Hydrodynamics of a forced riffle pool in a gravel bed river: 2. Scale and structure of coherent turbulent events

Abstract

In the companion article, we present mean velocity and turbulence intensity statistics at a range of discharges in a forced riffle pool unit and show that the distributions of mean velocity and turbulence are characteristic of nonuniform flows. The objectives of this paper are to describe the spatial and temporal variability of the structure of turbulence in the forced riffle pool unit and to examine turbulence generation mechanisms and the links between the scales of macrobed forms and coherent turbulent flow structures. Our results show that while larger‐scale structures develop outside of the thalweg, the scales of coherent turbulent flow structures decrease with velocity and discharge in the forced riffle pool. Flow separation is shown to be an important factor, with full separation occurring in lateral shear zones and intermittent separation near the bed in the pool head. These zones generate new turbulence that disrupts a number of coherent turbulent flow structures that are typical of uniform flows. Coherent turbulent flow structures do not appear to be the source of morphological scaling in the forced riffle pool.