269 - An Application of a 2D Sediment Transport Model to Assess Potential Changes to Physical Habitat Resulting from the 2017 Oroville Dam Spillway Incident

Oroville Dam in Northern California is the tallest dam in the United States, at 770 ft. In early 2017, extremely high inflows to the reservoir resulted in high outflows that were accompanied by significant erosion of both the main and emergency spillways. Impacts to physical aquatic habitat associated with the 2017 erosion incident on an 8-mile reach of Feather River below the dam were assessed by comparing simulation model outputs. Model simulations included two scenarios: one using the observed reservoir releases, and one with the reservoir releases that would have occurred without spillway erosion. Total bed-material sediment flux varied considerably along the study reach reflecting the heterogeneity in bed sediment sizes and local hydraulic conditions. Simulation indicated net scour and sediment export under both scenarios, with somewhat greater scour and export under the observed incident due to a larger volume of flow over the simulated period. Computed changes in the fractional sand content of surficial sediment at spawning sites were small and likely insignificant relative to model accuracy indicating minimal change in habitat quality and habitat type distribution.
Comparison of the observed sediment load to historic data collected prior to closure of Oroville Dam indicates that the LFC was subjected to total sediment load in during the 2017 event that was only about 5% of the load that would have occurred for this event during pre-dam conditions. The same figure for coarser (sand) sediments is about 2%.