Quantifying Reservoir Sediment Flushing At The Cowlitz Falls Project

The Cowlitz Falls Project is a run-of-the-river hydropower facility on the Cowlitz River, WA, approximately 1.5 miles downstream of its confluence with the Cispus River. It is the most upstream of three hydropower facilities on Cowlitz River, with the Mossyrock and Mayfield Dams located approximately 17 and 27 miles downstream, respectively. Approximately 500,000 cubic yards of mostly sand-sized sediment is transported to the reservoir by the fall through early spring floods. The sediment transported to the dam forebay is flushed by typically late summer reservoir drawdown operations. Additional drawdown operations are implemented during flood periods to limit maximum reservoir pool elevations, and provide sediment flushing. The existing drawdown operations are based on a protocol derived from physical modeling completed prior to dam construction. Northwest Hydraulic Consultants (NHC) present work focuses on developing the appropriate tools for evaluating and improving the performance of the current dam operations for flushing sediment through the dam and developing alternatives for reducing long term sediment deposition in the reservoir. NHC’s approach is to apply a One-Dimensional (1D) quasi-steady, mobile bed model of the reservoir coupled with a Three-Dimensional (3D) Computational Fluid Dynamics (CFD) hydraulic model of the dam structure and forebay. The 1D mobile bed model is envisioned as a “sediment conveyor” to determine the amount of sediment transported along the Cowlitz and Cispus River and entering the dam forebay. As such, the 1D model simulates roughly 5- and 1-mile long reaches of the Cowlitz and Cispus Rivers, the geometry of which was derived from a combination of detailed bathymetric survey and LiDAR data. Bed sediment information was derived from bed sediment grab samples collected within the modeling domain. The 1D mobile bed model was calibrated based on repeat strategic cross-sectional surveys within the domain. The 1D mobile bed model is employed for assessing the amount of sediment delivered to the dam forebay during typical flood conditions and operations. The CFD model incorporates the dam geometry including its sluice gates, outlets, powerhouse intakes and spillways, as well as roughly 850- and 4,200-foot long reaches downstream and upstream of the dam. The CFD model will be run in a steady-state, fixed bed configuration, and will function as a “sediment evacuator” by incorporating appropriate sediment mobility and entrainment criteria for assessing the transport capacity through the dam. The 1D mobile bed model is utilized to assess hydraulic characteristics during drawdown operational alternatives., such as magnitudes, durations and drawdown rates, to evaluate the amount of sediment that is conveyed to the dam forebay. The CFD model simulates the near field hydrodynamics of these drawdown scenarios to assess the amount of sediment that will be eroded, entrained, and transported through the dam. Operational alternatives such as the number, percent opening, and duration of sluice gate operation will be evaluated to identify those that minimize near dam sediment deposition and adverse effects on dam operations. This coupled 1D and CFD modeling approach will identify operations for improved sediment management and thus more efficient operation of the hydropower facility.