MODELING OF EROSION AND DEPOSITION BY TURBIDITY CURRENTS GENERATED BYRIVER MOUTHS

A numerical approach is developed to simulate sediment transport by tu rbidity currents, with particular application to hyperpycnal plumes, T he model extends the Chezy equation to explicitly include water entrai nment, sediment erosion and deposition, and internal grain friction, W ater entrainment is shown to be particularly important to the motion o f hyperpycnal plumes, wherein internal friction is greatly reduced and the plume can how even on small reverse slopes, Marine deposits assoc iated with a 28-day hood on the Saguenay River in 1663 A.D. are compar ed favorably to model simulations on the shape (runout distance, turbi dite thickness) and grain-size properties of the deposit, Properties o f the turbidite are down to be strongly linked to the duration and hyd rograph of the hood event, During the rising limb of the hood wave, wh en sediment concentration and flow velocities are on the increase or r emain 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 pha se leads to the formation of a deposit that thickens and then thins se award, in contrast to turbidites deposited from an ignitive surge, whe re deposit thickness simply decreases with distance, The deposit of a hyperpycnal flood event is initially inversely graded (finer to coarse r particles measured from the base of the deposit), in association wit h the period of increasing discharge, then normally graded in associat ion with the period of decreasing river flow.