Coastal Erosion Processes along Cape Cod Bay, MA, USA

Ocean-facing shorelines experience morphological changes on many temporal and spatial scales in response to various processes such as wave breaking, overwash, as well as wave- and wind-driven currents. The nature of coastline response can vary due to several factors, including the underlying sub-bottom stratigraphic structure, surficial sediment type, and local vegetation cover, among others. These eco-geomorphic changes are significant for both understanding coastal community hazards and infrastructure planning for short- and long-term shoreline stability.

 

Cape Cod Bay, MA, is a semi-enclosed embayment in the northeastern United States, open on the north to the Gulf of Maine. Typically, the coastline experiences the largest impacts from strong Nor’easter storms that occur in the late fall or winter months. Some sections of this coastline are affected more severely than others. We investigate the processes that cause spatial variability of coastal response to storm impacts by using geophysical surveys, shoreline-change analysis, and numerical modeling.

 

We simulated the Gulf of Maine and Cape Cod Bay from Jan – April, 2021, using the COAWST modeling system, including ocean, wave, infragravity wave (InWave), and sediment transport models, with an initial focus using a uniform seafloor sediment distribution. This time period included several strong Nor’easter events. The modeling used several grids to simulate bay-scale (order 100’s meters for Cape Cod Bay) down to nearshore-scale (order several meters for an 18km section of coast) processes. Bay-scale results produce storm-driven circulation of landward surface flows and seaward near-bottom currents, alongshore sediment fluxes, and sediment convergences at regional shoals. Nearshore modeling identified the largest impact from storm events when the surge, tide, and strongest waves all coincided. Analysis of the InWave model results reveal localized zones of increased wave heights, spaced along the 18km section of coast, due to bathymetrically induced alongshore convergence of wave energy flux. These locations correlate with observed regions of increased erosion and severe coastal impacts. Modeled and observed shoreline-change demonstrate locations of high correlation but the model does not capture all the variability. Computed net sediment fluxes for the modeled time period along the coast agree with regional sediment flux observations.