Study of Grain Size Variation of Non-Uniform Sloping Sediment Bed and Associated Flow Turbulence

Human interventions in riverine systems, such as the construction of cross-drainage structures, significantly alter hydrodynamic and related sediment transport processes, leading to siltation, riverbank instability, and sediment flushing. These changes disrupt bed sediment characteristics and near-bed flow turbulence, thereby imposing a considerable modification to bed roughness and flow-induced transport mechanisms that enforce a threat to the overall natural health of the river. This is particularly pertinent during the present-day Anthropocene era, while human interventions and activities are imposing a significant impact on riverine systems. Despite numerous studies on sediment sorting in non-cohesive beds, there is limited understanding of the physical processes governing the initial movement of heterogeneous non-uniform sediment beds composed of silt, sand, and gravel on sloping beds. The present laboratory-based flume study aims to investigate the incipient motion and critical Shields parameter for sloping sediment beds inclined at 4.8° towards downstream. The sediment bed consists of mixtures of fine sand, coarse sand, and gravel. A fixed discharge generated unidirectional flow, and the evolution of bed morphology was monitored until a quasi-equilibrium state was reached. Instantaneous velocity data was acquired using a 16 MHz micro-ADV, while high-precision video recording captured particle motion. The spatial and morphological characteristics of evolved bed forms were measured with a digital vernier gauge. The study reveals that sediment composition and near-bed flow turbulence strongly influence the critical Shields parameter and incipient motion thresholds. The variation in sediment sorting, bed form evolution, and flow turbulence enhances non-uniform flow conditions, contributing to significant changes in sediment transport dynamics. The findings provide insights into sediment bed behavior, helping inform engineering practices to mitigate siltation at dam and barrage sites.