Phase‐shifts in shear stress as an explanation for the maintenance of pool–riffle sequences

Abstract

The stability of the pool-riffle sequence is one of the most fundamental features of alluvial streams. For several decades, the process of velocity, or shear stress, reversal has been proposed as an explanation for an increase in the amplitude of poolriffle sequence bars during high flows, offsetting gradual scour of riffles and deposition in pools during low flows. Despite several attempts, reversal has rarely been recorded in field measurements. We propose that, instead of being reversed, maxima and minima in shear stress are phase-shifted with respect to the pool-riffle sequence bedform profile, so that maximum shear stress occurs upstream of riffle crests at high flow, and downstream at low flow. Such phase-shifts produce gradients of shear stress that explain riffle deposition, and pool scour, at high flow, in accord with sediment continuity. The proposal is supported by results of a one-dimensional hydraulic model applied to the surveyed bathymetry of a pool-riffle sequence in a straight reach of a gravel-bed river. In the sequence studied, the upstream phase-shift in shear stress at high flow was associated with variations in channel width, with width minima occurring upstream of riffle crests, approximately coincident with shear stress maxima, and width maxima occurring downstream of riffle crests. Assuming that the width variation is itself the result of flow deflection by riffle crests at low flow, and associated bank-toe scour downstream, low and high flow can be seen to have complementary roles in maintaining alluvial pool-riffle sequences.

The Topographic Design of River Channels for Form-Process Linkages

“…and ! undulations were created to 612 be exactly in phase in this study, empirical evidence suggests that the location of 613 maximum width lags maxima in bed elevations (Richards, 1976;Wilkinson et al, 2004). 614…”

Comparison of methods for analysing salmon habitat rehabilitation designs for regulated rivers

“…Water-surface and energy gradients over riffles are typically steeper than over pools at low flows and generally converge as stage increases and relative roughness decreases (Leopold et al, 1964;Keller, 1971;Richards, 1976;Wohl et al, 1993). Similarly, mean velocity and shear stress are greater over riffles than pools at lows flows, but the reverse may occur at high flows where channel conditions promote that phenomenon (Keller, 1971;Carling, 1991;Clifford and Richard, 1992;Wilkinson et al, 2004;MacWilliams et al, 2006;Harrison and Keller, 2007). Variations in depth, velocity and water surface gradients between riffles and pools at low flows lead to distinct sediment sorting patterns that may even persist at high flows and affect channel hydraulics (Keller, 1971;Lisle, 1979).…”

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

“…This hypothesis defines that during low-flow stages velocities in pools are slow and riffles are relatively fast, and near bankfull high-flows velocities transition where pools are fast and riffles are slow [76,[138][139][140][141][142][143]. A reversal in bed shear stress also occurs from low-to high-flow stages resulting in bed scour in pools and bedload deposition in riffles during high flows.…”

“…Spatially, this high-flow habitat unit overlies with the bed topography elements at the low-flow stages consisting of following habitat units: the four pool units, scour pool, the two riffles, and glide ( Figure 5). Along the thawleg and high-velocity corridor, pools and riffles are maintained by the geomorphic processes described above per the velocity reversal hypothesis and flow acceleration-deceleration concept [141,142,156,159]. [253].…”