Hydrologic Restoration of Welder Flats: Informing Engineering Design through Fine-Scale Hydrodynamic Modeling in a Hypersaline Salt Marsh

Abstract Coastal wetlands are among the most valuable and productive ecosystems, but human-constructed barriers can disrupt hydrologic connectivity, leading to hypersalinity and wetland degradation. At Welder Flats, a state-designated wildlife management area on the central Texas coast, roads have restricted tidal exchange causing elevated salinities on the western side of the marsh. The resulting hypersalinity has contributed to the decline of Spartina alterniflora , reducing suitable habitat for the endangered Whooping Crane ( Grus americana ) and other migratory birds. This study examines hydraulic connectivity in anthropogenically altered wetlands and evaluates the suitability of Delft3D flexible mesh (FM) for fine-scale restoration planning. There is a notable lack of literature demonstrating the application of Delft3D FM in shallow (<1 m deep), fine-scale marsh systems containing sinuous tidal creeks (1 m wide) while simultaneously encompassing a regional-scale domain. A modeling framework to improve the performance of Delft3D FM in complex coastal wetlands at fine spatial resolutions in tandem with coarse regional resolutions was developed. Using this framework, a hydrologic connectivity analysis of Welder Flats was conducted to identify culvert configurations that enhanced tidal connectivity at Welder Flats, while avoiding erosive flow magnitudes. The results showed that installing a 5.49-m-wide culvert at the road location would increase flow by a factor of 36 in a formerly stagnant marsh. This study helped to inform engineering design plans for the future restoration of Welder Flats and demonstrated the utility of using Delft3D FM as a restoration planning tool to be utilized in future studies and restoration projects.