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Memorandum Montgomery Associates 1I <br /> Page 3 Resource Solutions,uc ����� <br /> as moderately high to high,with rates varying from 0.6 in/hr to 2.00 in/hr.For this analysis an infiltration <br /> rate of 0.6 in/hr was used.These sources are attached in Appendix B. <br /> Modeling Summary <br /> WinSLAMM 10.1.6 was used to model TSS loading in the post-development condition.These calculations <br /> utilized Madison rainfall for the period of March 12, 1981 through December 2, 1981,as specified in NR 151, <br /> which has a total rainfall depth of 28.81 inches.The National Urban Runoff Program(NURP)50th percentile <br /> particle file was used for pollutant analysis. Standard parameter files specified by the Wisconsin <br /> Department of Natural Resources(WDNR) and obtained from the USGS website were used for other model <br /> inputs. <br /> The model runs continuously with hourly precipitation and daily average temperature data. Continual mass <br /> balance calculations are tracked between devices. Hourly rainfall data from Madison,Wisconsin(closest <br /> location to the site with adequate hourly rainfall data)was run between March 12, 1981 and December 2, <br /> 1981 as specified in NR 151, which equates to a total rainfall depth of 28.81 inches.The P8 model accounted <br /> for flows contributed to the rain garden from the disturbed area in the pre-developed and post-developed <br /> conditions. <br /> Model inputs and outputs are available in Appendix B. <br /> Compliance with Performance Standards <br /> WinSLAMM calculates a TSS load of 306.2 lbs/yr in the post-development condition with no controls.When <br /> accounting for the rain garden,the model calculates a TSS release of 180.5 lbs/yr.Thus,the rain garden <br /> provides a 41.05 percent reduction in annual TSS load and satisfies the Dane County performance standard <br /> for TSS removal. <br /> RECARGA and Dane County recharge equations were used to show the site design meets Dane County <br /> target recharge requirements of 7.6 inches per year. An annual recharge of 13.1775 inches per year was <br /> calculated for the rain garden controlled area of the site and a recharge rate of 7.093 inches per year was <br /> calculated for the uncontrolled area. The total recharge calculated for the site is 10.24 inches per year,which <br /> exceeds the target recharge requirement. <br /> Attachments: Permit application forms <br /> Figure 1 —Site Location Map <br /> Figure 2—Grading and Erosion Control <br /> Figure 3—Erosion Control Details <br /> Figure 4—Erosion Control Details <br /> Appendix A-Soil loss calculation <br /> Appendix B—Stormwater Management Modeling Summary <br /> • WinSLAMM Summary <br /> • RECARGA Summary <br /> P:\1741 Kieran Montgomery House\Reports\STORMWATER MEMO8-16.doca <br /> 119 South Main Street • Cottage Grove, WI 53527 ph 608.839.4422 fx 608.839.3322 • www.ma-rs.org <br />