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Silver Lake Road Flooding Analysis Results May 03, 2017 Page 2 Community Benefits The construction of these catch basins and 36 -inch pipe will increase public safety and reduce flooding risks in St. Anthony Village and neighboring communities downstream. These proposed improvements will: 1. Reduce flooding on Silver Lake Road a. Increase safety on Silver Lake Road by reducing the risk of Hydroplaning at the low point. b. Eliminate flood damage to adjacent properties in the 100 -year rainfall event. 2. Reduce flooding downstream on Penrod, Silver Lane, Fordham, Shamrock, 3911 Ave and Foss Road. a. Eliminate 59 cfs of overland flow onto Penrod and Silver Lane b. Eliminate 4 acre -feet of floodwater volume to the low point of Silver Lane and Shamrock Drive. c. Eliminates flooding of two homes and significantly increases flood protection to several additional homes. d. Reduce Flow to Ramsey County Ditch 3, which currently backflows to Foss Road system near Mirror Lake Condominiums and to the low point at 3911 Avenue and Fordham Drive. Other Alternatives Evaluated 1. 36 -inch Pipe Not Hydraulically Connected We evaluated a parallel pipe that was not hydraulically connected to the existing storm sewer system; however, this option is not suggested. The surface flow, directly tributary to the low point, peaks before the flow in the existing trunk system peaks. Therefore, disconnecting the low point from the existing trunk system would require a slightly larger pipe to convey the flows. 2. Pond Excess Volume from Direct Tributary Area We evaluated ponding the excess volume from the direct tributary area; however, this option is not suggested. Four acre feet of storage is needed to eliminate flooding to adjacent properties on Silver Lake Road in the 100 -year rainfall event. There are two potential ponding areas available. The first is the well house parcel which has about 0.31 acre feet of storage available. The second is to purchase 4017 Silver Lake Road (sold in 2010 for $152,500) which has about 0.37 acre feet of storage. Both properties combined do not provided the four acre feet of storage needed to eliminate flooding to adjacent properties on Silver Lake Road in the 100 -year rainfall event. 3. Buy and Remove Flooding Properties The City could buy the two properties that flood and continue to let the water flow from the low point in Silver Lake Road to the Penrod Lane. Based on the last sales dates of 2010 and 2015, the two properties are valued more than $600,000 dollars. This option is not suggested because: • This option does not reduce the flooding issues downstream at Penrod or Foss Roads • This option relocates two existing home owners • This option would permanently remove two home from the property tax rolls. Model Assumotions The NOAA Atlas 14, 24 -hour precipitation events were used along with the MSE3 rainfall distribution to model rainfall volumes. Drainage areas were delineated to each catchment. The catchments were placed the low point of Silver Lake Road. The SCS Runoff Curve Number (CN) method, with times of concentration ranging from 10 to 30 minutes, were used to calculate the surface runoff from each drainage area. The hydrologic soil group for the vast majority of this area is classified as "Urban Disturbed Soils ". Therefore, based on WSB's experience working in the area, a CN value of 74 was used. A CN value of 74 is representative of grass cover greater than 75 %, in good condition, on soils of Hydrologic Group C. City record drawings were used to model the storm sewer system along with LiDAR data to model over land flooding. K:I21 WWMInlp W21 M4 WSH.11.'Adll..di UIUM emo Wl7-0L3tl