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I I I i I I I I I I , I I I I I I I I I I DESCRIPTION OF EXISTING FLOOD PROBLEMS The flood problems at Rock Rapids result from rapid spring snowmelt as well as heavy, intense rains which primarily occur during the summer months. Nuisance flooding from frequent rainfall events does not seem to be a problem in Rock Rapids. However, the area known as Island Park is one of the most flood prone locations within the city and experiences occasional flooding. Overbank flows along the Rock River causing any significant flood damage to structures may occur only during more infrequent events. HYDROLOGIC ANALYSIS During the course of the previous Section 205 Reconnaissance Study for Rock Rapids, hydrologic analyses were performed in order to develop peak discharge-frequency relationships for the Rock River, Moon Creek, and Tom Creek at Rock Rapids, Iowa. Peak discharges were computed using several methodologies, and the results were compared to the values published in the May 1991 Flood Insurance Study (FIS) for Rock Rapids, Iowa. The Omaha District decided to use the peak flows estimated using the Iowa statewide regression equations (U.s. Geological Survey, 1993) even though there were only slight discrepancies between those discharges and the ones presented in the FIS. Peak discharges for the 1oo-year and 5oo-year events are shown in table 1 for both the FIS values and the selected regression equation values. The 500-year values were extrapolated on log-probability paper from the computed 10-, 50-, and 100-year values. Table 1 Comparison of Existing Conditions Peak Discharges Rock River at Rock Rapids, Iowa Flooding Source and Location Rock River just upstream from Tom Creek Rock River just downstream from Tom Creek Peak Discharges (ds) FIS Regression 100-yr 500-yr 100-yr 500-yr 36,210 58,800 35,560 57,000 37,660 61,150 37,440 60,000 4