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6/91 <br />DESIGN AND EVALUATION OF STORM WATER DETENTION <br />FACILITIES <br />USING A DESIGN STORM FREEBOARD EVALUATION STORM* <br />Rocky J. Keehn, * *Member ASCE <br />Mark L. Lobermeier, * *Member ASCE <br />Abstract <br />When designing a storm water detention facility, both a <br />design storm (DS) and a freeboard evaluation storm (FES) should <br />be considered. The DS is used to design the facility. The FES <br />is used to set a freeboard elevation. The DS and FES can be <br />based on recorded data or sensitivity analyses of the region <br />using a range of synthetic rainfall distributions and soil <br />moisture conditions. Sensitivity analysis using the TR -20 <br />program was utilized to evaluate the DS and FES as a part of the <br />Roseville, Minnesota Surface Water Management Plan (SWMP). The <br />Roseville SWMP also demonstrates how the DS /FES method provides a <br />cost effective approach for managing storm water runoff while <br />providing a sufficient protection level for the given recurrent <br />interval. The DS /FES design concept is a flexible method. This <br />makes it applicable to several design methodologies and usable as <br />a basis for design and evaluation in most surface water manage- <br />ment plans. <br />INTRODUCTION <br />Numerous computer models are available for the design of <br />storm water detention facilities. There are discrepancies and <br />disagreements on almost every parameter used. Most models can <br />analyze numerous storm durations, intensities, and soil moisture <br />conditions. They all produce an estimation of the hydrologic <br />response of a- watershed. Freeboard elevations, conveyance <br />systems and emergency overflows are then based on these results. <br />Often times, these = designs can be over conservative, and costly <br />improvements are unnecessarily undertaken. <br />*Published in "Hydraulic Engineering, Proceedings of the 1988 <br />National Conference" ASCE, August 1988 - Revised December <br />1988. <br />Presented at 1988 ASCE Hydraulic Engineering Conference, <br />Colorado Springs, CO and 1988 Water Resources Seminar, St. <br />Paul, MN <br />* *Water Resources Engineer, Short - Elliott - Hendrickson, Inc., <br />3535 Vadnais Center Drive, St. Paul, MN 55110 <br />