28-14 Amend Chp. 1339 Storm Water Mgmt

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Table 1 Runoff Coefficients Based on the Type of Land Use Land Use Runoff Coefficient Industrial and Commercial 0.8 High Density Residential ( >8 dwellings /acre) 0.5 Medium Density Residential (4 to 8 dwellings /acre) 0.4 Low Density Residential ( <4 dwellings /acre) 0.3 Open Space and Recreational Areas 0.2 (4) Where the land use will be mixed, the runoff coefficient should be cal- culated using a weighted average. For example, if 60 percent of the contributing drainage area to the storm water treatment structure is low density residential, 30 percent is high density residential, and 10 per- cent is open space, the runoff coefficient is calculated as follows (0.6)(0.3) + (0.3)(0.5) + (0.1)(0.2) = 0.35. (5) An additional volume equal to 20 percent of the WQv shall be incorpo- rated into the BMP for sediment storage and reduced infiltration ca- pacity. The BMPs will be designed according to the methodology in- cluded in the Ohio Rainwater and Land Development manual, ODOT post - construction storm water standards, or other manual that is ac- ceptable to Ohio EPA. (6) The BMPs listed in Table 2 below shall be considered standard BMPs approved for general use. However, the City may limit the use of some of these BMPs. BMPs shall be designed such that the drain time is long enough to provide treatment, but short enough to provide storage avail- able for successive rainfall events and avoid the creation of nuisance conditions. The outlet structure for the post - construction BMP must not discharge more than the first half of the WQv in less than one third of the drain tine. Table 2: Target Drain Times for Structural Post- Construction BMPs BMPs Suitable for Drainage Areas >_ 5 BMPs Suitable for Drainage Areas :5 5 Best Management Prac- Drain Best Management Drain tice Time Practice Time Dry Extended Detention 48 Hours Infiltration Basins 24 -48 Basin* or TrenchA Hours Wet Extended Detention 24 Hours Bioretention Ar- 24 Hours Basin ** ea/Cell Constructed Wetlands+ 24 Hours Sand and Other 24 Hours Media Filters Infiltration BasinsA 24-48 Enhanced Water 24 Hours Hours Quality Swales Pocket Wetlands# 24 Hours Vegetated Filter 24 Hours Strips Permeable Pave- 48 Hours 59