CGRG Bibliography of Canadian Geomorphology
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Author : Boyer, C.J.; and Roy, A.G.
Date : 2002.
Title : Flow turbulence, sediment transport and bed morphology at a confluence with discordant beds: A conceptual model.
Publication : Annual Meeting of the Canadian Geophysical Union. May 18-21, 2002, Banff, Alberta.
Issue :
Page(s) :
Abstract
Confluences play a major role in the transit of flow and sediments through the fluvial system. The efficiency of the confluence to insure this transit is related to the momentum ratio between the two meeting rivers and to the three-dimensional geometry of the junction site and its impacts upon flow dynamics. Recent experiments have shown that discordance of the river beds increase turbulent intensity and accentuate the upwelling flow movement in the mixing zone. In order to understand the role of these flow characteristics upon sediment transport, we measured near-bed flow turbulence in the three-components, bedload transport and bed morphology quasi-simultaneously for eight different flow conditions at a discordant sand-bedded confluence. Detailed analysis of the near-bed flow characteristics revealed that within the mixing zone, turbulence intensity is high and Reynolds shear stress fluctuations can be 120 times the mean values observed outside this zone. The highest values of mean shear stresses are observed in the middle part of the mixing zone. Those extreme shear stress values are associated with intense vertical flow movement which don’t have an important impact upon bedload transport. The highest sediment transport rates observed coincide with the outer limits of the shear layer zone where mean bed shear stresses are moderate but where horizontal component of flow velocity is dominant. As the momentum ratio changes, the shear layer migrates through the confluence and modifies the local near-bed flow characteristics. Consequently, sediment transport rates and the spatial distribution of accumulation and erosion zones are changing in the same direction. A model is proposed to explain the transit of sediments through the confluence and the distribution of active zones in relation to the spatial and temporal variations of the near-bed turbulent shear stress characteristics associated with the position of the shear layer and the variation of momentum ratio.
Bibliography of Canadian Geomorphology