USGS Western Coastal and Marine Geology
Pacific Coastal and Marine Science Center

Fate of Dredge Disposal Material and Polluted Sediment, Offshore Honolulu, Hawaii

Maps & Images
Raw Data


Color 3D Perspective Views Of Oahu Southern Shelf

Computer generated images of the seafloor and coastal area of south Oahu showing seafloor relief and backscatter patterns. The colored patterns on the seafloor are a measure of acoustic energy that was backscattered during a multibeam survey conducted in 1998. Generally, the patterns denote geologic character of the seafloor with reddish-brown patterns showing higher levels of backscatter or greater hardness (outcropping rock or sand), whereas the greenish-yellow patterns indicate less energy returned from the seafloor and therefore probably softer areas (muddy sediment). The seafloor relief in the image is based on very accurate bathymetric data collected by the USGS in 1998 with a 30 KHz multibeam system. Vertical resolution of the data is on the order of 50 cm; horizontal resolution is water depth dependent and varies from 6 to 16 m. The black indicates seafloor areas not imaged in 1998. The land image is derived from a USGS digital elevation model (DEM).

Western Mamala Bay
View direction is to the north and the black, top edge of the seafloor in the image is the shallow limit of mapping (about 60m). Points (A) and (B) delineate submerged reef terraces. The scale across the bottom of the image is appx. 7.5 km. The vertical exaggeration is 5x.
Ewa Wall
View direction is to the north. The top edge of the image is the shallow limit of mapping (about 60m); depths in the foreground exceed 600m. Points (A) and (B) delineate submerged reef terraces. Point (A), referred to as the Ewa Wall, is the location of a Pisces V submersible dive in August 1998 to examine the physical settings of habitats of commercially important bottom-dwelling fish. Location (C) is a zone of high back-scatter (red color) at high relief, indicating exposed reef limestone outcrops. Locations marked as (D) show typical examples of outcrops surrounded by a moat. Large-scale bedforms (E), probably sandwaves, are common along the South Oahu margin. The scale across the bottom of the image is appx. 4.5 km. The vertical exaggeration is 5x.
Central Mamala Bay
The submerged reef terrace (A) at 60 to 100 m depth is the dominant feature of the insular shelf edge. High backscatter patterns (reddish-brown) indicate rock exposures at (B), and patches of coarse dredge material at (C). Dredged material is a poorly sorted deposit of sand, gravel, and larger clasts of limestone and reef material bound by a cohesive matrix of grey mud. Dredge material is strongly contrast by the natural, light-colored muddy sand. The scale across the bottom of the image is appx. 12.5 km. The vertical exaggeration is 5x.
Disposal Area
Close-up view of the dredge and construction material disposal site (A) in Mamala Bay. The site has a high backscatter pattern from coalescing of numerous individual disposal operations. At the periphery of the disposal site are numerous individual high backscatter patches (B), each interpreted as the result of a single dump. Water depth is about 450m in the site. Note the large bedforms (C) and moats around individual outcrop blocks (D), both indicating strong bottom currents. The scale across the bottom of the image is appx 3 km. The vertical exaggeration is 5x.
Koko Head Trend
View is to the northeast. The insular shelf edge is dominated by a reef terrace (A) with numerous re-entrants. Seaward of Diamond Head is an extensive area of exposed rock and rubble (B), possibly the result of a landslide. Farther east is a volcanic ridge (C) built seaward in alignment with the Koko Head and Hanauma volcanic trend (C'). Diamond Head and Koko Head were formed very late in Oahu's history and the offshore ridge probably formed at that time, explaining why it is youthful in appearance and unburied by sediment. Numerous shallow channels (D) trend downslope from the reef escarpment. The channels are conduits for sediment transport off the insular shelf and out to the Mamala Bay seafloor. Transport in the form of sediment gravity flows likely occurred during lower sea levels. The scale across the bottom of the image is appx. 9 km. The vertical exaggeration is 5x.
East Oahu Bedforms
View is to the northwest. The scalloped reef terrace dominates the upper margin, and large bedforms are the principal feature on the slope and basin floor (A). The bedforms are long crested, straight to curved, and spaced several 100's of m. Heights typically range from 3 to 5 m. The bedforms, like those further to the west in Mamala Bay, indicate relatively strong near-bottom flows to the west and northwest. The scale across the bottom of the image is appx 3.5 km. The vertical exaggeration is 5x.

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For more information, please contact:
Mike Torresan

U.S. Geological Survey
2885 Mission Street
Santa Cruz, CA 95060

U.S. Department of the Interior | U.S. Geological Survey | Western Coastal & Marine Geology
Maintained by Laura Zink Torresan
Last modified 11 October 2005 (lzt)