Effect of River Network Geometry on Flood Frequency: A Tale of Two Watersheds in Iowa

AbstractAlthough numerous studies conducted over the past four decades have shown the significant role that drainage network geometry plays in determining the streamflow response and hence the peak flood magnitude, a review of regional flood frequency equations established for various hydrologic regions in the United States shows that drainage network geometry is largely excluded from regional regression equations. This study demonstrates how river network geometry affects flood frequency using two catchments located in the same hydrologic region and have the same drainage area, but with different drainage network geometry. The results from the analysis of historical peak discharge data obtained from streamflow gauging stations located at the outlet of the watersheds and the systematic numerical simulation of the watersheds show that the apparent difference in the drainage network geometry of the two watersheds accounts for the observed difference in the flood frequency. By showing the significant effects of drainage network geometry on flood frequency, the results highlight the need for a revision of the current approach to regional flood frequency equations to improve the accuracy of flood quantile estimation in ungauged basins.