Back-Barrier Water Level Response to Offshore Fluctuations

Water level in semi-enclosed bays, landward of barrier islands, is mainly driven by offshore sea level fluctuations. Inlet geometry is the major factor that regulates the magnitude of the transfer between the two systems. Tides and short-period offshore oscillations tend to be more dampened in the bays than longer-lasting offshore fluctuations, such as storm surge and sea level rise. We compare observed and modeled water levels at stations in mid-Atlantic bays (Great South Bay, Jamaica Bay, and Barnegat Bay) with offshore water level proxies. The differences in the frequency-dependent transfer functions are associated with the size of the inlet and the area of the bays, with larger bays exhibiting smaller transfers. The magnitude of the fluctuations at a specific frequency is not a factor controlling the water level transfer into the bays. The observed water level response in Great South Bay is characterized by transfers between 80-100% in the storm band (2-5 days) and between 20-40% at tidal frequencies. The transfer into Jamaica Bay of offshore storm-related fluctuations is close to 100% and tides are enhanced inside the Bay resulting in water levels greater than the offshore tidal amplitude. In Barnegat Bay, the observed water levels are combined with model results using the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system. The model water level transfers match the observed values at locations inside the Bay in the storm band (transfers raging 70-100%) and tidal frequencies (20-55%). The model provides transfer estimates for locations inside the Bay where observations were not available providing a complete spatial characterization and allowing for the study of the Bay response to alternative forcing scenarios (landscape changes, future storms and rising sea level). The approach can inform decisions on inlet management and contribute to the assessment of current and future natural hazards to back-barrier and mainland shorelines.