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FitzGerald and Pendleton
Storm Conditions.- The early January northeaster of
INLET FORMATION
1987 was not a record storm. However, it was slow moving
Long-Term Trends
and had sustained wind velocities of 40 km/hr, which
produced a 0.5 cm storm surge and waves approaching 3.0
The barrier spit system that extends south of Nauset
m in height (GIESE, 1990). The storm coincided with near
Heights to Chatham Harbor Inlet experiences periods of
perihelion, perigean, spring tidal conditions. The predicted
southerly accretion followed by a destructional phase
high tide was 60 cm greater than mean high tide conditions.
whereby the spit becomes segmented and portions of the
barrier migrate onshore (McCLENNEN, 1979; GIESE,
Tidal Head.- As explained previously, the long, narrow,
1988). Segmentation of Nauset Spit is related to a gradual
and shallow connection between Pleasant Bay and the
reduction in tidal exchange between Pleasant Bay and
ocean produced phase lags and tidal range differences
Chatham Harbor and the ocean, which is caused by a
between the bodies of water. The hydraulic head across the
restriction in tidal flow through the existing inlets (GIESE,
barrier was greater at low tide than at high tide
1988). The cycle of spit extension followed by barrier
(FRIEDRICHS et al., 1993). This is important because most
segmentation and destruction has a periodicity of
inlets are cut from the backside during a falling tide.
approximately 150 years (GIESE, 1988).
Shoreline Trends.- Historical shoreline trends for the
Historical changes in tidal exchange between the ocean
outer Cape are known from studies by GATTO (1978) and
and Chatham Harbor have been modeled by FRIEDRICHS
ALLEN et al. (1998). Their data show significant temporal
et al. (1993) using a one-dimensional nonlinear tidal
and spatial variability in cliff retreat rates north of Nauset
propagation model. Their model indicates that as Nauset
Spit suggesting that the supply of sand to the southern
Spit builds to the south, which lengthens and shoals the
barrier system may have been equally variable. Since 1938,
channel connecting the harbor and ocean, the tidal
the Nauset Spit shoreline south of Nauset Heights extending
amplitude gradually diminishes in Pleasant Bay. Applying
in front of Pleasant Bay has steadily retreated. The rate of
this model to the period between 1936 and a time just before
retreat increases with distance from Nauset Heights (Fig. 2).
the breach, they calculated that the tidal range in the harbor
Between 1938 and 1982 the barrier shoreline along northern
decreased from 2.0 m in 1936 to 0.8 m by 1986 (Fig. 3a).
Pleasant Bay eroded 70 m, while the shoreline at the 1987
The model also demonstrates that historical dampening of
breach retreated 340 m.
the tidal wave's propagation into the bay was concomitant
with a steady increase in tidal head between the ocean and
Bay Bathymetry.- Prior to the breach, flood-tidal deltas,
harbor (FRIEDRICHS et al., 1993). According to their
broad ebb spillover lobes, and intervening channels
model, by 1986 the tidal head across the barrier reached
characterized Pleasant Bay. The channels ranged in depth
0.86 m at low tide (toward the bay) and 0.70 m at high tide
from 2 to 6 m and were floored by ebb- and flood-oriented
(toward the ocean) (Fig. 3b). It is reasoned that under these
bedforms. The bay shoreline of Nauset Spit was lobate-
conditions the barrier is susceptible to breaching,
shaped and a product of recurved ridges building into the
particularly during storms when water level set up and set
bay during spit accretion. A narrow, shallow intertidal
down may increase the hydraulic head across the barrier.
platform abutted the bay shore. At the site of the eventual
breach, the bay was relatively deep and tidal shoals were
1987 Breach
absent.
During a severe northeast extratropical storm on 2
It should be noted that none of the factors by themselves
January 1987, Nauset Spit was breached forming New Inlet.
would have created the 1987 breach. As seen in Figure 3b,
After one day the inlet was approximately 100-m wide and
the modeled low tide reached a maximum hydraulic head
storm waves were breaking through the opening
across the barrier in 1966 and even the modeled high tide
(FRIEDRICHS et al., 1993, his figure 1). As the channel
neared its maximum by 1976. Thus, It can be argued that if
captured an increasingly larger portion of the bay tidal
differences in water level across the barrier were the
prism, the inlet grew in size from 0.5 km after two months
paramount cause of breaching, then it should have occurred
to almost 2.0 km wide by early 1988. A broad intertidal spit
10 to 20 years earlier. Rather, before the tidal head could aid
platform comprised 1.2 m of this width.
in cutting the barrier, an incipient channel had to be present.
At least four factors facilitated the breach: 1. Storm and
Formation of this feature was facilitated by long-term
astronomic conditions, 2. Tidal head across the barrier, 3.
erosion of Nauset Spit, which thinned the barrier eventually
Long-term shoreline trends, and 4. Bay bathymetry.
causing a segmentation of the foredune ridge. When the 1-2
January 1987 northeast storm struck the coast of Cape Cod,
the high astronomic tides coupled with a moderate storm
surge allowed waves to break high on the beach. Vestiges of
the frontal dune system funneled the ensuing wave surge
Journal of Coastal Research, Special Issue 36, 2002
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