Figure 2. Preplacement bathymetry taken in October 1998

ERDC TN-DOER-N6

March 2000

in the overwash area. Stone et al. (1999) reported waves exceeding 9.9 m (32.8 ft) off the

Mississippi/Louisiana coast with a storm surge between 1.9-2.9 m (6.1-9.8 ft). In October,

following the hurricane, a multibeam bathymetric survey was taken of the study area and served as

the preproject baseline survey. The survey identified a trench in the side of the ebb shoal that formed

during the hurricane. The trench was within the limits of the planned placement area, so the

placement area was relocated to the west (Figure 2).

Trench

Figure 2. Preplacement bathymetry taken in October 1998

After placement of the dredged material, three additional multibeam bathymetric surveys were

collected. The first survey was conducted in December 1998 immediately after the placement

operations terminated. This survey covered the same area as the predisposal survey of October

1998. The December survey did not capture the entire trench near the placement site, so the next

survey conducted in March 1999 was extended to include most of the trench. The third survey was

taken in May 1999 and covered a even larger area (Figure 3). The surveys show that the mound

created by the dredged material is on the southwest edge of the ebb shoal and along the -9-m- (-30-ft-)

mllw contour. The mound had an irregular footprint, and the highest peaks in the mound surface

were at -8 m (-27 ft) mllw.

The placement area was a square, roughly 305 m (1,000 ft) on each side. Figure 4 shows the change

in bathymetry before and after placement. The mound is characterized by several peaks showing

the difference in depth after placement. A notable flat area is the floor of the trench, which shows

that some sediment moved there between surveys. Little change in bathymetry can be detected

outside of the placement area. From the surveys, it also appears that the mound has changed little

in shape since placement. There has been only slight rounding of the peaks during the study period