Observed and Modeled Sediment Transport Around Katama Inlet, Martha’s Vineyard

Abstract:

Katama Inlet, connecting Katama Bay to the Atlantic Ocean on the southern shoreline of Martha's Vineyard, MA, has migrated eastward more than 2.5 km since it was breached during the Patriot’s Day storm in 2007. This morphological evolution, typical of the inlet’s decadal cycle of breach-migration-closure, is owing to sediment transport driven by wave-orbital velocities, breaking-wave-generated mean currents, and tidal flows. Here, the rapidly evolving shoreline near Katama Inlet and on the southern edge of Martha’s Vineyard is investigated using field observations and numerical model simulations. The bathymetry was surveyed in summer 2013 and 2014, and tides, waves, and currents were measured for a month in August 2013 and 2014 in the surf zone (~2 m water depth), on the outer edge of the ebb shoal offshore of the inlet mouth (~6 m depth), and on the inner continental shelf (~7 m depth). The model [Delft3D with coupled waves (SWAN) and currents] skillfully simulates observed wave heights, wave directions, and tidal currents, and is used here to estimate sediment transport rates. Model results suggest that during the relatively calm August conditions there is little transport on the inner shelf, but there is significant transport that changes directions with the tide on the outer ebb shoal. Transport rates in the surf zone decrease and become more unidirectional (wave-driven) with distance away (west) from the mouth of the inlet. In August, transport of suspended sediments is relatively more important on the outer ebb shoal than near the surf zone, where bedload transport dominates. The relative impact of these types of simulated transport on the migration of Katama Inlet will be discussed.

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