Sediment Analyses for Urbanizing Watersheds

An analysis of channel modifications for Harding Ditch, the main drainage canal for two urbanizing watersheds near St. Louis, Missouri, required an estimate of depositional changes between existing and future land use conditions, and the frequency and quantity of dredging required to maintain a modified channel configuration. A grid cell data bank was used to breakdown each watershed by land use category and the percent of the watershed in each category. Sediment washoff parameters were estimated for each land use category and input to the STORM computer program. Hourly values of discharge and tons of sediment washoff per event from each watershed were computed for 20 years of rainfall data for existing and future land use. STORM output was manipulated through an interface program and supplied to an HEC-6 sediment transport model of 11.5 miles (18.5 km) of Harding Ditch to evaluate depositional patterns. Comparison of 1982 and 2020 thalweg profiles showed less deposition under future land use conditions. Dredging analysis indicated a 10-year excavation interval was sufficient to maintain a with-project rating curve relationship.