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5.3 Evaluation of Tools 



5.3.1 Bathymetry 



Rather than providing a first order footprint of the dredged material deposit, as is 

 traditional with DAMOS monitoring, single-beam bathymetry in the Royal River study 

 provided only supplementary data due to the limitations over the complex bottom 

 topography. The REMOTS® sediment-profile images and core data were useful in 

 identifying some apparent depth changes as survey artifacts. In general, small volumes of 

 sediment are difficult to track with the use of single-beam bathymetry, and even more 

 problematic in an area like the PDS. The location of the mound over the relatively flat 

 Royal River Survey Area provided more confidence in the data, and a thicker dredged 

 material deposit would also strengthen the use of the single-beam tool. Changes in depth 

 of <0.25 m are generally undetectable due to the accumulation of errors originating from 

 the motion of the survey vessel, and the various correctors applied to the raw data. As 

 discussed in our recommendations (Section 7.0), the accuracy of mapping dredged material 

 deposits on a topographically complex area such as the PDS would be greatly enhanced 

 using multibeam bathymetry. 



5.3.2 Photography: Planview and REMOTS® Sediment-Profile Imaging 



The DAMOS Program has used REMOTS® sediment-profile photography for many 

 years as a method of detecting the distribution of dredged material, as well as mapping 

 benthic disturbance gradients and monitoring infaunal recolonization stams. During the 

 capping demonstration project, the sediment-profile camera was more instrumental than 

 usual because of the limitations of the single-beam bathy metric data. The images collected 

 by the REMOTS® camera demonstrated its ability to remotely visualize, differentiate, and 

 map thin layers of sediment deposited on the seafloor that would otherwise be undetectable. 

 The REMOTS® camera also was able to document the presence of historic dredged material 

 originating from disposal operations at the DG buoy from 1990 to 1991 as well as the 

 Harraseeket River material deposited in the fall of 1995. In fumre projects, the sediment- 

 profile images may prove to be useful in distinguishing project from historical dredged 

 material, although ideally the presence of historical material will be limited. 



5.3.3 Grab Sampling and Coring 



Grab sampling and coring provided a cost-effective means to characterize the 

 pseudo-UDM sediment, and to confirm the presence of pseudo-UDM in the postcap cores. 

 The coring survey was also successful in documenting the placement of two distinct layers 

 of dredged material over ambient sediments. One problem with the cores was the potential 

 for core-induced mixing of the different layers. Storing the cores vertically prior to 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



