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Benthic Habitat Mapping
Disposal Site Selection
An important consideration for any dredging project is what to do with the dredged material. Preparation of an Environmental Assessment or Environmental Impact Statement by federal agencies is crucial to the process of disposal site selection. These reports provide an evaluation of site characteristics and attempt to predict the suitability of each site being considered for the disposal of dredged material. Characteristics of the disposal site to consider include water depth, topography, physical and chemical nature of the sediment, and relative abundance of various benthic habitats (including the presence of submerged aquatic vegetation, reefs, or unique, rare, or critical habitats). Additional considerations are the disposal site's potential for recolonization by benthic organisms, the impact of any previous disposal operations, and whether the site is erosional or depositional in nature. A variety of benthic mapping techniques are used to choose disposal sites. Bathymetric and side-scan sonar surveys are useful for determining broad-scale physical characteristics and general water depth and topography of the seafloor. Sediment cores and benthic grabs can then be used at individual sampling locations to provide detailed information on sediment grain size, chemical composition (including contaminants), and benthic community structure. Photographic techniques are also used, especially sediment profile imaging (SPI).
How
is benthic mapping used in disposal site selection? One approach to selecting a dredged material disposal site involves first using SPI to evaluate whether the area under consideration is a depositional or an erosional environment. It is often desirable to locate disposal sites in low-energy, depositional seafloor environments. In these conditions, long-term containment of dredged material is expected, thereby limiting movement of dredged material and impacts to nearby resources. If SPI images show a silt-clay bottom, that area is likely to be a good candidate for retaining fine-grained dredged material. On the other hand, if the SPI images show a current-rippled sandy bottom, the area is likely to be a higher-energy, erosional environment. Once the depositional nature of an area is determined, SPI is further used in conjunction with sediment grab sampling, bathymetry data, side-scan sonar data, and current meter data to address a variety of other concerns regarding site suitability. A disposal site must have an area of bottom that is large enough to accommodate the dredged material, and the bottom sediment must be similar in grain size to the disposed sediment. The site also must not support large numbers of commercially important species, and the benthic habitat must not be unique, rare, or of critical significance. All of these factors can be evaluated with benthic habitat mapping techniques. A geographic information system (GIS) allows for the layering of different types of benthic habitat data to form a more comprehensive benthic habitat map. GIS was used in the Rhode Island and Massachusetts surveys of candidate disposal sites to show the relationship between topography and sediment types. First, candidate sites were chosen based on the presence of depressions in the sediment identified using bathymetric surveys. Such depressions indicate high-volume capacities and high potential for long-term containment. SPI and grab samples were used to determine sediment grain size. Sites with both depressions and soft, muddy, fine-grained sediments were considered preferable for disposal. The SPI and grab sampling also provided information on benthic communities and benthic habitat quality. This information was then used to select the most appropriate disposal sites. Existing benthic habitat maps can also be useful in selecting sites for dredged material disposal, especially if there are concerns about particular habitats. In Massachusetts, state law requires that eelgrass beds and the surrounding area (38.5 meters) be protected. With the use of GIS, the state's eelgrass data were combined with other information and incorporated into the site-selection process.
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