An Assessment of Changes To Physical Habitat Resulting From The 2017 Oroville Dam Spillway Incident: An Application of A 2d Sediment Transport Model To Characterize Potential Effects

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 around 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. Danish Hydraulic Institute (DHI) MIKE 21C two-dimensional numerical simulation models were constructed to simulate water and sediment conveyance in the 40-mile stretch of the Feather River between Oroville Dam and Yuba City, CA during the 2017 spillway erosion event (February 1, 2017 to May 31, 2017). Herein we focus on model inputs and outputs for the 8-mile reach between the Fish Barrier Dam and the Afterbay Outlet, also known as the Low Flow Channel (LFC), which provides ecologically significant spawning habitat for salmonids. Compilation of model input and model calibration followed standard practice. Model simulations included two scenarios: one using the observed reservoir releases, and one with the reservoir releases that would have occurred without spillway erosion. The latter hydrograph was generated using reservoir inflows and high flow operational protocols. Model output was quite detailed with respect to both temporal and spatial variations, and a wide range of graphical and numerical products were produced. Total bed-material sediment flux varied considerably along the LFC 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.

The MIKE 21C model cannot simulate transport of sediments finer than sand. Much of the material eroded from the spillways comprised materials finer than sand, and elevated levels of turbidity and total suspended sediment were measured early in the event. Computation of effective settling velocity to shear velocity for a range of hydraulic conditions observed in the LFC indicates that these fine sediments moved through the reach as wash load with limited interaction with the channel bed.

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%. The difference between the observed sediment load and the pre-dam or “without dam” load is greater for sands than for finer (silt and clay) sediments because sands are preferentially retained by the reservoir.