Modeling Headcut Erosion in a Proposed Fuse Plug Auxiliary Spillway Channel at Glendo Dam

Potential headcut erosion of a proposed unlined spillway channel excavated through Brule siltstone was studied using the SITES water resources site analysis computer program distributed by the USDA. Runs of the SITES model that utilize field-measured values of the detachment rate coefficient and values of headcut erodibility index derived from geologic investigations predict headcut erosion that will threaten to breach the spillway control structure, as presently proposed. Field tests used to determine the detachment rate coefficient exhibited high variability, due to the difficulty of producing measurable erosion and due to the extreme susceptibility of the siltstone to rapid weathering. Predictions of erosion from the SITES model were extremely sensitive to the value of the detachment rate coefficient. Reducing the detachment rate coefficient by what are believed to be plausible amounts caused the SITES model to predict minimal headcut erosion, with no resulting threat to the spillway control structure. Research needs and potential research approaches for improving the ability to model headcut erosion of rock materials are discussed. Terms: SITES, headcut erosion, erodibility, Pick-Sloan Missouri Basin Program Oregon Trail Division - Glendo Unit - Wyoming. Available online.