An Approach for Evaluating Flood Frequency Estimates for Ungaged Watersheds

Floodplain management and design of hydraulic structures, such as bridges and culverts, require flood frequency estimates for various return periods at ungaged watersheds. These estimates are usually made with regional regression equations or rainfall-runoff models. The regional regression equations are developed by relating flood discharges, such as the 100-year flood discharge, to watershed and climatic characteristics at gaging stations. For application at an ungaged site, one must determine the applicable watershed and climatic characteristics. Rainfall-runoff models are often based on a design storm where rainfall amounts are taken from applicable rainfall atlases and a time distribution is assumed. These two different approaches can provide widely differing estimates of the design discharge. Often flood discharges are available from both regression equations and rainfall-runoff models prior to developing the floodplain boundaries or designing the hydraulic structure. This paper describes an approach for comparing estimates from regression equations and rainfall-rainfall models to gaging station data and determining the most reasonable flood frequency estimate.