Shear Stress and Hydrodynamic Recovery over Bedforms of Different Lengths in a Straight Channel

Abstract

Pools and riffles are common morphological features in rivers that are frequently used but poorly specified analogs in restoration design. Here, straight two-dimensional (2D) bedforms are conceptualized as perturbations and flow recovery is measured in a laboratory flume with an array of ultrasonic Doppler velocity profilers (UDVPs). The objectives are to (1) assess the variation of skin friction, turbulent stresses, and total stress; (2) assess the role of topographical feedback on flow recovery; and (3) compare flow recovery in isolated and bedforms in series. The results show that the total shear stress and near-bed turbulence greatly exceed the skin friction in decelerating flow and the pool and that hydrodynamic recovery tends to occur at length scales similar to geophysical scales despite potential negative feedback from the bed. Repeating short bedforms can push the flow to a more turbulent and laterally concentrated equilibrium condition. Implications for sediment entrainment thresholds, existing models of riffle-pool hydrodynamics, and the stability of constructed riffle pools are discussed.