Origin of Flood Skew

Engineering analyses and designs that involve flood frequency often require an estimate of the log-space coefficient of skewness. The existing procedure may require the use of a flood skew estimate obtained from a skew map, which is known to be inaccurate. The map also lacks a conceptual basis. The goal of this research was to provide a method for estimating flood skew that is accurate and has a sound conceptual basis, then test the method with hydrologic data, and compare the accuracy of the approach to the accuracy of a flood skew map of the same region. The conceptual basis for flood skew estimation involves rainfall skew and watershed and channel storage. Rainfall skew represents an upper bound on the population of runoff skew. Flood skew decreases from the rainfall skew for the same location as storage increases. Using data from the coastal plain of Maryland and Delaware, a rainfall skew of 1.4 was estimated. A regression equation based on storage variables was found to be statistically significant with a correlation coefficient of 0.87. A map based on the same data only provided a correlation coefficient of 0.71, with a standard error that was 40% larger than that of the storage-based model. Greater accuracy was achieved using a storage-based modeling approach than using a skew map for the same region.