Salt Intrusion Affected by Large-Scale Human Interventions and Sea Level Rise: A 3D Modeling Study of an Engineered Estuary

Estuaries are heavily modified for human use while also being susceptible to sea level rise (SLR), changing estuarine geometries worldwide. In these estuaries, salt water intrudes inland, limiting freshwater availability, particularly during storms or low discharge periods. This paper studies how salt intrusion is influenced by three geometric changes that are prominent in estuaries worldwide in isolation and in combination: (a) channel depth, as channels are often dredged to improve port navigability, (b) intertidal wetland width, representing intertidal reclamation or restoration, and (c) lowering of the intertidal area due to SLR. A 3D schematized process-based model was employed (Delft3D-FM), reflecting conditions in the Rotterdam Waterway, the Netherlands. Salt intrusion lengths increased with channel depth, and the stratification generally increased with channel depth and intertidal area. However, results show that the latter dependency diminishes with larger intertidal areas, a novel finding. The salt intrusion length responds ambiguously to increasing intertidal wetland area, attributed to the balance between tidal flow and estuarine circulation flow: increasing intertidal areas, or lowering their elevation due to SLR, increases the tidal flow velocity amplitude as the tidal prism increases. This in turn suppresses the estuarine circulation flow. Consequently, the salt intrusion length decreases in strongly stratified estuaries while the salt intrusion length increases in well-mixed estuaries. In between these extremes, processes balance out and intertidal area do not influence the salt intrusion length. The findings how increases in tidal flow velocities, caused by intertidal areas, influences salt intrusion processes is novel.