Final report: Acoustic mapping of dredged material disposal sites and deposits in Mamala Bay, Honolulu, Hawaii
No abstract available.
Hawaii↗
Geology topics
J. H. Barber
Publications and source records attributed to J. H. Barber.
K1-95-HW: Cruise report 1995 - Preliminary results, phase III: Sediment chemistry and biological sampling survey
Mamala Bay, off the south shore of the island of Oahu, has been used as a repository of dredged material primarily from Pearl and Honolulu Harbors for over a century. The U.S. Geological Survey, U.S. Army Corps of Engineers, and the U.S. Environmental Protection Agency are conducting an integrated study on the distribution and character of dredged materials as well as the effects of dredged material on the marine environment. A three phase study is providing information to evaluate the effects on seafloor substrate and the benthic fauna. The studies include geophysical profiling and imaging, bottom photography, sampling, chemical and physical analyses of sediment, and evaluations of the benthic population, population density, and adverse impacts to the benthic fauna. Phase 1, conducted in 1993, inventoried the seafloor via remote sensing. Sidescan sonar and subbottom profilers characterized the seafloor in and around the disposal sites, and the resulting products reveal the character and extent of the dredged material. These data were used to plan Phase 2 in 1994, a sampling program that employed subbottom profilers, video and still photography, and seafloor sampling to ground truth the sonar mosaic and identify the seafloor substrates responsible for the various acoustic signatures on the sonar images and subbottom profiles. Box coring provided the samples necessary to distinguish dredged material from native sediment, and for the chemical analyses used to determine contaminant concentrations. Phase 3 studies conducted in June of 1995 consisted of box core sampling for chemical and biological analyses. Specific studies include: infaunal taxonomy and population density, bioassay/bioaccumulation, sediment chemistry, and post-disposal resuspension and transport. The 1995 survey, conducted June 14 through 17, resulted in the collection of 39 box cores from 20 different stations. Multiple box cores were composited at 7 different locations occupied in 1994, to provide the material required for the 7 bioassay and bioaccumulation analyses currently underway (Figure 1). Seventeen of the 20 stations occupied provided the biological samples for the benthic infaunal identification and population density study conducted by Dr. Julie Brock of the University of Hawaii, and the sediment chemistry analyses conducted (and completed) by Quanterra Environmental Laboratories (Figure 1). Seven of the 20 stations occupied in 1995 were occupied in 1994, and provide the data for direct comparison of sediment chemistry at the same sites from two consecutive years. The sum total of the data collected from all three phases of the monitoring program will provide the U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency with the information required to make informed decisions as to the management of the South Oahu disposal site in Mamala Bay.
Hawai'i↗
Gray whale and walrus feeding excavation on the Bering Shelf, Alaska
Sidescan sonar has been used to delineate benthic feeding structures of the California gray whale ( Eschrichtius robustus ) and Pacific walrus ( Odobenus rosmarus divergens ) on the northeastern Bering Shelf. The gray whales (average mouth length, 2.0 m), when suction feeding on infaunal amphipods, create shallow pits in the sea floor, typically 2.5 m x 1.5 m x 10 cm deep, which are distinct and mappable on sidescan sonographs. Similarly, walrus, when foraging for shallow clams, create long, linear feeding furrows that average 47 x 0.4 x 0.1 m (length-width-depth). The distribution of the whale pits over 22,000 km 2 of the Bering Shelf closely matches 1) sightings of feeding whales identified by mud plumes; 2) the distribution of ampeliscid amphipods, the gray whale's main prey; and 3) the distribution of a transgressive inner-shelf fine sand that serves as a substrate for the amphipods. The walrus' furrows are recognized over 6,600 km 2 of variable muddier or coarser-grained substrate with clam-rich benthic communities that surround the fine sand substrate of whale feeding areas. The whale feeding pits are commonly enlarged and oriented by seasonal storm-related scour. Nonenlarged pits (less than 5.3 m 2 in area) form a discrete statistical population that we define as fresh . We estimate that a minimum of 5.6 percent (1,200 km 2 ) of the feeding area of the northeastern Bering Shelf (22,000 km 2 ) was covered by fresh pits made by whales during the 1980 feeding season. Assuming that the average pit depth is 10 cm, a minimum of 120 x 10 6 m 3 (172 x 10 6 metric tons) of sediment, equivalent to about three times the yearly sediment load of the Yukon River, is excavated and injected into the water column by as many as 16,000 gray whales feeding in northeastern Bering Sea each season. As a result of 1) sediment resuspension by whales, 2) average current speeds of 10.7 cm/s northward during the feeding season, and 3) enhanced post-feeding current scour because of bottom roughening, the following occur: the majority of the clay fraction (4.3 x 10 6 metric tons) of resuspended sediment is advected to the Chukchi Sea each year; sand gradually is transported northward and fills old feeding pits; modern mud does not accumulate in this region; and the whale-disturbed sand lacks physical sedimentary structures and matrix mud. Walrus feeding features are smaller, formed in higher-energy environments, and modified more rapidly than whale feeding pits. The amount of sediment reworking by walrus feeding may nearly equal that of whale feeding, but this cannot be quantified accurately.
Journal of Sedimentary Petrology↗