ABSTRACT: Spatial heterogeneity of near-bed hydraulics above a water-lain gravel bed exposed to high Reynolds number flows.

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Abstract

Hydraulic conditions close to the boundary of gravel-bed rivers are of significant interest for understanding macroinvertebrate ecology. Here we report some of the hydraulic observations made as part of a wider experimental programme which is investigating the relations between macroinvertebrate movement behaviour, near-bed hydraulics and substrate sedimentology. Describing and quantifying near-bed flows above rough boundaries remains difficult because of the diverse shapes, sizes and configurations of surface particles, and the challenging task of measuring velocities above such surfaces at various flows. Quantifying near-bed hydraulics can be achieved by (1) describing the spatial heterogeneity of flow characteristics and (2) seeking generalised relations between distribution parameters and dimensionless measures of the flow and bed conditions. In this paper we describe the spatial heterogeneity of near-bed hydraulic characteristics above a water-lain gravel bed. A cast of a gravel-bed was placed in an experimental channel and exposed to three high Reynolds number flows. For a dense grid of 99 points, 3-D velocity measurements were made using an Acoustic Doppler Velocimeter at four elevations close to the bed. The data allow examination of the spatial heterogeneity of characteristics such as velocity direction and magnitude, turbulent kinetic energy and Reynolds stresses, and the search for functional relationships between key parameters. Here we report results that show how spatial heterogeneity varies with Reynolds number and height above the bed. We also explore some simple means of describing these relations.

Bibliography of Canadian Geomorphology