Managed Realignment and its Influence on Estuarine Salt Intrusion

Restoration of intertidal areas through managed realignment is increasingly implemented in estuaries to improve flood protection and ecological function. At the same time, environmental conditions and anthropogenic pressures are changing, and salt intrusion is expected to increase in most estuaries. Since managed realignment alters estuarine geomorphology, changes to estuary-scale hydrodynamics can be expected. Previous studies have shown that intertidal area extent can increase or decrease salt intrusion, with both the direction and magnitude of the effect depending on the degree of stratification in the estuary. However, a systematic analysis of how managed realignment influences salt intrusion has not yet been performed. We use a three-dimensional model, solving the shallow water equations in Delft3D-FM, to simulate the hydrodynamics and salt dynamics in funnel-shaped estuaries. Tidal range and river discharge are varied to generate base models with different degrees of stratification (salt wedge, partially mixed, and strain-induced periodic stratification (SIPS)). Managed realignment sites are implemented in the model with variations in surface area, inlet width, and along-estuary location. Results show that both surface area and inlet width influence the magnitude of the change in salt intrusion length, with larger areas and wider inlets leading to stronger effects. In contrast, the direction of the impact (i.e., whether salt intrusion increases or decreases) is mainly governed by the along-estuary location. For a 300 ha site with an open inlet, in the salt wedge system, intrusion decreased most at the baseline intrusion length (−5%) and least near the mouth (−1%). In the partially mixed system, intrusion increased near the mouth (+1%) and peaked at the baseline intrusion length (+31%), but decreased further upstream (−12%). In the SIPS system, all locations showed increases, from +7% near the mouth to +125% at the baseline intrusion length. While an increase in tidal prism is identified as one of the main contributors to changes in salt intrusion length, the inlet width influences the tidal trapping effect of the realignment site. These findings highlight the need to consider the potential impact of managed realignment on salt intrusion, alongside their flood safety and ecological objectives.