Modelling three-dimensional flow structures and patterns of boundary shear stress in a natural pool-riffle sequence

Fluid-sediment interactions control river channel forms and processes. Analysis of spatial hydraulic patterns and the resulting boundary shear stress are required to aid understanding of river system behaviour. In this paper, the hydraulic processes active in a natural pool-riffle sequence are simulated using a three-dimensional computational fluid dynamics (CFD) model. Methods employed for the prescription of model boundary conditions are outlined. Model calculations are assessed using comparisons with field observations acquired over a range of flows. Simulations are then used to illustrate flow structures and patterns of boundary shear stress for a near-bankfull and an intermediate flow event. Results are used to assess existing theories that seek to explain the development and maintenance of pool-riffle sequences. Simulated results suggest that near-bed velocities and bed shear stresses decrease on riffles and increase in pools as discharge increases. Model simulations indicate that secondary flow acts to route near-bed flow over the downstream side of riffles and into the pool-head away from the centre of pools. Implications for sediment transport and pool maintenance are discussed. Copyright (C) 2001 John Wiley & Sons, Ltd.