Conclusions

The results of the 1998 multibeam survey provide abundant evidence that anthropogenic dredged-material deposits are present at six of the seven Hawaiian disposal sites. The dredged-material deposits in Mamala Bay cover more than 100 km2 of seafloor and are more extensive than the area defined by the disposal site boundaries. In contrast, dredged-material deposits are not visible at the Port Allen Kauai site, may be visible at the Nawiliwili Kauai site, and are relatively sparse at the Kahului Maui and Hilo Hawaii sites, affecting less than 10 km2 of seafloor at each site.

The maps of the Mamala Bay seafloor depict areas of active sediment transport, drowned reef platforms and reef pinnacles, anthropogenic deposits, and volcanic features. The maps show three principal types of sea-floor material: low-backscatter natural sandy sediment and high-backscatter dredged-material deposits and drowned carbonate reefs. The disposal sites themselves appear as isolated, high-backscatter circular to subcircular features formed by individual disposal events. These high-backscatter features coalesce to a nearly continuous, high-backscatter blanket over the central portions of each disposal site. The Mamala Bay seafloor and disposal sites were the only Hawaiian disposal sites extensively sampled, and cores indicate that the low-backscatter natural sediment is muddy sand, composed of carbonate reef and microfauna debris with some volcanic grains. The high-backscatter dredged material comprises a nearly 40 cm thick, poorly sorted, cobble- to clay-size mixtures of reef, volcanic, and man-made debris. The mud-rich matrix of the dredged material is cohesive and binds the dredged material.

The high-backscatter signature of dredged material seen in Mamala Bay is not clearly defined at either the Port Allen or Nawiliwili disposal sites located off southern Kauai. The Kauai disposal sites have not been sampled. Thus, without samples, the apparent lack of visible dredged material can be explained in three ways. First, both the Port Allen and Nawiliwili sites receive about 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 multibeam system. Secondly, the dredged material may disperse during and after disposal, migrating away from the sites. Alternatively, the dredged material may be so similar in composition and particle size to the natural seafloor material that it has similar backscatter characteristics to the natural seafloor sediment that mantles the flanks of southern Kauai.

The multibeam data from the Kahului, Maui and the Hilo, Hawaii disposal sites show that dredged material disposal has produced high-backscatter features similar to those seen in Mamala Bay (Plates 5-8). But, when compared to the Mamala Bay deposits, the Kahului and Hilo sites have a relatively sparse cover of dredged material. The sparse nature of dredged material deposits at the Kahului and Hilo disposal sites is a function of the lower volumes of dredged material disposed of at those sites.

With the completion of the 1998 multibeam surveys, accurate base maps now exist for all Hawaiian disposal sites. The base maps provide information on the seafloor morphology and the distribution of dredged material and potential contaminants, and are a basis for further process-oriented investigations. Bottom sampling, seafloor photography, and oceanographic studies are now required in order to define the true extent and thickness of dredged-material deposits at the Kahului and Hilo disposal sites, and for verification of the presence of dredged material at both Kauai disposal sites. The georeferenced and coregistered digital maps derived from the surveys can be employed for site evaluation and change detection between temporally separated surveys over the same sites and for locating future sampling sites. The digital data are available on CD-ROM (Dartnell and Gardner, 1999) and the USGS website http://wrgis.wr.usgs.gov/dds/dds-55/. This report, including maps illustrations and much information regarding the Mamala Bay studies can be found at http://walrus.wr.usgs.gov/mamalabay/.

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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