|Acoustic Mapping of the Regional Seafloor Geology in and Around Hawaiian
Ocean Dredged-Material Disposal Sites

by Michael E. Torresan and James V. Gardner
U.S. Geological Survey Open-file Report 00-124
2000This report is preliminary and has not been reviewed for conformity
with U.S. Geological Survey editorial standards. Any use of trade names
is for descriptive purposes only and does not imply endorsement by the
USGS.
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|Summary
IntroductionA-1-98-HW Survey
Methods
Regional MapsStudy Areas
Kauai (1) (2)
Oahu
Maui
HawaiiConclusionsAppendix 1
Table A1
Table A2Figures

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 8AcknowledgmentsReferences (1) (2)Contact:
M.E. Torresan
J.V. GardnerInfobank Metadata
K-1-93-HW
A-1-98-HW | 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
(Figure 1). 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 3 and 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 (Plates 5, 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).continued
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--|URL: http://geopubs.wr.usgs.gov/open-file/of00-124/

Maintainer: Susan A. Cochran
Last modified: 12 Dec 2000
U.S. Department of the Interior,
U.S.
Geological Survey,
Western Region
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