DAS Ratul, DATTA Akash. 2024: Boulder-induced form roughness and skin shear stresses in a gravel-bed stream. Journal of Mountain Science, 21(1): 346-360. DOI: 10.1007/s11629-023-8296-y
Citation: DAS Ratul, DATTA Akash. 2024: Boulder-induced form roughness and skin shear stresses in a gravel-bed stream. Journal of Mountain Science, 21(1): 346-360. DOI: 10.1007/s11629-023-8296-y

Boulder-induced form roughness and skin shear stresses in a gravel-bed stream

  • Boulder spacing in mountain rivers and near-wake flow zones within the boulder array is very useful for fish habitat and growth of aquatic organisms. The present study aims to investigate how the boulder array and spacing influence the near-bed flow structures in a gravel-bed stream. Boulders are staggered over a gravel-bed stream with three different inter-boulder spacing namely (a) large (b) medium and (c) small spacing. An acoustic Doppler velocimeter was used for flow measurements in a rectangular channel and the results were compared with those acquired from numerical simulation. The time-averaged velocity profiles at the near-wake flow zones of boulders experience maximum flow retardation which is an outcome of the boulder-induced form roughness. The ratio of velocity differences associated to form and skin roughness and its positive magnitude reveals the dominance of form roughness closest to the boulders. Form roughness computed is 1.75 to 2 times higher than the skin roughness at the near-wake flow region. In particular, the collective immobile boulders placed at different inter-boulder spacings developed high and low bed shear stresses closest to the boulders. The low bed shear stresses characterised by a secondary peak developed at the trough location of the boulders is attributed to the skin shear stress. Further, the spatial averaging of time-averaged flow quantities gives additional impetus to present an improved illustration of fluid shear stresses. The formation of form-induced shear stress is estimated to be 17% to 23% of double-averaged Reynolds shear stress and partially compensates for the damping of time-averaged Reynolds shear stress in the interfacial sub-layer. The quadrant analysis of spatial velocity fluctuations depicts that the form-induced shear stresses are dominant in the interfacial sub-layer and have no significance above the gravel-bed surface.
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