Study Areas, Regional Setting, and Disposal Sites

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 Kahului Marine Disposal Site, Maui

The Kahului disposal site (OD0913) is located on the northeast, submerged flank of the West Maui Volcano. Regional slope is to the north, and the center of the disposal site is located about 1 n mi (1.85 km) inshore (south) of a breach in a 15-km long, east-west trending, drowned carbonate reef (Plates 5 and 6; Figures 17, 18, 19, 20, 21a and 21b). The reef appears as a linear scarp on the shaded-relief maps (Plate 6; Figure 17), and a high-backscatter lineament on the backscatter map (Plate 6; Figure 18). The shaded-relief map (Figure 17) and perspective view (Figure 19) show that there is a prominent fore-reef moat located at the base of the reef front on the down-slope side. The reef front slope is about 1:3 and is steeper than the regional slope. Perspective views of backscatter draped over the shaded-relief are shown in Figures 19 and 20, and show the breach from a high angle and low angle view respectively, looking inshore from beyond the seaward edge of the submerged reef. Note the steep outer face of the reef, the fore-reef moat and the breach between reef segments, behind which high-backscatter dredged-material deposits rest. Note that the fore-reef moat is non existent in the area of the breach.

The Kahului Ocean disposal site, has center coordinates 21° 04.7ï¿½N and 156° 29.00ï¿½W and is located about 12.5 n mi (21 km) north of the entrance to Kahului Harbor, Maui (Plates 5 and 6; Figures 17, 18, 19, 20, 21a, 21b). The disposal site is defined by a circle with a 3000-ft (914.6 m) radius about its center point and the site encompasses an area of 0.8 n mi2 (2,628,117 m2). Water depths in the disposal site range from about 320 m on the southern boundary to 400 m on the north boundary, and the depth at the center of the site is about 365 m (Plates 5 and 6; Figures 16 and 17). The EPA (1980) reports that the bottom topography of the site is smooth, undulating, and primarily composed of silty clay, whereas the dredged material from Kahului Harbor is classified as sandy mud composed of 50% silt and roughly equal amounts of sand and clay.

The reef north of the disposal site consists of two segments: a 10-km long east segment that extends from about 12°5.5ï¿½N/156°23.4ï¿½W to 21°5.5ï¿½N/156°28.8ï¿½W; and a 4-km long west segment that extends from 21°5.5ï¿½N/156°29.9ï¿½W to 21°5.5ï¿½N/156°32.2ï¿½W leaving a 2000 m breach in the reef. The breach may be an erosional channel created during a period of lower sea level. It is unknown whether modern-day oceanographic processes transport the dredged material through the breach and out onto the broad shelf north of the site.

The backscatter map and perspective views of the seafloor show that dredged material deposits are present in and around the Kahului disposal site (Plate 5; Figures 18, 19. 20, 21a, 21b). The locus of most disposal events (clustering of high-backscatter features) is about 1 km southeast of the designated center of the disposal site and outside the site boundaries (Plate 5; Figure 18). The characteristic high-backscatter signature of dredged material is present at the Kahului disposal site in the form of about 25 to 30 roughly circular, elongate, and comet-shaped high-backscatter features over a low-backscatter seafloor (Plate 6; Figures 18, 19. 20. Figures 21a and 21b shows that some of the high-backscatter features have relief, and are mounds that range from 0.5 to 1.5 m in height. It appears from the morphologies of the high-backscatter disposal features that the circular features are the result of individual disposal events produced while the dredge vessel was holding position whereas the elongate, comet-shaped features were produced while the vessel was underway.

Detailed bottom sampling, photography, and analyses similar to those conducted by Gardner et al. (1998) at the LA-3 disposal site off San Diego, are required to delimit the full extent and thickness of the deposits. Oceanographic studies are also required to better understand the processes that have potential for post-disposal transport of dredged material.

Top of Page

For more information, contact the PCMSC team.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Suggested citation:

Torresan, Michael E. and Gardner, James V., 2000, Acoustic Mapping of the Regional Seafloor Geology in and Around Hawaiian Ocean Dredged-Material Disposal Sites: U.S. Geological Survey Open-File Report 00-124, https://pubs.usgs.gov/of/2000/0124/.

U.S. Department of the Interior
SALLY JEWELL, Secretary

U.S. Geological Survey
Suzette M. Kimball, Acting Director