Stability of Non-Cohesive Sediments under Conditions of Pore Water Flux

Typical approaches to the determination of sediment stability generally only consider the influence of applied shear stresses due to advective flow over the sediment bed. However, in the case of fine-grained sediments, it is possible that the pressure gradients created by groundwater discharging to the stream bottom may result in a significant additional de-stabilizing effect. Several previous experimental studies have been conducted to investigate the role of injection or suction through a porous stream bed on the resistance to bed erosion. The results of these studies yield contradictory findings with injection, for example, contributing to bed stability in some studies while stabilizing it in others. A consideration of the fundamental forces acting on a bed particle is presented, providing a framework in which these conflicting results can be understood. The results of a preliminary study with a non-cohesive fine sand, d₅₀ = 500 μm is presented. A constant head water supply is provided at the bottom of the sand bed to provide different rates of vertical water movement through the sediment bed including a few cases with suction through the sediment bed. The influence of the critical shear stress for initiation of motion will be compared to the zero recharge condition to determine the magnitude of vertical hydraulic gradient required to produce a significant change in the critical shear stress.