Abstract:
New Bedford Harbor and its approaches form the
estuary of the Acushnet River in southeastern Massachusetts.
The estuary is weakly stratified and only partially mixed
because river discharge is very small. It appears to be
typical of the inlets of the coast of New England, and
is a branch of a larger estuary - Buzzards Bay.
Silt and clay are being transported into the estuary
in suspension by landward-moving bottom currents that
are driven by wave and tidal energy. These fine sediments
come from Buzzards Bay, but may originate out on the
continental shelf. Before the entrance to the harbor
was almost completely blocked by a hurricane barrier,
these sediments were accumulating in the harbor at rates
of about 1-2 cm/yr in the deeps, and less than 0.5 cm/yr
in the shallows. Construction of the barrier reduced the
efficiency of tidal flushing, causing the rate of siltation
to increase by a factor of 4-5. Outside the harbor, silt
and clay accumulate in the drowned valley of the Acushnet
and in related depressions at rates of 2-3 mm/yr.
In the water column, silt and clay sized minerals
are suspended together in organically bonded agglomerates.
During sediment transport, the silt and clay become partially
fractionated, probably by differential settling of
the agglomerates. Because fractionation is more effective
where wave and tidal energy are strongest, there is a
smaller proportion of clay relative to silt in the harbor
than there is seaward. Nevertheless, the net transport
of clay is still landward.
Fractionation due to differential settling also
appears to have formed a very thin, soupy layer of clay-rich
material at the sediment-water interface, that appears
to carpet the study area. This layer seems to form a
transition zone between the much more silty and less
mobile subsurface sediments and the highly mobile suspensates
of turbid near-bottom waters. Further study is needed
to ascertain precisely the nature and persistence of this
layer.
