Introduction

A-1-98-HW Survey
Methods
Regional Maps

Study Areas
Kauai (1) (2)
Oahu
Maui
Hawaii

Conclusions

Appendix 1
Table A1
Table A2

Figures
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
16 17 18 19 20
21 22 23 24 25
26 27 28 29 30

Plate 1
Plate 2
Plate 3
Plate 4
Plate 5
Plate 6
Plate 7
Plate 8

Acknowledgments

References (1) (2)

Contact:
M.E. Torresan
J.V. Gardner

Infobank Metadata
K-1-93-HW
A-1-98-HW

Backscatter from the southeast margin of Kauai between Makahu�ena Point and Nawiliwili is as complex as that observed from the southwest margin between Makahu�ena Point and Port Allen. The more elevated and lobate portions of the broad submerged terrace have high backscatter and the steep seaward edge of the submerged terrace is incised with gullies and canyons that lead to smooth, fan-shaped deposits on the lower slope (Figure 2 and Plate 1). The backscatter map in Plate 2 and Figure 3 shows a high-backscatter dendritic network that coalesces down slope into broader areas of high backscatter. The high-backscatter dendritic patterns in Plate 3 and Figure 3 correspond to the terrace-edge chutes and gullies and downslope sediment conduits visible in the shaded-relief maps (Plate 1 and Figure 2). The broad areas of high backscatter on the lower slope (A href="plate2.html">Plate 2; Figure 3) correspond to the broader, smooth, lower-slope sediment mantle visible in the shaded-relief image (Plate 1; Figure 2).

The Port Allen disposal site does not show the effects of dredged material disposal in the multibeam backscatter map (Plate 2 and Figure 3). The circular to subcircular high-backscatter features and high-backscatter blanket of dredged material seen at the Mamala Bay disposal sites are not visible at the Port Allen site (Plate2; Figure 3). Figures 4, 5 and 6 are perspective views of the Port Allen disposal site. Comparing these images to the images in Figures 12, 13, 14, 15, and 16 from Mamala Bay demonstrates the lack of detectable dredged material deposits at the Port Allen disposal site. A close-up multibeam image from the old Honolulu Harbor dredged material disposal site is shown in Figure 15. Note that the dredged material deposits off Honolulu are composed of high-backscatter circular to subcircular overprints that form a nearly continuous high-backscatter blanket over each site. Bottom sampling, photography or higher-resolution multibeam surveys are required to verify the existence of dredged material deposits at the Port Allen site.

Figures 7, 8 and 9 are close-up and perspective-views of the Nawiliwili disposal site. These figures do not show conclusive evidence of the high-backscatter features common to disposal sites seen in Mamala Bay and at other sites along the west coast (Gardner and Mayer, 1998). In contrast, Figure 10, a close up backscatter map of the Nawiliwili site shows that isolated 0.5 to 2.0-m high, high-backscatter mounds occur in the vicinity of the disposal site. In fact, the mounds are actually scattered over an area larger than the designated disposal site. These high-backscatter mounds may result from disposal activities, but, similar to the Port Allen site, bottom sampling, photography, or higher-resolution multibeam surveys are required to verify the existence of dredged material deposits at the Nawiliwili disposal site.

The lack of evidence for dredged material deposits at the Port Allen disposal site has three possible explanations. The site has received a small volume of material and the site is located in water depths greater than 1000 m. Thus, the Port Allen deposits may be so small and so deep that they are below the resolution of the 30-kHz multibeam mapping system. Second, the dredged material may be so similar to the natural seafloor sediment that they are acoustically identical. Data present in the EPA (1980) report show that the seafloor at the Port Allen disposal site is composed of silty clay. The dredged material from Port Allen and Nawiliwili Harbors is classified as mud (clayey silt/silty clay), thus dredged material contains less sand than natural seafloor sediment. It is hard to imagine that the disposal material is similar enough to the natural sediment to appear no different on the backscatter images. A third explanation for the lack of visible dredged-material deposits at the Kauai sites may be that the dredged materials disperse during settling or after reaching the seafloor, leaving behind no resolvable deposit.

continued

U.S. Department of the Interior, U.S. Geological Survey, Western Region Coastal and Marine Geology