133^ 



7.0 RECOMMENDATIONS 



7.1 Bathymetric Data Collection 



For areas of complex bottom topography (i.e., PDS, RDS, and CADS), a higher 

 resolution single-beam bathymetric survey grid (5 m to 10 m lane spacing) is needed to 

 reduce the effects of gridding (averaging) within a large cell, and in mm, minimize the 

 appearance of survey artifacts. This would require the collection of more soundings within 

 smaller cells, reducing errors attributed to averaging, but doubling or tripling the time 

 required to conduct the survey and process the data. 



For comprehensive, high resolution surveys, multibeam bathymetry systems have 

 the capacity to collect the necessary volume of data to provide depth difference 

 comparisons with significantly reduced error within a limited survey time. Shallow water 

 (< 100 m) multibeam systems emit up to 100 beams from a single transducer array to 

 provide complete coverage (100 to 200%) of the seafloor within a survey area. Survey 

 lanes are usually spaced 50 m to 100 m apart reducing survey time relative to single-beam 

 bathymetry. In addition, due to the 100% bottom coverage, multibeam data can be 

 gridded into cells as small as 1 m^. 



7.2 Operational Control 



Dredging and disposal operations before and during the Portland Disposal Site 

 Capping Demonstration Project contributed several variables that added to the level of 

 uncertainty in data interpretation: 



• Dredged material was already present in the area, both from historical disposal 

 towards the northwest of the Royal River Project Area, and from the 

 Harraseeket project; 



• The upper part of the CDM layer (last cap material placed) was essentially the 

 same material as the upper part of the UDM (last upper river material placed), 

 causing discrepancies in statistical analyses of core samples; 



• The relatively small volumes of both UDM and CDM made bathymetric 

 methods more subject to error, as well as increasing the uncertainty of 

 identifying thinner layers of sediment using tracer analysis. 



For the Royal River project, an area of PDS that was least impacted by historical 

 dredged material was selected prior to the initiation of the project. For fumre projects, this 

 will be the most practical approach, although among the disposal sites monitored by 

 DAMOS, finding an area completely free of historical dredged material is problematic. 

 Had the Harraseeket River material not been disposed at the site, the complication by the 

 presence of dredged material would have been restricted to the much older historical 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



