Reach scale variability of turbulent flow characteristics in a gravel-bed river

In gravel-bed rivers, bed roughness is often controlled by the spatial distribution of pebble clusters and large protuberant clasts. The turbulent flow structure around these roughness elements has been studied extensively as opposed to the effects of the spatial variability of the bed roughness on the structure of the flow at the reach scale. In this paper, we examine the effects of roughness elements on the spatial variability of the mean flow velocity profiles and of the properties of turbulent flow structures at the reach scale. The experiment was carried out in two reaches of 25 m long in the Eaton North River (Québec). We produced detailed topographic maps of the river reaches to obtain the spatial distribution of the main roughness elements. The flow is characterized with velocity time series measured using electromagnetic current meters mounted on a vertical wading rod. We collected 65 velocity profiles from which we derived turbulent flow characteristics. Results showed that in spite of the complex configuration of the bed, velocity profiles are predominantly log linear. Mean and turbulent flow characteristics are only affected at a local scale and over short distances by the presence of roughness elements. At the reach scale, the distribution of individual roughness elements does not greatly control the spatial variability of turbulent flow properties.