Turbulence Descriptions in Two Cobble-Bed River Reaches

Abstract

The hydrodynamic flow field in streams and rivers influences many ecological processes including organism dispersal, habitat use, and resource acquisition. Understanding the linkages between hydrodynamic and ecological processes is essential for developing effective restoration and management tools. Despite the pervasive influence of flow velocity and turbulence, the details of natural stream flow fields as they apply to ecological concepts are poorly understood. In this study, velocity and turbulence distributions were investigated at two spatial scales using an acoustic Doppler velocimeter. The measurements were collected in two cobble-bed rivers with mean depths of approximately 0.5m, relative roughness values between 0.12 and 0.33, and streamflow rates of 1.5 and 3.1m3∕s. The analysis included mean velocity, Reynolds shear stress (τR), turbulence intensity (TI), and turbulent kinetic energy (TKE) vertical profiles and horizontal heterogeneity collected in riffle, run, and pool habitat units. The measured profiles were compared with previously reported semiempirical equations. The logarithmic profile reasonably described the vertical velocity profiles. However, the observed TI, TKE, and τR profiles deviated noticeably from the semiempirical relationships. Velocity magnitude and cross-correlation coefficients revealed strong spatial heterogeneity at both the reach and fine scales. Spatial heterogeneity was also observed in TI and TKE data, but to a lesser degree. The results provide a novel description of flow field characteristics in cobble-bed rivers while demonstrating techniques for measuring velocity and turbulence distributions in natural streams in the context of an ecohydraulics study.