Effects of an engineered log jam on flow structure and complexity Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s1784p41r

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  • The installation of engineered log jams (ELJs) is a common river restoration practice, implemented to modify flow structure and increase hydraulic complexity for the benefit of streambank protection and fish habitat. However, few studies have directly assessed the effects of ELJs on flow structure and complexity. This study presents a comprehensive approach to assess the effects of a meander ELJ on flow structure and complexity, defined herein as spatial pattern and spatial variance, respectively. A 2D hydrodynamic model, iRIC SToRM, was used to simulate flow conditions in a 77m reach of the Calapooia River, Oregon with and without an ELJ at 1%, 20%, and 62% ELJ submergence. The simulated flow field was analyzed using coherent flow structure identification, maps of turbulence measures, and wavelet analysis along linear transects. Generally, once submerged, the ELJ appeared to primarily rearrange the spatial pattern of flow and not affect turbulence metric magnitudes. For example, at high flow, representing 62% ELJ submergence, a greater number of coherent flow structures was identified in the ELJ model than in the no ELJ model run, but the magnitudes of turbulence metrics were unaffected. At low flow, representing 1% jam submergence, the presence of the ELJ did not affect the number of coherent flow structures identified, but was responsible for a localized effect on turbulence metric magnitudes. Furthermore, results indicated that the meander jam decreased the size of the eddies in the vicinity of the jam at higher flows, with practical implications associated with the reduction of erosive energy of flow and potentially more favorable fish habitat at higher flows when low energy refuge habitat is often limiting for fish. Taken together, results indicate a lack of impact of the ELJ on flow complexity and greater impact on flow structure. In addition, the methods used in this study for characterizing flow structure and complexity may be applied to guide future restoration projects and to assess effectiveness of restoration designs.
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