Reclamation Information Sharing Environment (RISE)

The spatial distribution, volume, and relative grain size of sediment in reservoirs influences management plans through the operational lifespan and decommissioning stage of impounding dams. Reservoir sediment studies are infrequently conducted and historical data are lacking, however—leading to great uncertainty over how to manage infrastructure. Even if bathymetric surveys have been performed (often they have not) to estimate storage capacity, little may be known about the sediment characteristics. In August 2020, USACE and Reclamation jointly used a shallow acoustic chirp system to map the stratigraphy and pre-impoundment surface of two reservoirs in Colorado. Both reservoirs are rich with complementary data regarding the distribution of sediments and pre-impoundment surface, which provides for convenient assessment of chirp system capabilities. Qualitative comparisons suggest that the mapped sub-bottom surface elevations (from the chirp system) were consistent with known pre-impoundment surfaces prior to dam construction. Approximate volumes of accumulated sediment were calculated based on the mapping effort, and observations of the reflective surface were indicative of a gently sloping alluvial plane cut by numerous shallow fluvial channels. Overall, the results were useful in demonstrating the capability of the shallow acoustic technique in imaging pre-impoundment surfaces and assessing geomorphic conditions.