Formatted Title:

A Multi-Pronged, Adaptive Monitoring Approach to Identify and Assess Errant Suitable Dredged Material Placement on Native Seafloor Habitats

Background/Objectives:

Although infrequent, errant release of dredged material (often termed a ‘short dump’) can occur in areas that were not permitted for disposal. Mechanical failure, adverse sea conditions, or operator error can all lead to the unintentional release of dredged material outside the permitted target location. Although only dredged material that has been tested and determined to be suitable for unconfined open-water disposal can be placed at a U.S. Environmental Protection Agency (USEPA)-designated disposal site, an errant disposal raises concerns about significant and potentially long-term unintended impacts to native seafloor habitats. In addition to understanding the cause(s) of the errant disposal, monitoring to evaluate the potential physical and temporal scale of impacts related to the disposal is critical for the U.S. Army Corps of Engineers (USACE) and USEPA to assess compliance with permit conditions and potential enforcement actions. This presentation provides two recent case studies that demonstrate an effective, multi-pronged approach to identification of the scale of an errant disposal and assessment of the potential impacts to the benthic ecosystem. One event occurred in transit to a site off the Rhode Island (RI) coast in approximately 37 meters (m) water depth, and the other in transit to a site off the New Hampshire (NH) coast in approximately 95 m water depth. Both of these sites are managed and regularly monitored by the USACE New England District’s Disposal Area Monitoring System (DAMOS) program and by USEPA Region 1. Both sites were formally designated in areas that would intentionally minimize impacts associated with dredged material disposal. Adjacent to, or along nearby transit corridors are sensitive habitats, which drove concerns related to the errant disposals.

Approach/Activities:

At both the RI and NH sites, initial multi-beam echosounder (MBES) acoustic data (bathymetry, backscatter, side scan sonar) were collected around the coordinates reported for the errant disposals. The USACE Dredging Quality Management system records allowed for design of a limited MBES screening survey. The preliminary MBES data were reviewed to identify locations within the survey area that indicated the presence of errant disposal material, such as elevation changes within the bathymetry, and textural differences in the backscatter and side scan sonar, varying from the ambient seafloor. These acoustic indicators were used to direct subsequent sediment profile and plan view image (SPI/PV) collection. An adaptative SPI/PV field sampling approach was adopted to enable flexible sampling and to optimize spatial coverage to locate and identify evidence of dredged material on the seafloor at both locations. During adaptive SPI/PV sampling, referred to as Pogo SPI/PV, the SPI/PV camera system is raised and lowered while the survey vessel is slowly moving in order to collect multiple sediment profile and plan view images in rapid succession along a transect over a larger area than a typical station radius. This approach allows for high spatial density of imaging to maximize coverage over an area of interest.

Results/Lessons Learned:

The RI case study demonstrated the use of the monitoring approach for dredged material that was a similar sediment type to the native seafloor at the release point, differing only slightly in texture and color. Coastal bivalve shells visible on the surface in the PV imagery proved to be most diagnostic of the dredged material. Without this PV ground truth data, the slight acoustic anomaly identified in the MBES data would not have provided sufficient evidence of the location and extent of dredged material on the seafloor. At the NH site, the dredged material was distinct from the native sediments, allowing the MBES data (primarily backscatter) to clearly distinguish the distribution of non-native material on the seafloor and focus the location of the SPI/PV stations. This adaptive monitoring approach provided the agencies with clear and defensible data that was successfully used to inform permit compliance and enforcement actions for both of these cases. This approach can be implemented for other purposes to cost effectively delineate changes in sediment type including environments impacted by contaminated sediment.

Aaron Hopkins

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A Multi-Pronged, Adaptive Monitoring Approach to Identify and Assess Errant Suitable Dredged Material Placement on Native Seafloor Habitats