Progradational sand-mud deltas in lakes and reservoirs. Part 1. Theory and numerical modeling

Deltas are cone-shaped deposits formed at the confluence of rivers with standing bodies of water. One of the most common stream morphologies associated with deltas is that of the sand-bed stream. Such streams usually carry significantly more mud as wash load than they do sand as bed material load. Deltas typically form so that the sand deposits fluvially in the river channel and avalanches into deeper water to create a prograding delta face. The remaining muddy river inflow often plunges over the steep delta face and continues flowing downslope as a bottom turbidity current. The mud carried into deep water by the underflow settles out on the bed of the lake or reservoir. A variety of numerical models dealing separately with either fluvial deposition or depositional turbidity currents have been proposed to date. The work reported here pertains to an integral, physically-based, moving boundary model of deltaic sedimentation in lakes and reservoirs that captures the co-evolution of the river-delta morphology and the associated deposits. The formulation unites fluvial and turbidity current morphodynamics in a single numerical model. The model is tested against the results of the two experiments described in a companion paper.