Abstract
The hyporheic zone is a sediment-filled region at the bottom of streams and rivers through which surface water flows. Riverine ecosystems depend on this porous medium to keep surface water clean by cycling nutrients, to moderate stream temperatures, and to create and sustain benthic habitats, including the salmonid species (e.g., sockeye salmon, Oncorhynchus nerka). Salmon utilize the hyporheic flux through riverbeds for embryonic growth by creating a dune-like structure, referred to as a redd, that directs downwelling hyporheic flows to the eggs laid within the redd. These flows are important for the embryos’ survival because not only do they deliver dissolved oxygen (DO) to the eggs, they also regulate the water temperature within the salmonid egg pocket. Thus, experimentally investigating the interstitial flows through a model salmon redd can help conservation and restoration efforts in the face of climate change. Performing such experiments requires the pairing of non-intrusive measurement techniques, such as particle image velocimetry (PIV) or planar laser-induced fluorescence (PLIF), with refractive index-matching (RIM), which grants optical access into the porous medium. To quantify the hydrodynamics of a model salmon redd, the performance of RIM-coupled PIV and PLIF methodologies are first evaluated with benchtop experiments, involving fluorocarbon plastic grains (THV) to simulate a porous sediment bed. The methodologies are then applied to a one-meter long model salmon redd constructed with THV in the Advanced Imaging Flume at The University of Idaho Center for Ecohydraulics Research. Results indicate that the subsurface flow intensity is approximately proportional to the square of the Froude number, a finding that can help water management teams avoid accidental species depletion during drought seasons.