723 



5.1.2 Capping Dredged Material (CDM) 



Because of the small volume of CDM, survey artifacts associated with the 

 bathymetric data caused a higher degree of uncertainty in mapping the CDM deposit. 

 Therefore the CDM vi^as mapped using sediment-profile images and core data from the 

 postcap survey, so that a cap footprint could be estimated. Following analysis of the CDM 

 deposit is a discussion of the total mound thickness relative to that predicted by the 

 DAMOS Capping Model. 



REMOTS® data were most useful in mapping the areal distribution of CDM at the 

 project site, whereas core data often provided actual cap thicknesses at discrete locations. 

 The greatest cap thickness, of >20 cm, occurred again southeast of the buoy (Figure 5-2). 

 Core data showed the cap to range from a maximum of 35 cm near the center of the 

 mound (CTR, 508), to 20 cm within a circular area with a diameter of up to 200 m. 

 REMOTS® data indicated that CDM was present from the thinnest layers (1-2 cm) at the 

 peripheral stations located 300 m from the center, to full camera penetration at CTR. 

 Therefore, cap material was present over almost all of the survey area, except that the 

 material was not transported as far along the southern survey axis as the pseudo-UDM. 



Total thickness of both pseudo-UDM and CDM, from core data, indicated a 

 minimum of 60-70 cm in the center of the deposit. This value is a minimum because often 

 the gravity cores did not penetrate into ambient. If we assume that core penetration and/or 

 recovery was limited by the coarser ambient sediments, the thicknesses represent actual 

 total deposit thicknesses. 



The DAMOS Capping Model predicted a mound of material 1.2 m high, with a cap 

 thickness of 20 cm. The measured thickness of CDM was >20 cm in many areas, but the 

 overall mound appeared to have a lower relief than predicted. Compared to the predicted 

 spread of material from the model, the Royal River mound was about the same areal 

 dimensions, but the overall height was < 1 m. The material thickness on the seafloor, 

 however, was patchy so that discrete measurements may underestimate the overall 

 thickness of the material. To reliably approximate the thicknesses of the disposal mound 

 over the entire survey area in a region of strong seafloor topography, a much higher 

 resolution acoustic survey (i.e., multibeam) would be necessary (Section 5.3). In terms of 

 modeling a deposit on the seafloor of PDS, a more accurate prediction would require a 

 model that accounts for bottom topography and slope. In general, however, the deposit 

 was in good agreement with the model predictions. 



5.2 Tracking Dredged Material on the Seafloor 



A major goal of the Royal River Project was to select a tracer from the project area 

 that would be distinct enough to reliably identify pseudo-UDM and CDM on the PDS 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



