Our Seabed Frontier Challenges and Choices (1989) / Chapter Skim
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2. Seabed Processes and Activities
2. Seabed Processes and Activities
Pages 5-19
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From page 5... ...
The EEZ seabed encompasses passive and active tectonic margins and volcanic regions where a variety of tectonic processes, along with other environmental forces, such as currents, surface waves, tsunamis, earthquakes, and ice scouring affect the seabed and reshape and rework its sediments. Seabed materials include rock outcrops, boulders, coarse sands and gravels, biogenic sediments, carbonate reefs, phosphate deposits, silts, clays, gassy sediments, permafrost, hydrothermal crusts, and manganese nodules.
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From page 6... ...
include the area off the western coast of Florida, where there are carbonate sediments, and the arctic regions of Alaska, where permafrost, deepwater gas hydrates, and ice packs affect the seabed. EXTERNAL ENVIRONMENTAL EFFECTS Major environmental forces affecting the ocean floor include seismicity and active faulting at ridge crests and active margins; tsunamis, hurricanes and storm-related waves and currents in shallow waters; bottom currents in deeper water; and ice keel gouging in the high latitudes.
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From page 7... ...
~/ 1 HYDRATE GAS NEAR-SURFACE HILL SEEPS LANDSLIDE DEPOSITS GAS SEEPS SHELF EDGE /NEAR-SURFACE EROSION / DISTURBED I / SEDIMENT ROTATED BLOCKS , . / GROWTH FAULTS FIGURE 2-2 Principal geologic features of the outer continental shelf and upper continental slope offshore Louisiana and Texas.
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From page 8... ...
The topography in the EEZ, especially on continental margins and seamounts, influences the magnitude and direction of currents (Heezen and Hollister, 1984~. Episodic currents confirmed on the east coast continental rise are capable of suspending and transporting sediment by "benthic storms" that can occur several times a year (Hollister et al., 1984~.
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From page 9... ...
, the continental slopes off the Atlantic coast, in the Beaufort Sea, and off the west coast of the Bering Sea. Much of the mass wasting on the Atlantic margin occurred during low sea level stands (50- to 100-m lower than present)
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From page 10... ...
Although the bulk of sediment transport is caused by episodic events caused by large benthic storms, persistent currents result in significant long-term erosion and deposition. Knowledge of these currents is crucial to understanding dispersal of pollutants; local scour around structures, buried pipelines, and cables; stability of moored arrays or other bottom-mounted installations; the fate of dredged materials; and the possibility that sediment suspended by bioturbation, mining, or excavation will be transported elsewhere.
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From page 11... ...
Because of high turbulence in continental shelf regimes, any fine-grained suspended material tends to be well-mixed and dispersed rapidly. As discussed in greater detail later in this chapter, benthic organisms also play an important role in sediment dynamics because they may enhance the credibility of sediments by mixing and resuspending sediment or may undercut a slope by burrowing in steep canyon walls.
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From page 12... ...
. _ Shallow foundations Yes Yes Yes Yes Yes Yes Yes 1.5 to 2x foundation width Deadweight anchors Yes No No Yes Yes No No 1.5 to 2x anchor width Deep pile Yes Yes Yes Yes Yes Yes No 1 to 1.5x pile group foundations width, below tips Pile anchors Yes Yes Yes Yes Yes No No To depth of pile anchor Direct- Yes Yes No Yes Yes Yes No To expected penetration of embedment anchor; max 10 to 15 m anchors clay; 3 to 10 m sand Drag anchors Yes Yes No Yes No No No 10 to 15 m clay; 3 to 5 m sand large anchors Penetration Yes Yes No Yes Yes No No 10 to 15 m clay; 3 to 10 m sand Breakout Yes Yes Yes Yes Yes No No lx object width plus embedment depth Yes Yes No Yes No No No 1 to 5 m related to object size and water Slope stability Yes Yes Yes Yes Yes Yes No 10 to 30 m; more on rare occasions SOURCE: Rocker?
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From page 13... ...
Stability Displacements: Vertical, Horizontal Creep Scour FIGURE 2-S Interactions among environmental forcing effects, structural behavior, and foundation/sediment responses for a typical oil production gravity platform. The dynamic and static ocean structure-sediment interactions can lead to complex sediment behaviors, including degradation or enhancement of strength properties, depending on the nature of the sediment beneath and around the structure.
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From page 14... ...
Gas Hydrates Gas hydrates form when dissolved gas concentrations exceed thermodynamic solubility under local temperature and pressure. Much of the continental margin sediments could contain hydrates, although hydrates containing methane are less likely in depths less than 500 m and temperatures warmer than 7°C.
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From page 15... ...
Knowledge of bioturbation processes is important to predicting sediment response to activities such as waste and dredge disposal, since bioturbation can reintroduce materials from sediments into the overlying water and possibly disperse them over large areas. By itself, bioturbation generally acts to increase credibility of sediment by maintaining a high water content and physically moving material toward the sediment surface where it can be moved about by bottom currents.
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From page 16... ...
These detailed surveys will lead to improved terrain evaluation procedures using survey data for quantitative classification of areas. Sound scientific questions and problems related to present and anticipated uses of the EEZ require information on resource potential and assessment of bottom conditions that may inhibit or constrain development.
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From page 17... ...
Another important research area is the effect of sedimentary material passing through the guts of benthic organisms on remobilization of particle-bound materials, such as metals and organic pollutants. Feeding, irrigation, and burrowing activities of benthic animals are particularly important in shelf environments, where a substantial portion of organic matter remineralization takes place on the seabed, and nutrients are returned, with only minor time lags, to the photic zone.
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From page 18... ...
Bottom boundary layer studies relevant to erosion and sediment transport within canyons are examples of such research that can be envisaged to expand. The seafloor will also be used for deployment of sensors that measure processes and properties in the water column (Brink, 1987; Allen et al., 1987~: for example, acoustic current meters and inverted echo sounders look at topographic control of currents, warm core ring degradation, and internal wave generation.
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From page 19... ...
Research Activities Many government agencies and private industries support research related to the benthic boundary layer, the seabed, and the subseabed. Research on seabed processes is conducted by the National Science Foundation (NSF)
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