Twenty Years of Bathymetric Change in a Shallow Back-Barrier Bay

Bathymetric change in shallow coastal bays is intrinsic to the morphodynamic evolution of back-barrier systems given feedbacks between bay bathymetry, hydrodynamics, sediment resuspension, and marsh elevation gain. However, limited high resolution bathymetric datasets of shallow coastal bays exist, and quantitative assessments of change are further limited. A high-resolution bathymetric survey was completed in the late 1990s of Hog Island Bay, a shallow and relatively large back-barrier bay located along Virginia's Eastern Shore. The bay lacks fluvial sediment sources and features broad tidal flats with a mean depth of approximately 1 m during mean low water and a prominent channel dissecting the bay with depths often greater than 10 m. Relative sea-level rise is approximately 5 mm/yr. The track lines of the late 1990s survey were re-surveyed in the summer of 2022 using single-beam echosounding with integrated survey-level RTK positioning to assess bathymetric change after more than twenty years. Preliminary results show that bathymetric change is spatially variable across the bay, with areas of deposition and erosion likely influenced, in part, by proximity to channels and tidal inlets. On average, however, bay bed elevation has fallen since the late 1990s. Simultaneously, approximately 10 cm of relative sea-level rise has occurred. This combination of elevation change and relative sea-level rise has caused the bay to deepen. Due to feedback between processes, deeper bay depths may alter hydrodynamics and sediment resuspension, resulting in larger waves and accelerated rates of marsh retreat. Subsequent research will model bay hydrodynamics and erosion potential to explain the spatial variability of bathymetric change. A sediment budget for the back-barrier system will be constructed by integrating bathymetric change with rates of deposition on fringing salt marshes and sediment input through tidal inlets.