Observations and a linear model of water level in an interconnected inlet-bay system

A system of barrier islands and back-barrier bays occurs along southern Long Island, New York, and in many coastal areas worldwide. Characterizing the bay physical response to water level fluctuations is needed to understand flooding during extreme events and evaluate their relation to geomorphological changes. Offshore sea level is one of the main drivers of water level fluctuations in semienclosed back-barrier bays. We analyzed observed water levels (October 2007 to November 2015) and developed analytical models to better understand bay water level along southern Long Island. An increase (approximate to 0.02 m change in 0.17 m amplitude) in the dominant M-2 tidal amplitude (containing the largest fraction of the variability) was observed in Great South Bay during mid-2014. The observed changes in both tidal amplitude and bay water level transfer from offshore were related to the dredging of nearby inlets and possibly the changing size of a breach across Fire Island caused by Hurricane Sandy (after December 2012). The bay response was independent of the magnitude of the fluctuations (e.g., storms) at a specific frequency. An analytical model that incorporates bay and inlet dimensions reproduced the observed transfer function in Great South Bay and surrounding areas. The model predicts the transfer function in Moriches and Shinnecock bays where long-term observations were not available. The model is a simplified tool to investigate changes in bay water level and enables the evaluation of future conditions and alternative geomorphological settings. Plain Language Summary We analyze water level observations in the bays of southern Long Island (Jamaica Bay, Great South Bay and connected bays) to determine how the bays respond to the conditions in the open ocean. We focus especially on changes in time in the tides and in the response to storms. The tides and the water level relationship with offshore have been changing slightly in recent years (2008-2015). The changes occur at times during or immediately after inlet dredging and also as a result of the changing dimensions of a breach through Fire Island caused by Hurricane Sandy. We propose a simple model that takes into account the inlet and bay dimensions and friction in the inlet channels to predicts water level response to tides and storms in all bays including those for which long-term observational data were not available.