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I I I I I I I I I I I I I I I I I I I II. PROPOSED SOLUTIONS Three alternatives have been evaluated which address the regular flooding and sediment deposition problems generated by the hogback drainage basins. 1. No Action 2. Partial Detention (Hogback Detention Master Plan) 3. Full Detention No Action The consequences of "no action" have been considered in the evaluation of alternative flood mitigation measures. Summer thunderstorm events which resulted in flooding of residential and business areas below the hogbacks occurred in 1991, 1994, 1996 and 1997. Although little damage to structures was experienced during these events, cleanup and repair costs and inconvenience to local residents and business owners were substantial. In 1997, total cleanup and repair costs resulting from hogback drainage flooding were approximately $70,560 (documentation provided in Appendix A). As previously discussed, stormwater flows generated in the hogback basins are problematic both in their magnitude and in the large sediment loads which they typically carry. The high cost of cleanup and repair results mostly from sediment deposition in storm drains and on city streets and sidewalks. The magnitude of flow generated produces severe street flooding even for moderate storms. Residential and downtown streets are regularly turned into rivers flowing 6 to 12 inches deep. Figure II-I illustrates the extent of flooding and sediment deposition in the North Ninth Street Basin which has accompanied the notable storm events of the 1990's. The recurrence interval of these storms is difficult to ascertain because of widely varying rainfall reports and the highly localized nature of thunderstorm events. Reported rainfall depths for the 1991 storm ranged from I to 6 inches; 1 to 3 inches were reported for the 1994, 1996 and 1997 storms. Storm events resulting in substantial flooding and sediment deposition have occurred an average of once every two years in the 1990's. Detailed clean up cost data for the year 1997 and an estimate of average annual costs over the past seven years have been provided by the City. Including a 15 percent administrative overhead charge, clean up costs for 1997 were approximately $70,560. Based on the major storm of that year being representative of the 1994 and 1996 storms but less severe than 1991, 1997 clean up costs are thought to approximate the biannual cost for flood cleanup which should be expected under the No Action Alternative (see Section V). The City estimates that average annual clean up costs are approximately 75 percent of biannual costs or $52,920. If the No Action Alternative is selected, the extent of flooding and sediment deposition delineated on Figure II-I are expected to occur an average of once every two years. Annual clean up costs averaging $52,920 are expected to be incurred even in years without severe thunderstorms. II-I