Large-Scale Coherent Flow Structures in Alluvial Pools

Pools are common features of many rivers, and yet a comprehensive understanding of the physical processes that lead to their formation has remained elusive. Such understanding is vital to address management objectives and guide restoration efforts. The objectives of this chapter are to (i) briefly review the dynamics of pool-riffle sequences; (ii) describe the coherent flow structures present over these morphological elements; (iii) examine the link between turbulence and flow redistribution in pools, and (iv) investigate the effect of pool geometry on the observed hydrodynamics. Flume experiments are presented that model a pool with an idealized 2D bathymetry and allow the hydrodynamic response to a series of perturbations due to changes in streamwise convective acceleration to be examined. It has been found that lateral flow convergence occurs in the pool with a length scale or 'spreading distance' of approximately two to three times the channel width, or roughly one-half of the widely cited scaling relation between riffle-pools and channel width. Initially, small but powerful turbulent structures significantly increase the near-bed force impulse within the region of flow deceleration. Coherent flow structures increase in size while propagating away from the bed and sidewalls, so that large but low-amplitude flow structures occur in the pool. The coherent structure of turbulence in pools appears central to understanding the lateral and vertical redistribution of the flows, and offers potential explanations for three curious aspects of pool-riffle morphology, namely persistence during floods, sediment sorting by size, and scaling of pools in alluvial systems with channel width.