design volume for the wetpond design storm. Orifice calculations must show that the control structure is meeting both these requirements. City standards require a two-cell wetpond design, with 3 to 6 feet of dead storage in the first cell (called the open water cell in the City's standards) or in a wetvault. A maximum of 70% of the wetpond design volume may be associated with the open water cell or wetvault. A minimum volume equal to a 15 minute detention time for the peak flow from the design storm (approximately 160 cubic feet per acre of contributing impervious surface for a 6 month, 24-hour storm) must be associated with the open water cell or a wetvault. A minimum of 30% of the required permanent storage volume, with an average depth not exceeding 1.33 feet, is required to be associated with the second cell. This cell is called the wetland treatment cell. The design depth of dead storage in the wetpond and the percentage of the dead storage provided in cacti cell must he given in the calculation package. The required detention volume can be provided as live storage in addition to the dead storage. Live storage provided to meet wetpond design volume requirements counts towards required detention volume. The area of the pond bottom and the area of the pond at the dead storage elevation must be given in the calculation package to show that the required dead storage volume is provided. Oil/water separator/grit removal vat The MDNS requires an oil/water separator with 250 gallons of live storage to colic-t water from prodL.t transfer areas, if a vault with an oil/stop valve is not feasible. If a separator is provided, it should receive runoff only from product transfer areas, to the