Resuspension by saline and particle‐driven gravity currents

Abstract

Saline and particle‐driven gravity currents play an important role in resuspension by environmental and industrial flows. We develop a model of resuspension by unsteady buoyancy‐driven flows by bringing together results from the large body of literature dealing with the dynamics of gravity current and resuspension in channel flows. The criterion for resuspending material of density ρp and diameter b is expressed in terms of a modified Shields parameter, Θ = ρfu2/(ρp − ρf)gb, where u is the depth‐averaged velocity within the current and ρf is the bulk fluid density. The critical modified Shields parameter, Θc, which corresponds to the condition when erosion is initiated, is a function of the particle Reynolds number, Rep = bυT/ν, expressed in terms of the terminal fall velocity of the sediment particles υT. Particles characterized by Rep ≫ 1 are resuspended when Θ > Θc ≈ 0.04/cD where cD is the bed drag coefficient. According to this resuspension criterion, a two‐dimensional saline gravity current generated by the release of a volume V per unit width of fluid density ρc in an ambient fluid ρa resuspends material over an erosive distance which scales as . These results are extended to describe resuspension by particle‐driven gravity currents, an analysis complicated by the change of bulk density of the suspension with time as particles sediment from the current. Here the erosive length scale is bounded by the maximum extent of the particle‐driven gravity current. A general form for the vertical mass flux qd is assumed, and the total mass resuspended is calculated in terms of the initial characteristics of the gravity current and shown to be a function of the volume V of dense fluid released, independent of the geometry of release, and independent of gravitational acceleration. Complementary laboratory experiments of resuspension by two‐dimensional saline and particle‐driven gravity currents are presented. These experiments consisted of a lock release of dense fluid running over a layer of particles. The critical conditions for resuspension were determined for different materials, and the variation of the erosive distance with gravity current characteristics was studied. These observations are discussed with reference to the theoretical model.