Journal of Coastal Research
SI 36
290-299
(ICS 2002 Proceedings)
Northern Ireland
ISSN 0749-0208
Inlet Formation and Evolution of the Sediment Bypassing
System: New Inlet, Cape Cod, Massachusetts
Duncan M. FitzGerald and Elizabeth Pendleton
Department of Earth Sciences, Boston University
685 Commonwealth Avenue, Boston, MA 02215
dunc@bu.edu
ABSTRACT
Nauset Spit was breached during a moderate northeast storm on 2 January 1987, forming New Inlet. Factors
leading to the breach included a long-term narrowing of the barrier; a 0.5-m storm surge superimposed on near
perigean, perihelion, and spring tide conditions; restricted flow through existing inlets, and large differences in tidal
range and tidal phase between the ocean and Pleasant Bay. Ultimately, destruction of the foredune ridge and a large
hydraulic slope across the barrier (> 1.2 m) facilitated the development of an overwash channel, which led to tidal
exchange and inlet formation. As the channel captured an increasingly larger portion of the bay tidal prism, the inlet
grew in size from 0.5 km after two months to almost 2.0 km wide by early 1988. Opening of New Inlet increased
the tidal range from 1.2 to 1.5 m, which drastically changed the hydraulic character and sediment transport patterns
in Pleasant Bay.
The opening of New Inlet washed much of the sand from the eroding barrier into the bay. As the dimensions of the
inlet increased, the ebb-tidal delta grew in volume reducing the amount of sediment bypassing the inlet. Sand
continues to enter the backbarrier building sand shoals and bedforms. The decrease in sand supply to the downdrift
barrier caused 100 m to 300 m of shoreline recession immediately south of the inlet from 1990 to 2000. During the
past six years the rate of erosion has decreased slightly due to more active transport of sediment past the inlet. The
major mechanism of inlet sediment bypassing is accomplished through the breaching of hydraulically more
efficient channels through the outer portion of the ebb delta. This process results in the formation of swash bars
along the downdrift portion of the delta. These bars are 100 m to 300 m long and attach to the landward beach
every 1 to 3 years. Permanent breaching of the spit platform has not occurred at New Inlet due to rapid accretion
of Nauset Spit and backbarrier shoals directing ebb flow away from the spit platform.
Cape Cod, spit breaching, tidal inlet processes, hydraulics.
ADDITIONALINDEXWORDS:
INTRODUCTION
The 30-km long Nauset barrier system is composed of a
downdrift barrier shoreline (FITZGERALD, 1988). A
series of spits and barrier islands that form the southern
dramatic illustration of this process occurred at Assateague
outer coast of Cape Cod, Massachusetts. Barrier
Island when it was breached by a 1933 hurricane forming
construction has served to smooth an indented, irregular
Ocean City Inlet, Maryland. As the inlet accessed an
mainland coast composed chiefly of outwash sediments that
increasingly larger portion of the bay tidal prism, its channel
were deposited during deglaciation 17-18 ka BP (UCHUPI
dimensions also increased. The commensurate growth of
et al., 1996). Eroding bluffs north of the region supplied
the associated ebb-tidal delta depleted the downdrift
sand for southward spit progradation, enclosing Nauset
shoreline of sand leading to an onshore retreat of northern
Harbor, Pleasant Bay, and Chatham Harbor (Fig. 1). The
Assateague Island by more than a full barrier width
southern end of the spit complex is segmented due to
(STAUBLE, 1997).
several storm breachings. The most recent of these cuts
Breaching of Nauset Spit and formation of New Inlet in
occurred along the barrier fronting Pleasant Bay during a
January 1987 caused an influx of sand into Pleasant Bay.
moderate northeaster (extratropical storm) on 2 January
This process coupled with a change in backbarrier
1987.
hydraulics, due to an increase in tidal energy, drastically
Formation of a tidal inlet disrupts the longshore transport
altered lagoonal sedimentation patterns as well as beach and
of sediment commonly resulting in severe erosion of the
nearshore processes. New Inlet has provided an opportunity
Journal of Coastal Research, Special Issue 36, 2002
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