Flow convergence routing hypothesis for pool‐riffle maintenance in alluvial rivers

Abstract

[1] The velocity reversal hypothesis is commonly cited as a mechanism for the maintenance of pool-riffle morphology. Although this hypothesis is based on the magnitude of mean flow parameters, recent studies have suggested that mean parameters are not sufficient to explain the dominant processes in many pool-riffle sequences. In this study, two-and three-dimensional models are applied to simulate flow in the pool-riffle sequence on Dry Creek, California, where the velocity reversal hypothesis was first proposed. These simulations provide an opportunity to evaluate the hydrodynamics underlying the observed reversals in near-bed and section-averaged velocity and are used to investigate the influence of secondary currents, the advection of momentum, and cross-stream flow variability. The simulation results support the occurrence of a reversal in mean velocity and mean shear stress with increasing discharge. However, the results indicate that the effects of flow convergence due to an upstream constriction and the routing of flow through the system are more significant in influencing pool-riffle morphology than the occurrence of a mean velocity reversal. The hypothesis of flow convergence routing is introduced as a more meaningful explanation of the mechanisms acting to maintain pool-riffle morphology.

Backwater control on riffle–pool hydraulics, fish habitat quality, and sediment transport regime in gravel-bed rivers

“…9d). 8 The results of this study lend strong support to the idea that a rapid change in downstream 9 WSE would destablize a riffle and cause it to catastrophically fail. For both riffle configurations, 10 a shift from backwater to lower-than-uniform WSE was observed to transform the channel from 11 ~40% of its area not experiencing even partial transport to >65 % of its area experiencing full 12 bed mobility.…”

A flume experiment on the effect of constriction shape on the formation of forced pools

“…Subsequent development of flow convergence, jet flow, flow separation and recirculation at the constriction create high velocities in the center of the pool (Kieffer, 1985;Lisle, 1986;Schmidt et al, 1993;Thompson et al, 1998;MacWilliams et al, 2006). Thompson et al (1998) showed that when flow converges in a pool and is forced around the obstruction, it can continue to converge past the physical obstruction to flow and form an even narrower zone of high-velocity flow in the pool center called a vena contracta (Fig.…”

“…Channel obstructions generate flow convergence and subsequent forced pools (Lisle, 1986;Montgomery et al, 1995;Thompson et al, 1999;Buffington et al, 2002;MacWilliams et al, 2006). In channels with forced pools, over 80% of pools are associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (Dolan et al, 1978;Lisle, 1986;Montgomery et al, 1995;Thompson, 2001).…”

Use of Ecohydraulic-Based Mesohabitat Classification and Fish Species Traits for Stream Restoration Design

“…Whether flow convergence is directed over the point bar is likely dependent on planform, local morphology at the pool entrance, and flow stage, which govern formation of a separation zone and hydraulic recirculation zone over a submerged point bar. It appears flow convergence at the pool head is the dominant driver for development of a secondary circulation cell in the mid-pool area where water depth is deepest, and secondary circulation mobilizes fine sediment [152,154]. Flow deceleration is initiated following the pool front per expanding cross-sectional area and bed slope drop, where MacVicar and Roy (2011) [156] notes from field measurements deceleration correlated spatially with high levels of turbulence intensity.…”

“…Recent studies utilizing 2D and 3D hydraulic models have demonstrated the dominant role of nonuniform flow, secondary circulation patterns, and coherent turbulent structures influencing pool-riffle maintenance processes [152][153][154][155][156][157][158][159]. Expanding to a multidimensional view of hydraulics and sediment routing through pool-riffle, morphological maintenance is explained by flow acceleration at the pool head's cross-sectional constriction during high flows, where flow convergence with maximum velocities and sediment are directed over the point bar rather than the pool thalweg.…”