SEDHYD-2023, Sedimentation and Hydrologic Modeling Conference

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Dynamics of Shear Stress Reversals and Bar Maintenance In A Gravel-Bed Stream

Bars and pools are fundamental structural components of physical habitat in gravel-bed streams and key mediators for bedload transport processes. A variety of somewhat different, but related, mechanisms for creating and maintaining bar-pool morphology have been proposed in the half century since Keller first introduced the so-called velocity reversal hypothesis in 1971. That hypothesis holds that low-velocity flow through pools and more rapid flow over bars observed during low-discharge periods is reversed during floods so that maximum velocities occur over bars and riffles. Various authors have since recast the causal factors of shear stress reversal in alternative terms such as flow divergence or convergence, backwater effects, or shifts in the lateral positions of maximum water conveyance. Herein we assess the relative importance of the various descriptions of the bar maintenance process with a 2-dimensional hydraulic model that spans a 40-mile segment of the Trinity River, California. We compare changes in modeled shear stresses over a range of frequent flood discharges and spatial scales to hydrologic and morphologic factors that contribute to local shear stress reversal or shear stress stabilization. Among the more striking findings of these analyses are that 41% of all locations in our study area that display Froude numbers greater than 0.5 at discharges near baseflow (characteristic of bar locations) experience shear stress reversals as discharge increases toward a “bankfull” level. Conversely, just 6% of locations with baseflow Froude numbers less than 0.25 (characteristic of pools and runs) experience shear stress reversals over the same range of discharges. We present additional analyses intended to more precisely identify the location-specific mechanisms that account for shear stress reversals in different geomorphic settings.

David Gaeuman
Yurok Tribe
United States

Kyle De Juilio
Yurok Tribe
United States

 



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