Figure 4. Western edge of peat terrace, looking southeast.

OVERVIEW OF BREACHING PROCESS

WATER LEVEL MEASUREMENTS

Monitoring_Morphology_and_Currents
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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

washed away because of resistance of peat outcroppings that

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

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