Dredging Coastal Ports An Assessment of the Issues (1985) / Chapter Skim
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9. Environmental Issues
9. Environmental Issues
Pages 117-140
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From page 117... ...
A great deal of research has been undertaken in the past decade to improve our understanding of the actual physical, biological, and public health implications of dredoino and the disposal of dredged materials. _ _ _ _ This chapter reviews the accumulated knowledge and what it suggests for existing and future policies adopted to protect the marine and coastal environment, living marine resources, and public health.
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From page 118... ...
118 maintenance dr edg ing .
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From page 119... ...
Consideration of this factor often provides at least partial explanation for the substantive difference in the dredging frequency required to maintain one port as compared to another despite both having apparently similar flow and sediment supply characteristics. The combination of factors affecting sediment transport within coastal port facilities favors establishment of a controlling channel depth representing a condition of equilibrium between flow-associated transport energy and sediment supply.
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From page 120... ...
. Along several areas of the continental shelf with large estuaries, the disposal of dredged materials represents the dominant mechanism for transporting sediments from the continent to the oceans (Goldberg, 1975; Gross and Palmer, 1979)
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From page 121... ...
DREDGED MATERIAL DISPOSAL PROCEDURES Since the enactment of the variety of laws favoring reduction in the use of the ocean as a receiving area for wastes in the 1970s, the management philosophy governing disposal of dredged materials has emphasized selection of sites and procedures so as to minimize the
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From page 122... ...
Prior to passage of the Ocean Dumping Act, approximately 160 sites were used for the disposal of dredged materials within the open coastal waters or inner continental shelf of the United States. The majority of these sites are on the Atlantic and Gulf coasts (see following table)
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From page 123... ...
The availability of precision navigation and high-resolution acoustic profiling systems permits the management of ocean disposal sites to a degree not previously attainable. In combination, these systems allow sequenced placement of dredged materials at a number of specified points within the disposal area, avoiding development of prominent mounds or shoals, and permit quantitative determination of the amounts of materials actually placed within the disposal site during a given project.
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From page 124... ...
Finally, the development of precise placement procedures and associated follow-up surveys promises to provide a means of reducing the potential for biotic exposure or contaminant release from contaminated dredged materials by allowing placement of a clean "cover" or "cap" of sediments over these materials. This procedure is discussed in a succeeding section ("The Disposal Area".
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From page 125... ...
Both mechanical and hydraulic dredging operations introduce significant quantities of sediment into the water column immediately adjacent to the operating dredge. For mechanical operations in areas of moderately fine-grained cohesive sediments, concentrations of suspended materials adjacent to the dredge have been observed to exceed background levels by more than two orders of magnitude, as shown in Figure 10 (Appendix G)
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From page 126... ...
The rapid settling of these materials serves to confine the primary effects to the immediate vicinity of the operating dredge, resulting in zones of influence having characteristic spatial scales ranging from 100 to 1000 m2. The deposition of suspended sediments within this area affects particularly the filter-feeding organisms, including several species of commercial value such as oysters, scallops, and blue mussels.
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From page 127... ...
. In addition to the variety of relatively short-term effects, dredging operations may induce a number of longer-term effects associated primarily with modifications in local circulation and sediment transport following changes in channel depth and cross-sectional area.
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From page 128... ...
For large new construction dredging projects that would significantly alter channel cross-sectional areas, the potential for such changes should be carefully assessed. The Disposal Area Upland Sites and Sites Fringing the Shoreline Since the initiation of dredging in the United States in the late 1800s, upland sites and sites fringing the shoreline have been primary receiving areas for dredged materials.
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From page 129... ...
Determining the adverse effects associated with terrestrial disposal of contaminated materials and the advisability of using land sites rather than open-water sites is more difficult and controversial. Arguments favoring the use of terrestrial sites as receiving areas for contaminated dredged materials emphasize the combination of containment, the ability to observe closely any negative effects, and the relative ease with which corrective actions, such as removal and relocation, could be taken if unacceptable effects are observed.
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From page 130... ...
Before the burial options could be routinely used in a wide range of conditions and materials, however, generally applicable criteria need to be developed concerning, for example, the spread of dredged sediments along the seafloor during the discharge process, the geotechnical conditions that allow capping, and the migration mechanisms of specific contaminants. It is probably neither possible nor appropriate at this time to conclude categorically that either upland containment or subaqueous disposal is universally preferable for the management of contaminated dredged materials.
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From page 131... ...
This latter set of effects is by far the most difficult to assess, and consequently, is the least well known. As in the case of dredging-induced resuspension, a number of field studies have shown that the open-water disposal of dredged materials by hydraulic pipeline or hopper barge produces increases in suspended-material concentrations that are short-lived, and that the primary effects of these short -- lived increases are confined to the immediate vicinity of the discharge point.
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From page 132... ...
Efforts are generally made in the selection of disposal sites to avoid sensitive areas, including those that support submerged aquatic vegetation and significant concentrations of commercially important shellfish. The direct burial of the variety of benthic organisms resident within the disposal area represents the primary short-term environmental effect of dredged material disposal in open water.
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From page 133... ...
Considerations of particulate-phase contaminants often show weak correlation between sediment concentrations and body burden levels within the local biological community (Pequegnat, 1983~. A variety of studies conducted during the DMRP both in the laboratory and the field provided similar results and lead to the conclusion that for short-term effects "...impacts of dredged materials are primarily associated with physical effects and....biochemical interactions are infrequent and bioaccumulation of metals and hydrocarbons negligible" (Engler, 1981~.
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From page 134... ...
This review suggests that the major concerns remain with the disposal of contaminated sediments containing moderate to high concentrations of toxic materials. Since typically this contaminated fraction constitutes a relatively small percentage of the materials removed during maintenance of existing berths, channels, and maneuvering areas, and an even smaller percentage of the sediments associated with new construction dredging, their presence should not represent a major impediment to future port management or development plans if dredging and disposal methods can be matched to their location, type, and amount.
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From page 135... ...
Gunnison (1984) , "Efficiency of Capping Contaminated Dredged Material," Dredging and Dredged Material Disposal, Vol.
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From page 136... ...
" Wastes in the Ocean, Vol. 2, Dredged Material Disposal in the Ocean, D
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From page 137... ...
Cole (1978) , "Upland and Wetland Habitat Development with Dredged Materials: Ecological Considerations," Tech.
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From page 138... ...
(1984) , "Capped In-Water Disposal of Contaminated Dredged Material," Dredging and Dredged Material Disposal, Vol.
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From page 139... ...
(1979) , "A Model for the Estimation of the Concentrations and Spatial Extent of Suspended Sediment Plumes," Estuarine and Coastal Mar.
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