Modeling of Erosion and Deposition by Turbidity Currents Generated at River Mouths

ABSTRACT A numerical approach is developed to simulate sediment transport by turbidity currents, with particular application to hyperpycnal plumes. The model extends the Chezy equation to explicitly include water entrainment, sediment erosion and deposition, and internal grain friction. Water entrainment is shown to be particularly important to the motion of hyperpycnal plumes, wherein internal friction is greatly reduced and the plume can flow even on small reverse slopes. Marine deposits associated with a 28-day flood on the Saguenay River in 1663 A.D. are compared favorably to model simulations on the shape (runout distance, turbidite thickness) and grain-size properties of the deposit. Properties of the turbidite are shown to be strongly linked to the duration and hydrograph of the flood event. During the rising limb of the flood wave, when sediment concentration and flow velocities are on the increase or remain high, deposition of the turbidite shifts seaward. On the falling limb of the flood wave, deposition of the turbidite shifts landward, as sediment concentration and flow velocities decrease. This later phase leads to the formation of a deposit that thickens and then thins seaward, in contrast to turbidites deposited from an ignitive surge, where deposit thickness simply decreases with distance. The deposit of a hyperpycnal flood event is initially inversely graded (finer to coarser particles measured from the base of the deposit), in association with the period of increasing discharge, then normally graded in association with the period of decreasing river flow.

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