The effect of bottomset on fluviodeltaic land‐building process: Numerical modelling and physical experiment

Abstract

AbstractThe loss of land in coastal regions is an emerging topic across the scientific community, as many countries struggle to minimize the consequences of an accelerating relative sea‐level rise. Various methods have been attempted to mitigate land loss, and river diversion for the Mississippi River Delta, which makes use of the natural river delta‐building process, has been proposed and significantly examined. Prior delta‐building models predicted possible ranges of new delta‐building rates and verified the feasibility of reduction in land loss on the Louisiana coast by river diversion by only considering the delta topset and foreset deposition without incorporating the muddy bottomsets because sand was regarded as the main delta‐building sediment, and mud was treated as washload. Since sand flux is significantly smaller than mud flux in most coastal rivers and muddy bottomsets are common in most deltas, it is critical to understand the depositional processes of mud in deltas. Here, we present the results of a coupled numerical modelling and flume experiment that includes a moving boundary at the foreset‐bottomset break in addition to the shoreline. We find that bottomset aggradation can accelerate the shoreline progradation by decreasing the foreset length (i.e., depth at the delta front). We also apply our model to a field scale based on parameters taken from the Wax Lake Delta. When 10%–50% of the mud supplied to the delta is retained in the bottomset, the subaerial delta area increases by 4.4%–25.4% compared to that in a delta with no bottomset accumulation. Therefore, considering the bottomset in land‐building modelling can provide more accurate predictions for a new land‐building area by river diversion.