depths in the west channel were 7 to 10 ft NAVD88. The eastern
most breach channel was approximately 325 ft wide on the sound
side and 350 ft wide on the ocean side and was the main channel
as it captured a majority of the tidal prism. The unrestricted flow
through this channel created scour depths down to 20 ft
NAVD88.
The flood shoal is readily identifiable in Figure 3. The Survey
1 DEM showed that a flood shoal formed within 2 weeks after
the breach opened with the centroid approximately 1,750 ft from
Highway 12. The shallowest water over the flood shoal was less
than 0.5 ft NAVD88 and had to be surveyed by wading, as depths
were too small even for the waverunner system. The second
survey showed little growth of the flood shoal. A volumetric
analysis indicates that the flood shoal gained on the order of
10,000 cu yd of sand over the 10-day period. A well-defined ebb
shoal also formed by 5 October (Figure 3). The ebb shoal
extended offshore as far as 1,250 ft from the former location of
Figure 4. Western edge of peat terrace, looking southeast. The
the highway. Depths over the ebb shoal were 4 to 6 ft NAVD88.
peat extends approximately 1.5 to 2 ft above the adjacent sandy
Waves were often observed breaking on the ebb shoal.
bottom.
The DEM analysis indicates rapid morphology change. Figure
6 is a comparison plot of breach cross-sections from Surveys 1
and 2. Comparisons are made only where sufficient coverage of
the channel was captured by both surveys. Due to debris and
wave conditions, the seaward end of the west breach channel was
not measured in Survey 1. As Figure 6 shows, there was little
change in the west channel, because of the substantial amount of
armoring by debris and peat. Flow velocities were significantly
weaker than observed in the main channel. The sound end of the
Figure 5. West breach channel, looking east, 5 October 2003.
Note the old bridge pilings on the right and the extensive road
debris in the channel.
best coverage of the nearshore region just seaward of the breach
openings and, combined with the ebb shoal data, provides the
most comprehensive data set. Figure 3 is a DEM created for
illustrative purposes from Survey 2 data combined with the
multi-beam ebb shoal data. Three breach channels, separated by
breach islands, are well defined. The breach islands were not
fronted the islands on the ocean side (Figure 4). The peat acted
as an erosion-resistant barrier, revetting the island at this
location. The broad peat terrace extended from the eastern most
breach island, across the middle breach channel and west breach
island (Figure 3).
The peat terrace restricted flow in the middle breach channel,
which was about 225 ft wide. Water flowed through the middle
channel and over the peat terrace only at higher tide elevations.
The restricted flow resulted in little scour of this channel.
Maximum depths in the middle channel were about 5 ft NAVD88
(1988 North American Vertical Datum). The west channel was
about 350 ft wide and littered with debris (Figure 5). Pilings
from the bridge partially constructed in the1930s extended across
the channel on the ocean side. Large chunks of asphalt and
roadbed material were visible at lower tide elevations,
particularly on the west side of the channel. Near the center of
the channel was a peat outcropping with large chunks of asphalt
Figure 6. Comparison of Survey 1 and Survey 2 breach channel
resting on top. The flow through the west channel was somewhat
cross-sections (North Carolina Department of Transportation
greater than that of the middle channel. Survey 2 maximum
(NCDOT) photograph).
Shore & Beach Vol. 72, No. 2, Spring 2004, pp. 9 - 14
11
Previous Page
