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Mayor and Council • May 14, 1996 Page 3 remove three homes from the 100-year floodplain. This estimate is based on the topographic mapping that was done for the 1992 study, and is therefore approximate. No field survey was done. The approximate flood damage cost to homes and out buildings in the 100-year flood plain is also shown in-Table 2. -The figures assume one story homes, valued at $175,000, with contents valued at $25,000.-o No allowance-for basement values was considered. The flood damage estimates were estimated using Table 1 of the 1992 report;.although the costs are approximate, the difference between the estimate for existing conditions and the estimate for conditions with the proposed relief storm sewer is probably representative of the damage reduction which might be achieved -with the relief storm sewer. Over a 20-year period, the estimated annual flood reduction benefits average $38,000 per year. Comparing the estimated average annual cost for the relief storm sewer with the estimated average annual reduction of flood damages shows that the project would cost about 4 times the estimated benefits. Therefore, partial implementation of the diversion-storage alternative in the 1992 report cannot be recommended based on strictly economic criteria. Larger Pond and Relief Storm Sewer System We also considered another alternative that would require much of the trailer park parcel to build a very large pond, and a larger.relief system to provide capacity to carry overflow from areas north • of 29th Avenue, plus local flows, so that no damages result from the design storm. A preliminary analysis suggests that such a pond would need to provide a volume of about 38 acre-feet, and the relief storm sewer would need to provide about twice the capacity of the system described above. It would require about 14 acres of the parcel to be regraded as pond. The entire system is estimated to cost approximately $3 million. The average annual cost would be over 4 times the estimated annual value of reduced damages. As before, implementation of this alternative is not recommended based on strictly economic criteria. Other Considerations If the city chooses not to implement any part of the diversion-storage alternative in the 1992 report, and to permit development of the area identified as necessary for ponding in the report, then there will no longer be an opportunity to implement the diversion-storage alternative. .As a minimum, any developer of the trailer park parcel should be required to provide storage for runoff which originates in subwatershed 6A-5 (about 4 to 5 acre-feet in size), which will require 1 to 2 cres of area. Our calculations show that this pond can be about the same size and serve to also hold runoff from subwatersheds 6A-7 and 6A-8 as well. This should be considered when a development plan is reviewed for the site, and would not necessarily result in added cost to the city, depending on details of the development agreement. I hereby certify that this report was prepared by me or under my direct supervision, and that I am a duly registered Professional Engineer under the laws of the State of Minnesota. 0(?�AA�t'4__� • Dennis E. Palmer May 14, 1996 Registration No. 9632 2327467\37777-1