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WSB 701 Xenia Avenue South I Suite 300 1 Minneapolis, MN 55416 1 (763) 541 -4800 Memorandum To: Mark Casey, City Manager Jay Hartman, Director of Public Works Molly Churchich, Ramsey County From: Todd Hubmer, PE, City Engineer Tony Miller, EIT Date: May 03, 2017 Re: Silver Lake Road Flooding Analysis Results WSB Project No. 2170 -420 Existing Conditions There is a low point on Silver Lake Road that is currently overtopping and flooding the properties on the east side of the road. This low point is located approximately 250 feet south of the intersection with Silver Lane Northeast. An XPSWMM model was created to evaluate the flooding and used the 2 -, 10 -, and 100 -year National Oceanic and Atmospheric Administration (NOAA) Atlas 14, 24 -hour precipitation events along with the MSE3 rainfall distribution. In this area, the 2 -, 10 -, and 100 -year events correspond to 2.83, 4.24, and 7.38 inches of rain in 24 hours. Based on the MSE3 rainfall distribution, approximately 60 percent of the rain falls between the 1211 and 1311 hour of this 24 -hour period. Therefore, the 2 -, 10 -, and 100 -year events are approximately equal to 1.7, 2.5, and 4.4 inches of rainfall in 1 hour. The storm sewer pipe trunk system and inlet capacity on Silver Lake Road at this low point is currently undersized and flood the east properties during the 2 -year event. Improvements for Consideration To reduce the frequency of flooding at this low point in Silver Lake Road the following improvements are proposed for your consideration. 1. Install eight high capacity catch basins at the low point in Silver Lake Road to replace the five existing catch basins. This will allow the water on the road surface to more quickly enter the pipes. 2. Construct a new 36 -inch pipe from the low point north that will discharge into Silver Lake through the retaining wall on the south bound side of the road. This pipe will be constructed in the current location of an abandoned 24 -inch cast iron force main, and will be hydraulically connected to the existing 60 -inch stone sewer pipe in Silver Lake Road. This design is anticipated to provide adequate storm sewer capacity to prevent flooding of the east properties during a 100 -year event. Figure 7 provides a plan view illustration of this design. Building a legacy — your legacy. Equal Opportunity Employer I wsbeng.com K1021 ]0-020WEminlpau` 021I 0-0 20_ SiMhnYeRUFbotli ,gReMlaMa„o_M1)- 0503.dou