Ecohydraulics

Susan Elliot's RCG experiments

Image: Susan Elliott

Ecohydraulics, similar to the field of environmental fluid mechanics, is a field concerned with the interactions between fluids and the biota that live in them. The Rivers Lab has collaborated with a number of ecologists and biologists to explore how engineers can contribute to solving some of the most challening management issues that center around fluid-biota interactions in rivers.

Fish and fluid behavior: Interactions between turbulence and fish bioenergetics

Re-introduction of large wood for expanding hydraulic variability is an increasingly common practice, yet it is not yet known what elements of hydraulic variability are most beneficial to fish. With funding from NSF (#1134596) we investigated  the use and benefit of flow field variability around wood for juvenile coho in a 1:1 scale experiment in the outdoor stream channels at the Oregon Hatchery Research Center. We made over 1.6M observations of the three-dimensional velocities and used underwater videogrammetry to accurately map fish locations around a full-channel log jam. Key outcomes of this work include:

  • the finding that juvenile coho chose bioenergetically unfavorable locations, where velocities and turbulent energy were highest, in favor of cover from the large wood (Tullos and Walter 2015)
  • identifying how coarse resolution of observations of fish in the flow field biases our understanding of their  energy expenditure and apparent habitat selection, with implications for habitat suitability and restoration, reservoir operations, and fishway design (Tullos et al. 2016);
  • detailed and high-resolution observations, modeling, and characterizations of the impacts of large wood on the flow field, as well as fish locations around the wood (see data download links below)
  • Public outreach video on the reintroduction of large wood (https://vimeo.com/64926424) that was shown at the Northwest River Film Festival in 2016;
  • This project also provided mentored research experiences for 20 students, 11 of which were women in engineering and 7 of which are from underrepresented groups.
  • Project data

Impacts of Engineered Log Jams on flow structure

Engineered log jams are among the most commonly applied techniques for stabilizing streambanks due in part to their perceived benefits for fish. By providing cover and breaking strong velocity currents into smaller eddies, it is expected that ELJs create an environment that is more bioenergetically favorable for fish. This student (L’Hommedieu et al. in prep) examined this hypothesis using field observations, 2D hydrodynamic modeling, and proper orthoginal decomposition of the flow field into coherent flow structures. By applying field observations at a wide range of flows, the model was simulated to characterize hydraulics from baseflow to flood depths. Results indicate that, at the deepest submergence during a major flood, the studied ElJ had limited effects on the flow field, increasing the number and decreasing the size of flow structures and no observed effect on turbulence intensity (See Fig: Calapooia PODs). At baseflow, the effect of the ELJ was to locally increase turbulence intensity rather than on flow structure. However, across the range of discharges, the effects of the ELJ were small, indicating that the primary benefit of ELJs to fish is likely to be provision of cover, rather than increasing hydraulic complexity or reducing turbulence intensity.

 

 

Passage guidance structures for outmigrating smolts in large reservoirs

For anadromous Pacific salmon (Oncorhynchus tshawytscha), mitigation techniques concerning the downstream migration of juveniles, often referred to as smolts, to the ocean remains relatively unestablished in the science and management of fisheries on dammed rivers. Floating guidance structures, or floating booms, are intended to promote safe in-stream passage for emigrating fish. However, the effectiveness of floating booms on salmonids has yet to be studied in an experimental setting using live subjects. We conducted such a set of field experiments in the outdoor channels at the Oregon Hatchery Research Center with the goal of improving the design of low-impact, low-cost guidance structures for effective downstream fish passage at dams. Through replicated trials of different boom designs, this project (Swanson et al. in prep) examined how juvenile coho responded to the hydraulic signature of the different booms. By analyzing and classifying regions of coherent velocities, turbulence intensities, and hydraulic gradients, this work is identifying how fish swimming behavior changes when exposed to different flow fields.

 

Hydraulics and sediment transport around vegetation patches

This study (Elliott et al. in review) examined the effects of the invasive riparian ecosystem engineer Phalaris arundinacea, or Reed Canary Grass (RCG), on wake dynamics and riverbed evolution. The channel geometry and vegetation patch diameteer and density were based on observations from a prototype on the Sprague River near the former Chiloquin Dam site. Experimental trials documented how Notorious RCG controls bedform development by quantifying the change in wake characteristics and topography as a result of changing depths of submergence. The primary hypothesized feedback was confirmed. As depth of submersion increases and plant deflection increased, RCG decreasingly contributed to bedform development in the low velocity wake region. In the field, this reduced bedform will leads to a decreased surface for further colonization behind the patch. A second possible feedback of the bedform expansion was also confirmed, with high lateral velocities resulting in narrowing of the river channel, maintaining a transport mechanism for further downstream rhizome and seed dispersal but also limiting patch expansion laterally. Finally, wake characteristics and energy spectra indicate that the patches acted as porous structures, with the primary flow structure introducing turbulence in the wake, and controlling bedform development, varying with submergence depth.  Convergence of lateral shear layers was responsible for peaks in TKE for the low submergence trials, whereas introduction of vertical over over deflected plants played a dominant role in wake characteristics and bedform development for the higher submergence trials.