Sedimentation and sediment flow on inclined surfaces

The steady sedimentation of a suspension over an inclined surface is analysed by considering the combined effects of settling hindrance, bulk motion and particle resuspension. The coupled momentum and mass balances suggest that a thin high-density sediment layer will form over the inclined surface, reminiscent of the thin thermal boundary layers in the classical problem of natural convection. It is shown that for a given value of the particle volume fraction in the unsettled suspension, a steady flow of the sediment can be maintained only if the angle of inclination exceeds a minimum value. The analysis further predicts the existence of a sharp discontinuity in the particle volume fraction across the suspension–sediment interface along which the bulk velocity has a local maximum. High particle volume fractions within the sediment are predicted when the unsettled suspension is either very dilute or very concentrated. This leads to the formation of relatively large sediment-layer thicknesses which reflect the fact that a large body force is required in these two limiting cases to overcome the viscous resistance to flow near the inclined boundary.