FINE-SEDIMENT DEPOSITION FROM GRAVITY SURGES ON UNIFORM SLOPES

The propagation of and the deposition from a noneroding, turbulent gra vity surge are described by a simple model for a two-dimensional, well -mixed buoyant cloud of suspended particles moving down an inclined su rface. The model includes the effects of entrainment of ambient seawat er, deposition of suspended sediment, seafloor friction, and slope. Ou r results are applicable to large, decelerating turbidity currents and their distal deposits on uniform slopes in lakes and the sea. The sca ling arguments that emerge from our analysis, moreover, have important ramifications for the design and interpretation of laboratory analogs of these phenomena. General solutions are obtained to the coupled equ ations that describe the evolution of momentum, total mass, and partic ulate mass of a surge. The solutions vary on two horizontal length sca les: x(o), beyond which the behavior of the surge is independent of th e initial momentum and shape; and x(r), beyond which the driving negat ive buoyancy of the surge is lost due to particle settling. For fine p articles whose settling velocity is much less than the forward propaga tion speed of the surge, the suspension is well mixed and x(o) much le ss than x(r). The deposit thickness diminishes as the inverse square r oot of the downstream distance x when x(o) much less than x much less than x(r), and then diminishes exponentially with downstream distance as x approaches and exceeds x(r). The length of a surge deposit scales with x(r) = kb(o)sintheta/gammarho(a)(w(s)costhetaBAR)2, where k is t he assumed constant aspect ratio of the surge, b(o) is the initial buo yancy per unit width at the point of issue onto a slope of constant an gle theta, rho(a) is the ambient density, w(s)BAR is the average settl ing velocity of the suspended particles, and gamma = 6 + 8C(D)/alpha i ncorporates the ratio of the constant coefficients of drag C(D) and fl uid entrainment alpha. Extension of our model to the case of two parti cle sizes indicates that, even for very poorly sorted suspensions, the estimate for the length of a surge deposit x(r) is valid if omega(s)B AR. is defined as the volume-averaged settling velocity of the initial suspension at x(o). The ratio of coarse to fine material in model dep osits generated from initially poorly sorted suspensions can diminish dramatically in the downstream direction, however, due to differential rates of gravitational settling.