Size Stratification in a Laboratory Gilbert Delta Due to a Varying Base Level: Measurement, and Numerical Modelling

Abstract:

In the present study we numerically model the progradation and size stratification of a Gilbert type delta subject to base level fluctuations. A foreset-dominated Gilbert delta is a delta that is dominated by sediment avalanches over its front. It forms when a river flows into a basin or sea characterized by a flow depth that is much larger than the one in the fluvial reach and the sediment is relatively coarse. A Gilbert delta is typically described in terms of a low-slope topset, a steep-slope foreset, and a bottomset. Our model consists of three main submodels: a sediment mixture mass conservation model suitable for mixed sediment in the fluvial reach, a delta progradation model, and a lee face sorting model. In order to calibrate and validate the model under base level variation conditions, we reproduce three laboratory experiments in which the base level varied between the experiments: (I) constant base level, (II) a gradually rising base level, and (III) a slowly varying base level. The model is able to properly reproduce the main characteristics measured in the laboratory experiments. The active layer coarsens at the start of the runs similar to the mobile armor that formed in the laboratory experiments. Base level rise induces an M1 backwater curve over the fluvial reach. This results in an increasing flow depth in streamwise direction and so a streamwise decrease of the sediment transport capacity. This then induces aggradation over the fluvial reach and a slower delta progradation rate. The opposite happens when base level fall is imposed. The model captures the slightly coarser-than-fed grain size distribution of the topset deposit. The predicted size stratification in the foreset results to be weaker (i.e. less sorted) than measured.