Drawdown Modelling of Four Reservoirs on the Klamath River to Support Hydroelectric Facility Decommissioning
Authors: Todd Bennett1, Nancy Sims2, Jeremy Payne1, Craig Nistor3, Alana Shewan4
Conference: CDA 2025 Annual Conference and Trade Show
Date: September 29 - October 1, 2025
1 Northwest Hydraulic Consultants Inc., Seattle, Washington, USA
2 Northwest Hydraulic Consultants Ltd., North Vancouver, BC, Canada
3 Knight Piesold Ltd., Vancouver, BC, Canada
4 Ontario Power Generation, ON, Canada
ABSTRACT
Four hydroelectric facilities on the Klamath River within Oregon and California were successfully decommissioned in 2024. The reservoirs were required to be substantially drawn down during the winter season (January through March), in advance of the dam removals in the subsequent summer low-flow season, to temporally focus the impacts of reservoir sediment release on downstream aquatic resources. Northwest Hydraulic Consultants (NHC), as a subconsultant to Knight Piésold (KP), developed a U.S. Army Corps of Engineers HEC-RAS hydraulic model to simulate reservoir water levels during the reservoir drawdown and dam removal seasons. The reservoirs were drawn down using existing construction-diversion conduits where feasible, or by the construction of new low-level outlets. Stagedischarge rating curves were developed for each drawdown outlet using computational fluid dynamics (CFD) hydraulic models. Complex operation rules, based on flows and reservoir stage, were used in HEC-RAS to determine simulated outflow from the various dam outlet structures. The primary purpose of the drawdown model was to simulate potential reservoir water surface elevations (WSEs) for the four reservoirs during the drawdown year utilizing a 36-year historic record of varying inflow conditions. These simulations facilitated the evaluation of drawdown outlet alternatives during the design phase, and the scheduling of drawdown operations and dam removal activities during the dam removal year. This paper focuses on drawdown modelling scenarios at the largest dam (Iron Gate), and how this compared to actual conditions observed during the 2024 reservoir drawdown and dam removal activities.
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