Summary

During January and February 1998 the U.S. Geological Survey Coastal and Marine Geology Team (USGS) conducted regional high-resolution multibeam mapping surveys of the area surrounding EPA-designated ocean disposal sites located offshore of the Hawaiian Islands of Oahu, Kauai, Maui, and Hawaii.

Location of Hawaiian ocean disposal sites

Figure 1. Location of Hawaiian ocean disposal sites.

The sites are all located within 5 nautical miles of shore on insular shelves or slopes. Regional maps were required of areas much larger than the disposal sites themselves to assess both the regional seafloor geology and the immediate vicinity of the disposal sites. The purpose of the disposal site surveys was to delimit the extent of disposal material by producing detailed bathymetric and backscatter maps of the seafloor with a ± 1 m spatial accuracy and <1% depth error. The advantage of using multibeam over conventional towed, single-beam sidescan sonar is that the multibeam data are accurately georeferenced for precise location of all imaged features. The multibeam produces a coregistered acoustic-backscatter map that is often required to locate individual disposal deposits. These data were collected by the USGS as part of its regional seafloor mapping and in support of ocean disposal site monitoring studies conducted in cooperation with the US Environmental Protection Agency (EPA) and the US Army Corps of Engineers (COE).

The multibeam shaded-relief and acoustic-backscatter maps produced from these surveys are shown in Plates 1-8. The characteristic high-backscatter signature of dredged material so evident at the disposal sites located in Mamala Bay, Oahu (Plate 4) is not seen on the backscatter images of the Port Allen and Nawiliwili sites, Kauai (Plate 2). Because of the lack of core samples from the two Kauai sites, the lack of resolvable dredged-material deposits on the backscatter maps can be explained in three ways. (1) The Port Allen and Nawiliwili sites received less than 1% of the volume of material disposed of in Mamala Bay and both sites are located in water depths greater than 1000 m (EPA 1980). Thus, the Kauai dredged material deposits may be so small and situated in water so deep that they are below the resolution of the EM300 multibeam system. (2) The dredged material may have been dispersed away from the sites during and after disposal. (3) The dredged material may be so similar to the natural sediment in composition and particle size that they have similar 30-kHz backscatter characteristics.

The South Oahu disposal site has received over 90% of all Hawaiian dredged material (EPA, 1980). The South Oahu disposal site is located in Mamala Bay adjacent to Honolulu (Plates 3 and 4). The USGS mapped all of Mamala Bay and the south coast of Oahu from about 50- to over 1000-m water depths, extending about 8 nautical miles offshore. The resulting seafloor maps depict areas of active sediment transport, drowned reef complexes, dredged-material deposits, and volcanic features (Plates 3and 4).

The 1998 multibeam data reveal the same seafloor features shown in the 1993 sidescan sonar data (Torresan et al., 1995a; Hampton et al., 1997) collected over the same Mamala Bay disposal sites. Both multibeam and sidescan sonar depicts three principal types of sea-floor material: low-backscatter natural sandy sediment, high-backscatter dredged-material deposits, and high-backscatter drowned carbonate reefs. The disposal sites appear as isolated, high-backscatter circular to subcircular features formed by individual disposal events. The disposal deposits coalesce to a nearly continuous blanket over the central portions of each disposal site. Large bedforms, imaged in 1993 with sidescan sonar and 3.5-kHz profiles, show up in greater detail in the multibeam maps. Cores show that the dredged material is composed of a poorly sorted cobble- to clay-size mixture of reef, volcanic, and man-made debris. The dredged material is bound together by cohesive gray mud. In contrast, natural sediment is muddy sand, composed of carbonate reef and microfauna debris with some volcanic grains.

The multibeam data collected from the region surrounding the Kahului, Maui, and Hilo, Hawaii sites show that dredged materials display high-backscatter features similar to those seen in Mamala Bay deposits (Plates5, 6, 7, and 8), and disposal sites off southern California (Torresan et al., 1995; Hampton et al., 1997; Gardner and Hughes Clarke, 1998; Gardner and Mayer, 1998). The Kahului and Hilo deposits (<10 km2) are extremely small and relatively sparse when compared to the Mamala Bay deposits (>100 km2). This is probably a function the lower volumes of dredged material disposed of at the Kahului and Hilo sites, which account for only about 1% and 2% respectively of the total material dredged from all Hawaiian harbors (EPA 1980).

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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