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Mayor and Council May 14, 1996 • Page 2 limit stormwater retention at this site, the city wanted to consider whether a partial implementation of the diversion and stormwater storage alternative suggested in the 1992 study . offered any flood reduction benefits. This study reports the benefits of constructing a stormwater pond on this parcel and a relief storm sewer along Wilson Street (shown on Figure 1), without construction of the other elements of the diversion-storage alternative illustrated in the 1992 report. Partial Implementation of the Diversion-Storage Alternative in the 1992 Report To provide relief, any improvements will have to make maximum use of available storm sewer capacity, and provide a place for excess runoff to accumulate (other than in the areas now experiencing some flooding): The existing storm sewer in Wilson Street is already overloaded when it reaches 29th Street, so maximum use of capacity can be achieved by simply eliminating any connections to the existing storm sewer system along Wilson Street between 29th Avenue and 27th Avenue; flows from areas north of 29th Avenue will continue to use the existing storm sewer system to flow past those low-lying areas. Meanwhile, the areas between 29th Avenue and 27th Avenue will be drained by an additional storm sewer system along Wilson Street, from Murray Avenue south to the alley south of 27th Avenue NE, and then west to the proposed storage pond in watershed 6A-5 (see Figure 1). The new relief storm sewer system would collect runoff from the watersheds between 29th Avenue and 27th Avenue and direct it to the new stormwater pond, thus providing relief to the low-lying areas between 29th Avenue and 27th Avenue. The storm sewer on St. Anthony Boulevard at Coolidge Avenue would be bulkheaded to direct runoff west of Coolidge Street into the new storm sewer system. The proposed pond, located in the east end of the existing trailer park parcel, would provide approximately 19 acre-feet of storage, and would require acquisition of approximately 6.5 acres of land. The estimated order-of-magnitude cost of the relief storm sewer and pond is summarized in Table 1. Assuming a total cost of$1.5 million and an annual interest rate of 8 percent, the estimated annual cost for construction of the system over a 20-year period is about $152,000 per year. With the additional storm sewer system in place, the model predicts a reduction in 100-year flood levels of only 0.3 to 0.8 feet. The larger flood level reductions are near Murray Avenue; the smaller reductions occur near Pahl Avenue (the existing depth of flooding from the 100-year storm event in this area is typically less than 2 feet). While some reduction in damages would be achieved, and there would apparently be slightly fewer homes affected, greater reductions of the flood level are desirable. Greater reductions cannot be achieved because: Runoff from the area north of 29th Avenue that cannot find room to flow in the existing storm sewer system, flows overland and accumulates in the problem areas, adding to the water that must be handled by the new relief storm sewer. The proposed ponding area is simply not big enough to completely accept and store all the local runoff plus the overflow runoff from the watershed north of 29th Avenue. The approximate number of homes and outbuildings located in the existing and revised 100-year • floodplain are listed in Table 2. It appears that construction of the new storm sewer system would