How Managed Realignment Modifies Salt Intrusion in Short Estuaries

Abstract The global loss of intertidal habitats undermines ecosystem services. To restore ecological value and enhance flood protection, managed realignment sites (MRSs) are increasingly implemented along estuaries worldwide. At the same time, estuarine salt intrusion intensifies due to climate change and anthropogenic impacts. While it is known that the extent of the intertidal area influences salt intrusion, the effects of MRS location and configuration remain understudied. We adopt a hydrodynamic model to simulate an idealized funnel‐shaped short estuary, where the salt intrusion length is comparable to the tidal excursion length. Different stratification regimes are simulated by varying the tidal range and river discharge. MRSs vary in along‐estuary location, surface area and inlet width. Results show that the impact of MRSs on salt intrusion depends on both the global stratification regime and site location: a salt‐wedge regime exhibits minimal changes, a partially mixed (PM) regime shows either increases or decreases depending on location, and a strain‐induced periodic stratification regime experiences increases. In a PM regime, coupling an MRS at the baseline intrusion length increases salt intrusion, while positioning it further up‐estuary reduces it. Tide‐averaged stratification decreases in all scenarios, though a local maximum appears near the inlet of the MRS. These effects arise from estuary‐scale shifts in dominant salt transport mechanisms, a changing tidal prism and local tidal trapping, modulated by MRS geometry and location. Our findings demonstrate the key role of along‐estuary location on salt intrusion response for a given estuarine stratification. This underscores the need to explicitly consider salinity dynamics in MRS design.