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Torresan, M.E., Hampton, M.A., Gowen, M.H., Barber, Jr., J.H., Zink, L.L., Chase, T.E., Wong, F.L., Gann, J.T., and Dartnell, P., 1995, Final report: acoustic mapping of dredged material disposal sites and deposits in Mamala Bay, Honolulu, Hawaii: U.S. Geological Survey Open-file Report 95-17.


Introduction 1, 2
  Study Area
  Previous Studies
  Seafloor Materials

K1-93 Survey
  Scope of Work
  Sidescan Sonar

  Sonar, 3.5kHz 1, 2, 3


  1   2   3   4   5
  6   7   8   9 10
11 12 13 14 15
16 17 18 19 20

Plate 1

Apx 1: Statistics 1
Apx 1: Statistics 2
Apx 2: Equipment 1
Apx 2: Equipment 2

References 1, 2, 3

Sidescan sonar and subbottom profiling shows that the dredged material disposal leaves a distinct imprint on the seafloor and affects an area of about 100 km2 (figures 4, 6, and plate 1). The sonar images show that dredged material deposits are characterized by high­ backscatter, circular to subcircular footprints that are spaced up to 300 m apart at the extremities of the deposits, and coalesce to form a high­backscatter blanket over the center of disposal areas (figure 6 and plate 1). The high backscatter that characterizes the dredged material is probably due to the irregular seafloor at the sites, the number of coarse limestone, coral, and basalt clasts, and to the general hardness of the deposit. This blanket of high­backscatter dredged material covers the natural, low­backscatter sediment that mantles the Mamala Bay seafloor in the area. Examination of the sonar images (figures 5, 6, and plate 1) shows that within the high­backscatter blanket are subtle variations in backscatter that probably correspond to variations in the dredged material, especially in the abundance of coarse clasts. Associated with some of the primary dredged material footprints are lower­backscatter aprons that may represent the finer components of individual dredged material deposits (figures 5, 6, and plate 1). These aprons likely settled after the main body of a particular disposal action, forming a secondary deposit or apron. A number of other anthropogenic targets such as tires, wire cable, military ordnance, and various barrels or drums, litter the seafloor, and these are seen on bottom photographs collected in the 1977­1978 site­designation studies conducted in and around the disposal sites (Chave and Miller 1977a, 1977b, 1978a, 1978b, Neighbor Island Consultants, 1977; Tertra Tech, 1977, Goeggel, 1978). These targets are likely associated with ocean disposal activity in Mamala Bay, but most fall below the resolution of the sonar mosaic. Bedforms having wave lengths from 5 to 20 m are visible on the sidescan images (plate 1). They are primarily located in the western half of the survey area, but also occur within the disposal sites.


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Maintained by: Michael Diggles
Author: Florence L. Wong
Last modified: May 27, 2005 (mfd)

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