07/25/2001 Env Bd Packet - Laserfiche WebLink

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2/90 A sensitivity analysis was performed with the SCS TR -20 model for an urban district which contains a variety of detention areas and a large recreational lake. A range of synthetic storm distributions was used to model the watershed. They included the SCS Type I and II, several Huff rainfall distributions and a snowmelt event. SCS antecedent moisture conditions (AMC's) for dry, normal, and wet soils (1, 2, or 3) were modeled for each distribution. The analysis indicated that the Type I distribution with an AMC of two tended to represent the average water surface elevation for all conditions analyzed. Based on these results and our confidence on the Type I AMC 2, it was used as the DS. The FES was either the SCS Type II with an AMC of three or a snowmelt event. This rainfall distribution and soil condition appeared to be the most critical. The snowmelt modeled the 10 -day runoff for the region. Two FES's were used since the snowmelt event is critical in certain cases which cannot be accurately modeled by the Type II distribution. The existing facilities were checked based on this criteria. Freeboard evaluations were determined to be either of the DS results plus two feet, or the FES results. The two foot freeboard is the standard flood insurance requirement. This concept will be used to design new facilities. The study indicated that several building elevations were below the DS -plus- two -feet elevation. To provide protection, storage volume and /or outlet capacity would need to be increased at considerable cost. By applying the FES rather than the DS- plus -two, it was demonstrated that adequate protection exists. The DS /FES concept provides the means to perform a cost /benefit analysis prior to undertaking improvements. CONCLUSIONS The DS /FES concept was developed to provide for a cost - effective method to design storm water detention facilities. It is flexible, satisfies local review agencies criteria, provides for hydrologic prediction of freeboard elevations, and can easily be applied to local watershed plans. The Roseville SWMP illustrates the practicality of concept. APPENDIX Huff, F.A., (1967) "Time Distribution of Second Quartile Storms;" Water Resources Research, 3, No. 4, pp. 1007 -1019. Maclay, L.M., (1965) "Computer Program for Project Formulation Hydrology," Technical Release No. 20, PB- 233778, Soil Conservation Service, Department of Agriculture, Washington, D.C. D - 10.