Progradational sand-mud deltas in lakes and reservoirs. Part 2. Experiment and numerical simulation

Rivers deposit deltas wherever they reach standing water, i.e. a lake or reservoir. Here the case of a sand-bed river carrying mud as wash load is considered. In general the sand tends to deposit out to form a fluvial topset and an avalanching foreset, and the mud tends to deposit out as a bottomset. During floods, many sand-bed rivers carry sufficiently high concentrations of washload to render the river water heavier than that of the body of standing water. In such a case the mud-laden river fow plunges to form a bottom turbidity current. In a companion paper a I-D numerical model of a prograding delta was presented. In this model fluvial, avalanching and turbidity current deposition are all linked in terms of a moving boundary formulation. Here the model is compared against two experiments on delta progradation. The experiments reveal an intriguing interaction between the three elements of the model, with foreset progradation burying the bottomset and pushing its upstream point ever downstream, and with bottomset deposition raising the toe of the foreset, so increasing the rate of foreset progradation as sand is delivered from the topset. The numerical model of the companion paper captures this interaction with a minimum of adjustment of input parameters. The resulting description of delta morphodynamics is very similar to that observed in e.g. the delta in the Colorado River, USA where it reaches Lake Mead.