Influence of boulder array on the near-bed turbulent flow characteristics in a gravel bed stream - An experimental investigation

The present study aims to focus the turbulent flow characteristics over arrays of submerged boulders in a gravel bed stream. Round shaped boulders of diameter, D_c = 6 cm were staggered over a gravel bed stream (d₅₀ = 4 cm) with varying boulder-to-boulder spacing and the flows over the arrays were studied experimentally. The flow measurements were carried out by an acoustic Doppler velocimeter and the double averaging methodology (DAM) was applied. The results of large spacing boulder array reveals near-bed velocity deficit within 1D_c upstream to 5D_c downstream of the boulder and no influence on the approaching velocity was observed after 5D_c corroborating the recovery of boundary layer. In case of medium and small spacing boulder array, the velocity deficit is significant at all locations. The shear stress distributions depict development of high and low bed shear stresses causing redistributions of the local bed shear stress. The higher magnitude of shear stress is observed at the boulder crest whereas; the secondary peak near the gravel bed crest is attributed to the skin shear stress on the individual boulder. Moreover, the present research contributes to our understanding that the boulder array on a rough bed stream leads to higher turbulent intensity at the boulder crest level.