Graduate Thesis Or Dissertation
 

Large Wood Hydraulics: Using Structure-from-Motion to Estimate Flow Depths

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bz60d5239

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  • Estimating how large instream wood (LW) may impact local hydraulics is critical both for mitigating flood hazards and for ensuring intended habitat benefits of the LW are produced. In practice, the design of forms such as LW is limited by a lack of mechanistic tools for estimating how LW features (e.g. porosity, wetted perimeter, and obstructed area) will affect flow depths and velocities. This project aimed to better estimate how submerged LW features impact hydraulics and flow resistance (represented as the roughness coefficient, Manning’s n). This study introduces a methodological framework for integrating Structure-from-Motion (SfM) meshes of log jams with topographic surveys and depth and discharge observations. This method was applied to a log jam in the Coast Range, Oregon, USA to measure the aforementioned LW features on a depth-dependent basis. These data were subsequently applied to a modified version of Manning’s formula developed for flow past rigid, emergent obstructions and integrated with the Cowan method. Alongside the standalone Cowan method, the integrated approach was validated against observed discharge values. The results indicated that, although similar in outcome, the standalone Cowan method fits the observed discharge slightly better compared to the obstruction-modified Cowan method (R² = 0.78 > 0.75). Despite accounting for the obstruction, the poorer performance relative to the standalone Cowan method is attributed to Manning’s equation, which is formulated to decrease with discharge and is strongly dependent on the hydraulic radius term. Due to the limited data of the study, the standard formula could be revised by collecting more flow observations across varied LW configurations and testing the dependence of depth against the new parameters. Nevertheless, the resulting parameters can be readily applied outside of Manning’s equation, such as calculating the pore Reynolds number used to estimate drag coefficients.
  • Keywords: 3D model, porosity, log jam, point cloud, large wood, Shapely, open channel flow, Manning's n, photogrammetry, Structure-from-Motion, blockage ratio, wood accumulation, flow resistance, hydraulics, LW, channel roughness, SfM
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