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

The five active Hawaiian Island ocean disposal sites shown in Figure 1 service the major harbors for the islands of Oahu, Kauai, Maui, and Hawaii. Each of the disposal sites is located on the submerged flanks of a volcano, and the Hilo disposal site is the only disposal site located near an active volcanic rift zone.

The Port Allen and Nawiliwili Marine Disposal Sites, Kauai

The two northern-most disposal sites are located south of Kauai and service both Nawiliwili and Port Allen harbors (Plates 1 and 2; Figures 2 and 3). Both sites are defined as circular areas with a 3000-ft (914.6 m) radius about their center points so that each site encompasses an area of 2,628,117 m2 (0.8 n mi2). The Port Allen disposal site (OD0905) has center coordinates 21° 50.00�N and 159° 35.00�W and is located about 4.8 n mi (8 km) south of Port Allen harbor (Plates 1 and 2; Figures 2 and 3).  Water depths range from about 1410 m to 1630 m with the regional slope falling away towards the southwest. The disposal site is located on steep slopes within the headwall region of a gigantic submarine landslide, the South Kauai Debris avalanche, described by Torresan et al. (1989), and Moore et al. (1989). Bottom photographs show that the site consists of a flat, sandy bottom with rocks, boulders, and cobbles (EPA 1980).

The Nawiliwili site (OD0904) has center coordinates of 21° 55.00�N and 159° 17.00�W and is located about 4 n mi (7.4 km) southeast of Nawiliwili Harbor (Plates 1 and 2; Figures 2 and 3). The site is situated on the outer edge of a narrow, northeasterly-trending shelf. The narrow shelf slopes towards the southeast and water depths over the site range from 840 to 1120 m. Bottom photographs show a rolling topography strewn with rocks, boulders, and debris (EPA 1980).

The entire southeast margin of the island of Kauai, between Nawiliwili on the east and Port Allen on the west, was surveyed with the Kongsberg Simrad EM300 multibeam system in February 1998. Both high-resolution bathymetry and quantitative backscatter were obtained over the entire margin from water depths of 100 to 3500 m (Plates 1 and 2; Figures 2, 3, 4, 5, 6, 7, 8, 9, and 10). The multibeam data reveal that the submarine flanks of the island within the study area are characterized by two distinct morphological styles that reflect processes associated with island construction and evolution.

The morphology of the region between Port Allen and Makahu�ena Point on the southwest flank of Kauai reflects the effects of gigantic submarine landslides that have shaped the Hawaiian Islands (Moore et al., 1989). The slope between Port Allen and Makahu�ena Point is the headwall region of a gigantic debris avalanche (Torresan and Shor, 1989; Moore et al., 1989). Much of the original slope material was transported away into deeper water at the time of the failure, leaving behind scars and isolated blocks surrounded by a smooth mantle of sediment (Plates 1 and 2; Figures 2 and 3).

The seafloor morphology of the upper southeast margin and flank of Kauai between Makahu�ena Point and Nawiliwili appears much different in character from the southwestern margin (Plates 1 and 2; Figures 2 and 3). The effects of extensive and catastrophic submarine slope failure are not evident along this portion of the study area. The fringing reefs situated off the southeastern corner and along the southeastern side of southern Kauai are intact (Plate1; Figure 2). The major morphologic feature that characterizes the upper-most portion of this submarine flank consists of a relatively broad and gently sloping submerged terrace (Plates 1 and 2; Figures 2 and 3). The terrace dips towards the southeast, trends northeasterly about 45 km in length, and is 10-13 km wide. Backscatter and shaded-relief images show the seaward edge of the terrace slope changes downward into a much steeper regional slope. The major submarine break in slope that occurs at the seaward edge of the submerged platform marks the position of the shoreline when the main period of shield building ceased (Mark and Moore 1987; Moore and Campbell 1987). The shaded-relief map shows that below the terrace the steeper slopes is incised and gullied (Plate 1; Figure 2). Multibeam backscatter patterns imply that sediment has migrated downslope via a dendritic pattern of conduits (canyons, gullies and chutes) incised into the slope (Plate 2; Figure 3).

continued

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