The effect of sand to mud ratio on shoreline progradation rate: Numerical modeling of a coupled fluviodeltaic topset to bottomset system

Abstract:

Recent deltaic land loss caused by sea level rise, subsidence, and other anthropogenic effects has become a serious problem for many countries. There have been various attempts to restrict land loss, or even build new deltaic land. Among several methods employed by engineers and scientists, natural processes to supply enough sediment to the deltaic area are thought to be most important. Thus, many previous studies have attempted to determine whether delta building is feasible by creating quantitative models to predict natural delta building rates. However, those models have only considered sand as an input sediment, exempting mud-sized grains as washload. It is known that the topset and foreset of a delta consist of mainly sand, and bottomset growth of a delta is mainly produced by the settling of mud from settling and gravity flows. Bottomset growth reduces the length of the foreset; thus, mud undoubtedly has an effect on the rate of shoreline progradation. In this study, the numerical model, which handles sand and mud together as input sediments and has three depositional surfaces (foreset, topset, and bottomset), was run with varying boundary conditions (basin water depth and mud-to-total-sediment ratio) to investigate the relationship between mud proportions and shoreline progradation of the delta. The total amount of sediment flux was kept constant, and the progradation distance was measured at various run times (~50 years) and depths (from 10m to 100m), changing only the mud-to-total-sediment ratio. We found that (1) there was a tendency for progradation to decrease as the mud ratio increased gradually for the shorter total run times and (2) eventually inversion occurs for the longer total run times, so that progradation increased as the mud ratio increased. This inversion occurred earlier at shallower water depths because shallower basin depth is more likely have a greater effect on mud accumulation rates. These results indicate that progradation rate differs by sediment ratio, as mud can cause acceleration of the shoreline progradation. The inversion times associated with various basin depths obtained can provide an optimal sediment ratio for building new land, which helps with designing delta restoration. In essence, this study suggests the importance of mud in delta restoration studies.