Fate of cohesive sediments in a marsh-dominated estuary

Salt marshes are among the most valuable coastal landforms in the world, supporting productive ecosystems and buffering the shoreline against violent storms. In recent years, salt marshes have experienced increasing pressure from Sea Level Rise and human activities (e.g. a decrease in riverine sediment supply due to dams). Deposition of fine sediments on a marsh platform favors accretion that counteracts sea-level rise. However, it is difficult to assess the sediment trapping capacity of a marsh given the heterogeneity of sediment sources and the geometric complexity of the system, with a network of dendritic and meandering creeks dissecting the intertidal area. Here we use a numerical model to study the sediment trapping capacity of a marsh-dominated estuary, Plum Island sound, USA, and its variations across the landscape. The results highlight the importance of the sediment releasing instant and show that sediment discharged from tidal rivers deposit within the rivers themselves or in adjacent marshes. Most sediment is deposited in shallow tidal flats and channels and are unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platform. Trapping capacity of sediment in different intertidal subdomains decreases logarithmically with the ratio between advection length and the typical length of channels and tidal flats. Moreover, sediment deposition in the marsh decreases exponentially with distance from the channels and marsh edge. This decay rate is a function of settling velocity and the maximum value of water depth and velocity on the marsh platform.