Abstract
Barrier island-bay-marsh systems must accumulate and redistribute sediment to maintain elevations and shoreline positions under sea-level rise (SLR). Although vertical accretion of salt marshes has been extensively studied, volumetric change at bay-marsh boundaries and erosion and deposition across adjacent shallow coastal bays remain poorly quantified. These measurements are necessary to construct sediment budgets and understand the long-term morphodynamic evolution of coastal systems. This dissertation quantifies the rates, patterns, and drivers of morphodynamic change across bay-marsh systems in the Virginia Coast Reserve using repeat morphologic surveys, change detection, uncertainty analysis, and hydro- and sediment-dynamic numerical modeling.
At the bay-marsh boundary, unoccupied aerial system Structure-from-Motion photogrammetry and boat-mounted echosounding were integrated to resolve three-dimensional morphology and quantify sediment loss associated with marsh retreat. Lateral shoreline retreat averaged approximately 1.5 ± 0.2 m yr⁻¹ and was notably consistent across space and time, whereas volumetric erosion was more spatiotemporally variable. Variations in volumetric erosion of the marsh edge were more strongly related to variations in marsh elevation than lateral retreat rates, demonstrating that variations in marsh topography drive variations in the volume of sediment released during marsh retreat when rates of lateral shoreline retreat are spatiotemporally consistent.
Repeat bathymetric surveys across unvegetated Hog Island Bay documented significant decadal-scale deepening of shallow tidal flats at 1.9 ± 0.3 cm yr⁻¹ due to both erosion and SLR, while a deep tidal channel accumulated sediment. The volume of sediment eroded from shallow tidal flats substantially exceeded the estimated volume of sediment required for marshes to vertically keep pace with relative SLR. Model simulations further indicated that sediment export through ocean inlets during fair-weather conditions exceeded sediment import during storms, suggesting a sediment deficit at the bay scale.
In vegetated South Bay, repeated bathymetric surveys revealed strong spatial and temporal variability in sediment erosion and deposition. This variability was, in part, controlled by depth and distance into the seagrass meadow. Model simulations indicated that spatial differences in fine-sediment availability influenced whether regions were erosional or depositional under low-density seagrass conditions. A seagrass depth limit of approximately -1.8 m was identified along a depth-controlled meadow edge. Significant deposition occurred along this boundary while its position remained stable, indicating that sediment accumulation can maintain suitable water depths and potentially offset habitat loss associated with SLR.
Together, the results of this dissertation highlight the value of large, high-resolution morphologic datasets for quantifying morphodynamic change and evaluating the long-term evolution of coastal systems. These datasets are critical for constructing numerical models that investigate the short-term processes driving change. The findings also have broader implications for the long-term evolution of bay-marsh systems under SLR conditions. Erosion of the shallow bay over decadal timescales in Hog Island Bay indicates a deficit in the bay-scale sediment budget that may be broadly true of Virginia’s coastal bays and other similar systems with limited external sediment supply. As such, sediment retention in shallow coastal bays may be critical for mitigating the effects of erosion and SLR. Results from South Bay demonstrate that seagrass may contribute to sediment retention by stabilizing the bay bottom and promoting deposition, thereby helping to maintain depths as sea level rises. Collectively, these methods and findings provide a framework for quantifying morphodynamic change and improve our understanding of the processes governing the evolution and resilience of bay-marsh systems.
