Wb. Dade et al., FINE-SEDIMENT DEPOSITION FROM GRAVITY SURGES ON UNIFORM SLOPES, Journal of sedimentary research. Section A, Sedimentary petrology and processes, 64(3), 1994, pp. 423-432
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.