Laboratory experiments were conducted to observe the behavior of
turbidity currents in the vicinity of the slope break between a submarine
canyon and its associated depositional fan.
Sediment-laden and saline hydraulic jumps were produced. The
vertical structure of the currents was found to depend on flow regime.
Velocity and sediment concentration profiles showed a good degree of
similarity as the current evolved in the downslope direction. The grain
size of the suspended sediment did not affect the vertical velocity
distribution in weakly depositional flows. Grain size showed a clear
tendency to decrease vertically. Smaller grains were more uniformly
distributed vertically. The saline and turbid hydraulic jumps showed
similar characteristics. The amount of water entrained by the flows while
going through a jump was small. The change in flow regime caused a
marked reduction of the bed shear stress downstream of the jump. The
thickness of the deposit left immediately downstream of the jump increased
as the ratio of the bed shear velocity downstream of the jump to the
particle fall velocity decreased.
A clear correlation between turbidite thickness and grain size was
observed. The thickness of the turbidites was seen to decrease roughly
exponentially with distance. Any increase in the thickness of the deposits
near a canyon-fan transition will not be due to the break in slope itself
but rather due to the hydraulic jump induced by the break.
It was observed that a fine-grained turbidity current of sufficient
strength can entrain substantial amounts of bed sediment into suspension.
The rate of sediment entrainment showed a clear tendency to increase with
current velocity. Prominent bedforms developed during the experiments.
The bedforms were found to have an important effect on boundary shear
stress. The effect of the bedforms was removed successfully by modifying
the technique of Nelson and Smith (1989) for open channel flows.
Two formulations to evaluate sediment entrainment rates were
obtained using data from equilibrium open channel suspensions, one for
well-sorted sediment and the other for poorly-sorted sediment.
An integral model for steady, spatially developing turbidity currents
driven by poorly.sorted sediment was developed and used as a basis for the
analysis of the experimental results.
Garcia, Marcelo H..
Depositing and Eroding Sediment-Driven Flows: Turbidity Currents.
St. Anthony Falls Hydraulic Laboratory.
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