Flood Skew in Hydrologic Design on Ungaged Watersheds

Abstract

The skew of a peak‐discharge frequency curve is an important determinant of the magnitude of events having small exceedance probabilities. Unfortunately, methods for estimating skews for ungaged locations provide highly inaccurate estimates. This appears to be the result of a lack of understanding of the primary causes of variation in skew. An analysis of existing hydrologic methods indicates that the skew assumed with these methods for ungaged locations is different than the mean skew for gaged sites in the same area. Furthermore, the design methods suggest that watershed storage is the primary factor affecting skew, with an increase in skew as the volume of watershed storage is decreased. Variables that show greater potential for providing accurate estimates of skew are discussed, and a relationship for estimating the effect of imperviousness on skew is provided.