NORMALIZED UNIT-HYDROGRAPH AGGREGATION

The unit hydrograph is a common tool in hydraulic design. Used correct ly, it allows a design engineer to estimate a runoff hydrograph from a drainage basin given a rainfall event. The typical method for estimat ing a unit hydrograph for a gaged watershed is by deconvolution. Howev er, distinct storms produce different unit hydrographs for a single wa tershed. Consequently, a design engineer usually develops a composite, or average, unit hydrograph based on several recorded storm events. C ommon methods for estimating this composite unit hydrograph include cu rve fitting, simple aggregation, and multistorm optimization technique s. This paper introduces a new method to perform aggregation of unit h ydrographs. The method is an extension to the simple averaging techniq ue, in which prior to averaging, the individual unit hydrograph time o rdinates are normalized with respect to the average time to peak. The normalization method is compared to a simple averaging technique and t wo multistorm aggregation techniques at six rural watersheds in Alabam a. The results indicate that on average the normalization method predi cts runoff nearly as accurately as the multistorm techniques, and disp lays improvement for 60 percent of the storms tested when compared wit h the simple averaging technique.