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b. Rainfall Hyetograph: <br />The enclosed hyetograph of rainfall versus time shows a distri- <br />bution of rainfall in 20 minute increments for a 6 hour storm <br />event (100 year). If it is assumed that the 24 hour 100 year storm <br />event includes all shorter duration 100 year storm events as given <br />in TP--40, then rainfall prior to and after the 6 hour storm event <br />shown will be of low intensity. The hyetograph shows horizontal <br />lines for infiltration rates of 111 and 2 inches per hour. These <br />appear to be reasonable rates based on the very sandy mature of <br />Mounds View soils. The two infiltration rates give the following <br />runoff values. <br />Infiltration Rate Runoff -inches <br />Vi in./hr. 1.59" <br />2 in./hr. 1.29. <br />Based on the above information, a.runoff rate of 1.45 inches is used <br />in our calculations for all duration runoff events. <br />EXISTING STORAGE IN SCOTTLAND GREEN POND <br />Topographic information for the pond area is based on a site sur- <br />vey by Egan, Field and Nowak in 1971 and selected shots by our field crews <br />in 1980. The additional work was required to determine elevations along <br />the north line of the apartment porject. The control elevation at the <br />present outlet of the marsh area is 891.5 (invert of culvert). However <br />the ice level in the outlet channel for the marsh was at 892.2. <br />The following table shows elevation versus area and volume for the <br />existing marsh area. <br />Contour <br />Area <br />Acres <br />A + A <br />1 2 <br />L/2 <br />Volume <br />AF <br />Cumulative <br />Volume AF <br />892.2 <br />0.00 <br />0.00 <br />0.85 <br />0.9 <br />0.77 <br />894.0 <br />0.85 <br />0.77 <br />10.77 <br />1.0 <br />10.77 <br />896.0 <br />9.92 <br />11.54 <br />898.0 <br />19.58 <br />29.50 <br />1.0 <br />29.50 <br />41.04 <br />POND OUTLET RATING CURVE <br />Under the City of Mounds View ordinance, after development discharge <br />cannot exceed predevelopment discharge. We assumed tailwater control for <br />the 36 inch diameter outlet pipe. The tailwater elevation used was 894.0 <br />which is the case that has existed in the past when flooding occurred at <br />Long Long Lake Road and Ridge Lane (this assumes 0.8 feet of water at <br />the centerline of the road at Ridge Lane). <br />