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Predicting Impact of Storms on Circulation in Massachusetts Bay

 Massachusetts Bay is a semi-enclosed embayment in the western Gulf of Maine about 50 km wide and 100 km long and extends from Cape Ann southward along the eastern coast of Massachusetts (Figure 1). The Bay is open to the Gulf of Maine along the eastern boundary between Cape Ann and Cape Cod, but the opening is partly blocked by Stellwagen Bank, a shallow bank that rises to within 20 m of sea surface at its southern end. Stellwagen Basin, located to the west of Stellwagen Bank, is the deepest part of Massachusetts Bay with maximum water depth of 95 m. Cape Cod Bay is located between Cape Cod and the eastern coast of Massachusetts; the deepest water of Cape Cod Bay is 40 m at its center. Details of the mapping project are located at (http://woodshole.er.usgs.gov/project-pages/coastal_mass/).

Sudy area map

Figure 1. Physical setting of Massachusetts Bay.

 

Numerical Simulations of Massachusetts Bay

Bottom sediment resuspension is controlled predominately by storm-induced surface waves and transport by the tidal and wind-driven circulation. Because the Bay is open to the northeast, winds from the northeast (‘Northeasters’) generate the largest surface waves and are thus the most effective in resuspending sediments. We use the three-dimensional oceanographic circulation model ROMS (Regional Ocean Modeling System) to explore the resuspension, transport, and deposition of sediment caused by Northeasters. The model transports multiple sediment classes and tracks the evolution of a multilevel sediment bed. The surficial sediment characteristics of the bed are coupled to one of several bottom-boundary layer modules that calculate enhanced bottom roughness due to wave-current interaction. The wave field is calculated from the model SWAN (Simulating WAves Nearshore). Two idealized simulations were carried out to explore the effects of Northeasters on the transport and fate of sediments, as described below (Warner, et al., in press).

Transport pathways
A set of simulations explored sediment-transport pathways caused by storms with winds from the northeast quadrant by simulating release of sediment at several locations in the Bay. Results from a simulated release of sediment in Boston Harbor are shown below in Figure 2. Storms with winds from the north cause transport southward along the western shore of Massachusetts Bay (left 2 panels), while storms with winds from the east (and southeast) drive northerly nearshore flow (right 2 panels). All simulations suggest Stellwagen Basin and Cape Cod Bay as long-term sediment sinks.

 

Figure 2. Animation showing trajectory of material originating from a point source in Boston Harbor for
two different storms events, each repeated 10 times. Left panels are for the March 1997 simulation with winds from the
north. Right panels are for the October 1996 storm with winds from the east. Top panels animate suspended sediment. Bottom
panels animate deposition on the sea floor. Time series represent magnitude and timing of the repeated storm patterns.

 Seafloor evolution.
Another experiment was performed to simulate the evolution of sediment grain distribution on the sea floor. The model was initialized with a spatially-uniform bed of seven mixed sediment classes. The model was subjected to a repeated Northeaster storm (December 1992) and the sediment distribution on the sea floor evolved to a pattern similar to the existing surficial sediment distribution.

Figure 3. Animation showing bottom suspended sediment concentration, bottom stress, significant wave height,
bathymetric change, and mean grain size for a repeated storm (December 1992). The final panel of the animation
for the mean grain fraction is shown in Figure 4 and compared to observations

 

Model simulation

Figure 4.  Results from model simulation of the evolution of a mixed sediment bed
in response to a sequence of 10 idealized northeast storms, modeled after the December
1992 storm with winds from 50º. Left panels shows the modeled final mean surficial grain
size following the 10-storm sequence. Right panel shows the observed surficial grain
size distribution (Poppe, et al., 2003) and qualitatively matches the evolved sediment texture.

References:

Warner, J.C., Butman, B., and Alexander, P.S. (in press). “Storm-driven sediment transport in Massachusetts Bay.” Continental Shelf Research.

Poppe, L.J., Paskevich, V.F. , Williams, S.J. , Hastings, M.E. , Kelly, J.T. , Belknap, D.F., Ward, L.G. , FitzGerald, D.M. , and Larsen, P.F.  (2003). Surficial Sediment Data from the Gulf of Maine, Georges Bank, and Vicinity: A GIS Compilation. U.S. Geological Survey Open-File Report 03-001, http://pubs.usgs.gov/of/2003/of03-001/index.htm.

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This page last modified on Monday, 24-Nov-2014 13:13:46 EST