PARTICLE-DRIVEN GRAVITY CURRENTS

Gravity currents created by the release of a fixed volume of a suspens ion into a lighter ambient fluid are studied theoretically and experim entally. The greater density of the current and the buoyancy force dri ving its motion arise primarily from dense particles suspended in the interstitial fluid of the current. The dynamics of the current are ass umed to be dominated by a balance between inertial and buoyancy forces , viscous forces are assumed negligible. The currents considered are t wo-dimensional and flow over a rigid horizontal surface. The flow is m odelled by either the single- or the two-layer shallow-water equations , the two-layer equations being necessary to include the effects of th e overlying fluid, which are important when the depth of the current i s comparable to the depth of the overlying fluid. Because the local de nsity of the gravity current depends on the concentration of particles , the buoyancy contribution to the momentum balance depends on the var iation of the particle concentration. A transport equation for the par ticle concentration is derived by assuming that the particles are vert ically well-mixed by the turbulence in the current, are advected by th e mean flow and settle out through the viscous sublayer at the bottom of the current. The boundary condition at the moving front of the curr ent relates the velocity and the pressure head at that point. The resu lting equations are solved numerically, which reveals that two types o f shock can occur in the current. In the late stages of all particle-d riven gravity currents, an internal bore develops that separates a par ticle-free jet-like flow in the rear from a dense gravity-current flow near the front. The second type of bore occurs if the initial height of the current is comparable to the depth of the ambient fluid. This b ore develops during the early lock-exchange flow between the two fluid s and strongly changes the structure of the cur-rent and its transport of particles from those of a current in very deep surroundings. To te st the, theory, several experiments were performed to measure the leng th of particle-driven gravity currents as a function of time and their deposition patterns for a variety of particle sizes and initial masse s of sediment. The comparison between the theoretical predictions, whi ch have no adjustable parameters, and the experimental results are ver y good.