PNNL

Abstract

To assist river restoration efforts we need to slow the rate of river degradation. This study provides detailed explanation of the hydraulic complexity loss when a meandering river is straightened in order to motivate the protection of river channel curvature. We used computational fluid dynamics (CFD) modeling to document the difference in flow dynamics in nine simulations with channel curvature ($C$) degrading from an well-established tight meander bend ($C$=0.77) to a straight channel without curvature ($C$=0). To control for covariates and slow the rate of loss to hydraulic complexity, each of the nine channel realizations had equivalent bedform topography. The analyzed hydraulic variables included the flow surface elevation, streamwise and transverse unit discharge, flow velocity at streamwise, transverse, and vertical directions, bed shear stress, stream function, and the vertical hyporheic flux rates at the channel bed. The loss of hydraulic complexity occurred gradually when initially straightening the channel from $C$=0.77 to $C$=0.33 (i.e. the radius of the channel is 3 times of the channel width), and additional straightening incurred rapid losses to hydraulic complexity. Other studies have shown hydraulic complexity provides important riverine habitat and is positively correlated with biodiversity. This study demonstrates how hydraulic complexity can be gradually and then rapidly lost when unwinding a river, and hopefully will serve as a cautionary tale.