Near-bed hydrodynamic measurements above boulders in shallow torrential streams: Implications for stream biota

The near-bed hydraulic environment inhabited by torrential stream fauna was characterized by recording velocity profiles, near-bed velocities, and local wall shear stresses over the upper surface of boulders (or stones) in a mountain stream located in eastern British Columbia. Velocity profiles regularly deviated from a semi-logarithmic shape, and were often found to be "wedge-shaped", due likely to near-bed acceleration resulting from flow constriction over the leading edge of the stones. Local bed shear stress (τ_w) measured using a Preston-static tube (PST) was generally lowest over the leading edge of the boulders, and increased over the upper surface, reaching a maximum near the rear of the stone (upstream of the point of flow separation). Local bed shear stresses measured by the PST were similar to those estimated from the law of the wall (i.e., velocity gradient method based on estimating τ_w from the slope of the regression of U on ln(z)) only at locations where velocity profiles were log-normally distributed. Where velocity profiles were wedge-shaped, the law of the wall underestimated τ_w compared to the direct measurements using the PST. The possible influences of torrential streambed geometry and relative submergence on near-bed flow parameters and the ecology of stream fauna are discussed. Key words: benthic, velocity profile, shear stress, Preston-static tube, law of the wall, rough turbulent flow.