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

摘要

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 pool-riffle 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.