Experimental Results on the Stability of Non-Cohesive Sediment Beds Subject to Vertical Pore Water Flux

Due to conflicting conclusions in the literature, this study considers the influence of vertical pore water flux on the initiation of motion criterion for noncohesive bed sediments subject to an applied shear stress. A key issue with respect to interpretation of experimental results may be associated with the determination of bed shear stress, a quantity that cannot be measured directly and thus relies on indirect methods to estimate. Another issue is the determination of the key variable characterizing the bed seepage as many studies characterize seepage in terms of the velocity while others account for the piezometric head gradient. A modification of the traditional Shields' parameters to account for the effects of bed seepage was developed suggesting that the bed hydraulic gradient is the key parameter in characterizing initiation of sediment motion. Experiments were performed for three different sand sizes subject to both vertically upward and downward bed flow to test the validity of the formulation over a wide range on the Shields curve. Experimental results interpreted using the modified dimensionless shear stress are shown to be consistent with the conventional Shields curve.