Hec-Ras Sediment Simulation of A Snake River Dam Removal Alternative

A 2019 Environmental Impact Statement was updated the Columbia River System management, a system that includes fourteen federal, water-regulation projects in Idaho, Montana, Oregon, and Washington. One multiple objective alternative included structural measures for breaching of the four Lower Snake River dams (Ice Harbor, Lower Monumental, Little Goose, and Lower Granite). This alternative explored potential impacts of breaching the earthen embankments for these four dams. The analysis included a sediment impact assessment of a two-stage dam removal. This talk will describe the HEC-RAS, 1D, sediment model constructed to assess these impacts.

The modeling team developed and balanced the sediment budget and then calibrated the model against three evaluation metrics. First, the modeling team calibrated sediment-flux volume by calibrating reservoir deposition over two time windows. Second, we calibrated the flux gradation, comparing model results to reservoir gradation trends. Third, we evaluated the model performance in an erosion regime by simulating a 1992 partial drawdown that eroded the sediment delta in the upper two reservoirs, comparing the drawdown model results to repeated bathymetries and a few mid-drawdown concentration measurements.

Then the modeling team ran long-term forecast simulations, that included two-phases of mid-simulation dam removals to evaluate the sediment concentrations this alternative would introduce, downstream deposition, as well as temporary and persistent flood risk impacts from downstream deposition. The study also explored the sensitivity of sediment impacts to the post-removal hydrology. This talk will describe the modeling approach, data analysis, calibration effort, and lessons learned.