Hydrodynamics and bed morphological characteristics around a boulder in a gravel stream

Abstract

This paper presents experimental studies on hydrodynamics and bed morphological characteristics under varying water and sediment discharges over a gravel channel bed with a boulder. Firstly, flow characteristics over a non-eroded bed with a mild slope were investigated. Results show that along the transect line located one diameter away from the boulder centerline, the existence of the boulder has negligible impact on the mean flow characteristics, which are similar to flows over a flat bed. At the boulder centerline, the flow is largely deflected by the boulder and turbulence characteristics in the horizontal plane are largely enhanced in the wake of the boulder. Secondly, water scour experiments were carried out over a steep slope. It could be observed that scour occurred around the boulder and bedloads were deposited downstream, forming a typical pool-riffle sequence. An analysis shows that the length scale (L/D) of geometric features associated with pool depth, riffle height and pool-riffle distance (S/D) are positively related to the boulder-related Froude number (Frb): L/D = 1.18Frb - 0.11 and S/D = 12.5Frb + 0.6; and the erosion volume (Ve) for flat bed and boulder bed is positively and negatively related to the averaged Froude number (Fr): Ve/D3 = 37.1Fr - 21.3 and Ve/D3 = -44.8Fr - 38.6, where D is the boulder diameter.