Sediment Transport by Storms

Strong storms with winds from the northeast resuspend fine sediments from western Massachusetts Bay and transport them offshore and toward Cape Cod Bay. Northeasters, with winds that blow across the Gulf of Maine, generate large waves that enter Massachusetts Bay from the east. The oscillatory currents associated with these waves cause resuspension of the bottom sediments in water depths less than 40 to 50 m over areas exposed to the northeast, principally along the western shore of Massachusetts Bay (fig. 7). Typically only a few millimeters of sediment are resuspended from the seabed during each storm. The currents driven by winds from the northeast flow southeastward parallel to the coast (with an offshore component near the bottom) and carry the suspended sediments toward Cape Cod Bay and offshore into Stellwagen Basin (figs. 8 and 9). Sediments settle to the sea floor along this transport pathway following each storm.

Relative strength of wave-induced current and suspended sediment concentrations

Figure 7. Above: Plot of the relative strength of the wave-induced current (black line) and suspended sediment concentration (red line) at the bottom at the long-term monitoring station near the site of the future outfall. Increased sediment concentrations occur in the water whenever wave-induced currents are large.

Below: Sequence of bottom photographs taken before, during, and after the April 1, 1997, storm. During the storm (middle photograph), material in suspension obscures the sea floor from view. By the end of the storm, several of the large cobbles on the sea floor have moved (the frame holding the camera also moved during the storm, slightly changing the field of view).

Mean wind and currents Figure 8. Mean wind (in m/s) and currents (in cm/s) measured near the future outfall site during strong storms that resuspend and transport bottom sediments. Winds from the northeast drive near-surface currents at this location to the south toward Cape Cod Bay and near-bottom current to the southeast offshore toward Stellwagen Basin.

Sediments that reach the sea floor in Cape Cod Bay or Stellwagen Basin are likely to remain there. In this coastal system, currents caused by surface waves are the principal cause of sediment resuspension. Cape Cod Bay is sheltered from large waves by the arm of Cape Cod, and waves are rarely large enough to resuspend sediments at the seabed in the deep Stellwagen Basin. Thus, once sediments reach Stell- wagen Basin or Cape Cod Bay, carried either by the mean flow or transported by storms, it is unlikely that they will be re- suspended and transported again by waves.

Modeled wind-induced currents and contours of near-bottom wave current speed

Figure 9. Modeled wind-induced currents (arrows) and contours of near-bottom wave current speed driven by a northeasterly wind of 14 m/s (28 knots). Near-bottom wave speeds in excess of about 10 cm/s are sufficient to resuspend fine-grained sediments. During major northeasters, fine sediments along the western shore of Massachusetts Bay are resuspended by the wave currents and transported by the wind-driven flow to the southeast toward Cape Cod Bay, where they settle. They are protected from the influence of subsequent storms by water depth and and basin geometry. The numerical circulation models provide predictions of the basinwide storm response, which would be very difficult to observe directly.

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Last revised 6-24-98