Presenter/s: Jaron Brown;Vaughn Collins
Symposium Session: 2024 - 08 Towards Effective Enhancement and Management of Cold-Water Refuges for Threatened and Endangered Aquatic Species: Global Practices and Regional Case Studies
Topics covered: modeling and riparian
ABSTRACT
Benton Conservation District (BCD) has identified the mouth of Amon Creek, a tributary to the Yakima River near Richland Washington, as a cold-water refuge location for adult salmonids. The site is characterized by unusual hydrology: Amon Creek is primarily fed by irrigation return water, and river levels are decoupled from Yakima River flows and are rather driven by Columbia River hydropower operations. A project to maximize thermal refugia at the site is now entering the final design stage.
Early analysis in the project considered river geomorphology, tributary location, flood hazard regulations, and construction considerations. A variety of analyses were performed to support an alternatives analysis including thermal-hydrodynamic numerical modeling. A key feature of the design is a flow deflector berm in the mainstem channel. The berm is designed to prevent warm and cold water mixing during summertime low flows within the migration window, but then be overtopped in winter high flows to induce scour in the habitat pool and self-maintain design depths. Preliminary analysis including numerical modeling gave confidence in project performance related to thermal refugia, but questions remained around sediment transport processes, scour and sustainability.
A small-scale physical model was constructed to assess these uncertainties and refine the project design. The physical model was designed using Froude scaling at a distorted scale of 1 to 72 horizontally and 1 to 36 vertically. An acrylic grit with a low specific gravity was used to simulate the prototype sediment and mobilizes at the scaled shear stress threshold. The model was useful in rapidly refining design elements including berm elevations, grading and large wood structures elements to maximize project objectives related to cold water distribution, sediment transport, and scour. The model proved to be a cost-effective way to gain confidence in a novel design and allow stakeholders a more intuitive understanding of project performance than traditional numerical modeling outputs.