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I II I ,I I II I I , I I I I I I I I I I I STUDY AREA DESCRIPTION The City is located in Pipestone County in southwestern Minnesota, approximately 38 miles northeast of Sioux Falls, South Dakota, The majority of Pipestone lies to the west and south of the Main Ditch. The primary north-south highway serving Pipestone is U.S. Highway 75, while State Highway 23 provides access from the northeast and southwest. DESCRIPTION OF EXISTING FLOOD PROBLEMS The flood problems at Pipestone result from rapid spring snowmelt as well as heavy intense rains which primarily occur during the sununer months. The City is flood prone due to the relatively flat topography in the areas to the east and to the north. Generally, some nuisance flooding will occur each spring, Some areas experience flooding as a result of blocked drainageways and backed up floodwaters along small tributaries. HYDROLOGIC ANALYSIS During the course of the previous Section 205 Reconnaissance Study for Pipestone, hydrologic analyses were performed in order to develop peak discharge-frequency relationships for the Main Ditch and County Ditch A. Peak discharges were computed using several methodologies, and the results were compared to those values published in the 1991 Flood Insurance Study (FIS) for Pipestone, Minnesota. The discharges for this study and the previous study were determined using the Iowa statewide regression equations (U.S. Geological Survey, 1993), which compared favorably to the values published in the FIS. The 500-year peak discharges were extrapolated on log-probability paper using the 10-year, 50-year, and 100-year computed discharges. Table 1 shows the existing conditions peak discharges that were used in the hydraulic model for this study. 3