USGS Coastal & Marine Geology
Wong, F.L., Hamer, M.R., Hampton, M.A., and Torresan, M.E., 1996, Bottom Characteristics of an Ocean Disposal Site off Honolulu, Hawaii: Time-based Navigational Trackline Data Managed by Routes and Events: Redlands, California, Environmental Systems Research Institute, 1996 ESRI Users Conference Proceedings (cdrom), approx. 15 p.

GIS Analysis
Results and Conclusions
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This report focuses on the use of geographic information systems (GIS) tools to manage and display data collected from geologic surveys in Mamala Bay (offshore of Honolulu, Hawaii), a dredged disposal area. One data set, still photographs of the sea bottom collected in 1994, is used to demonstrate these tools. Observations from the photographic data provide information about current and circulation patterns in the area. These patterns, in turn, are used to infer sediment movement and help address questions about the fate of the dredged material.

Pearl and Honolulu Harbors, on the south coast of the island of Oahu, Hawaii, are dredged intermittently to maintain navigability for commercial and strategic purposes. These dredged materials have been disposed of just offshore in Mamala Bay for more than a hundred years. Since 1993, the U.S. Geological Survey (USGS), U.S. Army Corps of Engineers (USACE), and the U.S. Environmental Protection Agency have studied the dredged materials and any associated contaminants for their impact on the marine environment (Hampton and others, 1995; Torresan and others, 1995, Torresan and others, 1996).

Shaded relief map of Mamala Bay. Sidescan-sonar image. Mamala Bay consists of a narrow insular shelf bounded by a steep escarpment that drops from 50 to 250 m water depth on the seaward edge of the shelf. This escarpment borders a broad southwest-trending trough that gently deepens from 250 to 650 m depth in the study area (Figure 1). USACE has designated three areas within Mamala Bay for the disposal of dredged material. The South Oahu site (SO in Figure 1) is the only currently active area. The examples in this report, however, focus on one of the previously active sites, Old Honolulu (OH in Figure 1). The distribution of the dredged material is easily identifiable in sidescan-sonar images (collected by the USGS) because of its high-backscatter signal (Figure 2). Deposits of the dredged material range from coalesced masses in the target disposal sites to solitary blobs on the fringes of the disposal areas where individual unloadings are evident as circular patches 25-150 m in diameter (Torresan and others, 1995).


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Maintainer: Florence L. Wong
Last modified: 01 Oct 97.

U.S. Department of the Interior, U.S. Geological Survey, Western Coastal and Marine Geology