Date of Award
1-1-2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Oceanography
Specialization
Physical Oceanography
Department
Oceanography
First Advisor
John P. Walsh
Abstract
Coastal areas in southern New England are vulnerable to a range of impacts from extreme storms and sea level rise. The most frequent extreme storms are extratropical storms, commonly referred to as nor’easters, named for the direction of the strongest winds, though these storms also have strong winds from other directions.
Storm winds generate large waves and push water up along the coast in the form of storm surge. The combined impacts of increased water levels and waves include flooding and morphological change. Sea level rise (SLR) will exacerbate these processes, amplifying the associated impacts, which may pose increasing threats to safety, infrastructure, and ecosystems. This dissertation presents a series of applied numerical modeling studies focused on evaluating the unique impacts from extreme storms and SLR at different coastal locations within southern New England. The two main study areas are the eastern coast of Cape Cod, including its coastal embayments, and Moonstone Beach along the south coast of Rhode Island.
Chapter 1 presents a study of the effects of waves and SLR on water levels during an extreme nor’easter in coastal Cape Cod. This study uses the Advanced Circulation hydrodynamic model (ADCIRC) coupled with the Simulating Waves in the Nearshore (SWAN) wave model. The model application was first validated for the study area and then used to conduct a set of experiments under different scenarios: with and without wind forcing, with and without wave coupling, and with and without 1 m of SLR. These scenarios were designed to isolate the effects of waves on water levels and the effects of SLR on coastal impacts. The spatially and temporally varying impacts are analyzed and discussed with respect to the driving forces. The study demonstrates that radiation stress gradients from waves are responsible for up to 20% of the total surge along the open coast and have potentially greater impacts in coastal embayments, where the response is more complex. These stress gradients increase the water levels and generate alongshore currents that directly and indirectly impact the water levels and currents across broad scales. Additionally, the study reveals that SLR effects are nonlinear, altering tides, surge, and waves. The magnitude of the effects is spatially and temporally variable, with coastal embayments exhibiting a more pronounced response compared to the open coast.
Chapter 2 presents a modeling study focused on the effect of shoreline change on the impacts of nor’easters in Cape Cod and performs a sensitivity analysis on the effect of marsh characterization in the model application. The outer coast of Cape Cod, Massachusetts, is exposed to a variable tide range, storm surge, and wave exposure. Conditions within coastal estuaries are further influenced by the estuary shape, bathymetry, and inlet configurations. The coastline is morphologically dynamic in response to short- and long-term coastal processes, resulting in variable shoreline configurations over time. We use the ADCIRC-SWAN hydrodynamic-wave coupled model system to investigate the effects of shoreline configurations and tide stages on storm surge and waves during an extreme nor’easter within two coastal estuaries along the Cape Cod shoreline: Pleasant Bay Estuary and Nauset Bay Estuary. Model simulations are performed for the January 2018 Nor’easter, incorporating variations in shoreline configurations for each estuary, as well as changes in the timing of peak winds relative to tidal phases. Three different shoreline configurations are evaluated for each estuary based on a review of the shoreline over the past ~40 years to represent potential extreme scenarios. Analysis of the results highlights the differences in the magnitude of impacts between the open coast and the embayments, emphasizing the need to resolve the embayment systems in models. The results demonstrate that within the embayments, storm surge is not solely determined by meteorological conditions and is not simply additive to tidal water levels. Tidal forces play a large role in determining the timing and extent of storm surge propagation within the embayment, which subsequently affects water levels and significant wave heights. While the tidal cycle dominates the impacts, overall water depth also plays a critical role, making embayment systems sensitive to changes from natural (e.g., inlet shifts, sea level rise) or man-made (e.g., dredging) changes. The model results are not sensitive to the characterization of the marsh roughness for this regional model application.
Chapter 3 presents a numerical modeling study of morphodynamics at Moonstone Beach in Rhode Island during winter storms. Located along the southwest coast of Rhode Island, Moonstone Beach undergoes constant morphological changes driven by geological, meteorological, and oceanographic conditions, as well as human influences. Morphodynamic models used for hindcasting or forecasting morphological changes in coastal areas rely on extensive parameterization to represent uncertain physical processes. These models are often highly sensitive to parameter values and their performance is frequently constrained by limited observational data. In this study, we use coastal observations and the XBeach model to examine the morphodynamics of Moonstone Beach between December 2, 2023, and January 12, 2024. This period coincides with a coastal field campaign that captured multiple winter storms of varying intensity, enabling us to test the sensitivity of the model predictions to key numerical parameters (facua and beta) and modeling input (grain size, initial bed elevation) for different conditions.
Overall, the model successfully recreated the observed trends, and the simulations across different periods appropriately reflected the relative trends as observed. The best agreement with observations was achieved with different combinations of facua and beta for each modeling period. The trend of the degree of erosion varied across different combinations of facua and beta depending on the modeling period, indicating that the modeling period and storm intensity are not the sole, and perhaps not the dominant, factors in determining the overall trend.
The modeling predictions of the inundation area and the presence or absence of overwash were very similar regardless of the combination choice of facua and beta, although the magnitude of underlying bed elevation changes varied substantially. The model accurately reproduced the absence of overwash during the low intensity period and significant overwash during the high intensity storm, as observed. However, it underestimated the extent of overwash during the moderate-high storm intensity.
Modeling results from simulations of the low-intensity storm period were highly sensitive to changes from medium to fine sand but showed little sensitivity to changes from medium to coarse sand. Similarly, while the overall trends were consistent when varying the initial bed elevation based on different data sources, differences in magnitude indicated that the results are influenced by the characterization of elevations at a broader spatial scale than the immediate study area.
Recommended Citation
Crowley, Deborah, "NUMERICAL MODELING STUDY OF COASTAL IMPACTS FROM EXTREME STORMS AND SEA LEVEL RISE IN SOUTHERN NEW ENGLAND" (2025). Open Access Dissertations. Paper 4449.
https://digitalcommons.uri.edu/oa_diss/4449
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