The Effects on Bay Circulation of Changing Geometry and Location of Katama Inlet, Martha’s Vineyard, MA

Abstract:

Circulation in bays with one or more connections to the ocean depends on the location and morphology of the inlets. Here, field observations and the numerical model ADCIRC are used to investigate circulation in the small (~7 km² surface area) and shallow (~2 m deep) Katama Bay, Martha’s Vineyard, MA, which is connected to Vineyard Sound in the north by the long (~2.5 km), stable, maintained Edgartown Channel, and to the Atlantic Ocean in the south by the variable and ephemeral Katama Inlet, which changes shape in response to storms, as well as to daily waves and tides. Katama Inlet was initiated by a breach in 2007, and has migrated almost 2.5 km to the east where it is now adjacent to Chappaquiddick Island. In a typical decadal cycle, the inlet eventually closes until another storm breaches the sand barrier. As it migrates, Katama Inlet varies in width, length, depth, and orientation. The bathymetry near the inlet was measured pre- and post Hurricane Irene (2011), and in the summers of 2012-2014. In addition, sea levels, waves, and currents were measured in the ocean, the inlets, and the bay from August until October 2011 (including during Hurricane Irene), and in August 2013. Between 2011 and 2013 the inlet migrated 1 km to the east and changed alignment from roughly north-south to east-west. Pressure data from the Atlantic Ocean and northern Edgartown Channel are used to drive ADCIRC using bathymetry measured (1) pre- and (2) post-Irene in 2011, and (3) in 2013. The model is run over month-long periods using a variable Manning’s n for friction and including wetting and drying of the coast. The model simulations are consistent with the observations, including the observed changes to the circulation caused by the evolving inlet channel. The results suggest that changes in the geometry, orientation, and location of one inlet may have significant effects on hydrodynamics throughout the bay and in the other channel.

Supported by ASD(R&E), ONR, and NSF.