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• II. EXECUTIVE SUMMARY A. Introduction /Background This engineering analysis of selected flood problem areas within the City of St. Anthony was commissioned as a follow-up to a rainfall event experienced on July 1. 1997, when many structures within the City experienced flood damage. During and after the event,the City received 57 storm water complaints. These complaints were brought to the City's attention by telephone calls, citizen contact with staff, council representatives, or public works employees. A list of these storm water complaints/issues is included in Appendix B. The City followed up by sending a survey on July 8`h to residents experiencing problems, in an effort to gather more information on the specific cause of flooding and associated damage. Responses to the surveys are included in Appendix D. The 57 complaints/issues were either resolved, removed from consideration(see discussion below), or included in one of 13 individual study areas. These areas are shown in the attached figure. This report provides an analysis of the rainfall event that occurred, a conceptual design for recommended improvements and preliminary cost estimates for those improvements. The report also outlines council response options available to address these problems. • B. Analysis of July 1, 1997 Rainfall Event On July 1, 1997, the City of St. Anthony received 3.34 inches of rain over a 50-minute period. Probability tables developed by the National Weather Service (based on rainfall data from 1853 through 1961)predict there would be less than a 1% chance in any given year that more than 2.9 inches of rain would fall in 60 minutes (Table 2). The Weather Bureau tables also indicate that a storm having a 1% chance of occurrence (i.e. a 100-year event) would generate 3.5 inches of rain in 2 hours. Therefore, the amount of rainfall that normally would have occurred in 2 hours for a 100-year event occurred in approximately 50 minutes on July 1. A rainfall analysis for the Midwest over the past 30 years (1961 through 1991) indicates the severity of rainfall events appears to be increasing. Utilizing this data, the tables generated by the National Weather Service could be updated and rainfall amounts for a given probability event could be increased by 25 to 30%. The recent data suggests that a 60-minute storm having a I% chance of occurrence would generate approximately 3.70 inches of rainfall (up from 2.9 inches based on original tables). Since the July 1 event generated 3.34 inches of rain in 50 minutes, this event still exceeded a 1% chance storm event. Most cities within the Twin Cities Metropolitan area currently attempt to design storm. • drainage systems to accommodate 100-year storm events without allowing water to inundate structures. As discussed above, the 100-year event is equivalent to 2.9 inches of rain in 60 minutes from the older probability tables and 3.7 inches of rain in 60 minutes City of SL Anthony WSB Project No. 1065.11 Pape 3 3