Sand Dunes as a Nature‐Based Solution to Mitigate Salt Intrusion in Stratified Estuaries

Salt intrusion in estuaries can pose a threat to drinking water extraction, irrigation, industries, and ecology. Key factors influencing the salt intrusion length include freshwater river flushing and vertical salt mixing. This study investigates the potential of estuarine sand dunes, that is, large-scale rhythmic patterns, to serve as a nature-based solution to mitigate salt intrusion. To this end, we adopt a two-dimensional vertical (2DV) modeling approach, which numerically solves the flow and salt transport in an idealized single-channel estuary. By varying the tidal amplitude and river discharge, three types of estuaries are modeled; a salt wedge, a strongly stratified and a strain-induced periodic stratification estuary. Two types of dune configurations are considered: ’depth-neutral’ dunes (preserving mean water depth) and ’deepening’ dunes (crest level equal to original bed level, rest of dune profile entirely below). Model results demonstrate that the presence of dunes can effectively reduce the salt intrusion length by several kilometers, compared to the scenario without dunes. Systematically varying dune geometry demonstrates that steeper (higher and shorter) dunes yield a stronger reduction in the salt intrusion length. Deepening dunes have the potential to generate sufficient mixing to overcome the adverse effects of channel deepening and, as such, can mitigate salt intrusion while maintaining navigability. As a naturally existing bedform, estuarine sand dunes present a promising nature-based solution against salt intrusion without compromising accessibility to seaports.