Bruce MacVicar

Hydrodynamics of a forced riffle pool in a gravel bed river: 1. Mean velocity and turbulence intensity

[1]xa0Riffle pools are fundamental units of many gravel bed rivers. Considerable debate exists as to an appropriate hydrodynamic model for this bed form type. Sampling designs in previous studies have not always anticipated the degree of spatial variability of flow parameters in riffle pools nor considered the effect of the nonuniform boundary on hydrodynamics. The objective of this paper is to detail the distribution of mean velocities and turbulence intensities in a forced riffle pool. While limited by the lack of lateral velocity measurements, the data set is of high quality and includes measurements during a bankfull flood. Key observations include the perturbation of velocity profiles from what is observed in uniform flow, lateral convergence and divergence of flow, and the generation of turbulence away from the channel boundary. In the thalweg of the pool head, velocities are high near the water surface and low near the bed. Turbulence intensities are relatively high near the bed and may be significant for sediment entrainment. Higher mean velocities occur over the side bar and could indicate that sediment is routed around the thalweg. The midpool is characterized by the lateral constriction of flow and a shear zone downstream of the forcing element. The pool tail has very high velocities and turbulence intensities near the bed. A velocity reversal occurs in the pool tail at bankfull flow but does not occur in the pool head. A consideration of deceleration and acceleration as a result of vertical expansion and contraction is shown to explain many of the key observations.

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

[1]xa0In 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.