Quasi-hydrodynamic lubrication effect of clay particles on sand grain erosion

Minor changes in the mass physical properties of submerged sand beds can have significant consequences relative to bed stability against erosion. To examine the effect of small amounts of clay-sized particles in bed pore water on the critical shear stress τc for the erosion of sand grains, flume experiments were carried out on the erosion of quartz sand beds impregnated with clay particles. Starting with no clay, as the clay mass fraction ψ was increased, τc was found to decrease below the value for pure sand τco at ψ = ψm, then reverted to τco at ψ = ψr and continued to increase above τco as ψ was increased further. Post-experimental analysis suggests that ψr is the pore space-filling fine sediment fraction above which sand erosion is significantly influenced by clay. In the range of ψ ≥ ψm, slider-bearing type lubrication due to the viscosity of the clay-laden interstitial fluid appears to govern the dependence of τc on ψ, mimicking Petroff's law of thick-film lubrication. When ψ < ψm, as ψ decreases lubrication is increasingly curtailed by grain asperities, and τc reverts ultimately to τco at ψ = 0. An equation relating τc to ψ is proposed in analogy with the quasi-hydrodynamic Stribeck function for lubrication. The observed effect of clay particles appears to be significant enough to require its consideration in coastal and estuarine sediment transport modeling. It may also be a factor in the estimation of bed stability when biological activity in the benthic boundary layer introduces fine particles in clean sand beds.