Urban Flood Frequency Characteristics

This paper presents the results of a study concerning the occurrence and factors affecting floods from small urban watersheds. Based on Log-Pearson Type III flood frequency analysis on four urban watersheds located in Texas, urbanization has significantly increased peak discharge. However, data on the Waller Creek watershed (Austin, TX) and Wilbarger Creek watershed (near Austin, TX) suggest that the effects of urbanization on peak discharge decreases after a certain recurrence interval. Data from 27 urban watersheds in Texas, combined with 33 urban watersheds located in Virginia, Maryland, Delaware, Illinois, and Mississippi, form the basis for the derivation of empirical flood frequency equations for small urban watersheds. These equations relate the flood discharge for recurrence intervals of 2.33, 5, 10, 20, and 50 yr. to physiographic, urban, and climatic factors. The significant urban factors were found to be the impervious cover and a channel urbanization factor.