A WATERSHED MODELING ANALYSIS OF FLUVIAL GEOMORPHOLOGIC INFLUENCES ONFLOOD PEAK ATTENUATION

Flood peak attenuation caused by storage of flood water on overbank su rfaces effectively reduces the magnitude of peak discharges in some, b ut not all watersheds. Several geomorphic factors that affect the stor age and conveyance of flood water were investigated to assess their qu antitative influence on downstream peak discharges. The MIKE11 rainfal l-runoff and hydrodynamic models were calibrated for the Grant River w atershed, southwestern Wisconsin. Alternative geomorphic conditions we re modeled and compared to the original case. Results indicate that ch annel-floodplain-terrace morphology, valley width, stream slope, and h ydraulic roughness each influence peak discharges, especially for mode rate magnitude (5- to 50-year recurrence interval) floods. Peak discha rges varied by as much as 49% between simulations depending on geomorp hic conditions. Watersheds that effectively attenuate produce peak dis charges that are strongly correlated with total runoff. Watersheds tha t attenuate little produce peak discharges that exhibit greater varian ce due to storm intensity and duration.