SP 



overlying pre-existing topographic features, were affected by survey artifact. The 

 sediment-profile data were examined closely to establish the thin stratigraphy of pseudo- 

 UDM and CDM in these areas. 



4.5.2 REMOTS® Sediment-Profile Photography 



Due to the small volume of capping material (22,200 m^) and the limitations of 

 sequential bathymetric surveys over the irregular seafloor of PDS, sediment-profile 

 photography was crucial in documenting the existence of a thin but distinct CDM layer 

 over the Royal River pseudo-UDM deposit. A gray silt with a high sand fraction 

 characterized the first layers of cap material extracted from the lower reaches of the river. 

 As dredging operations proceeded up-river, a dark, homogeneous silt component became 

 prevalent. Therefore, the presence of the CDM layer in the REMOTS® images was 

 determined by 1) appearance of a coarser grained sand fraction within the sediment matrix; 

 2) detection of an RPD or an identified sedimentary sequence from the previous pseudo- 

 UDM survey buried by a homogeneous, dark layer; or 3) average dredged material 

 thickness over ambient sediment (or historic dredged material) for a station during the 

 CDM survey that exceeded the depth encountered during the pseudo-UDM survey. 



We noted CDM deposition indicators at all stations within a 200 m radius of the 

 PDA buoy position, except at 200W. As in the pseudo-UDM survey, the apparent 

 thickness of the cap was often controlled by the depth of camera penetration (Figure 4-22). 

 Average cap material thickness ranged from 1 to 2 cm at the peripheral stations to full 

 camera penetration at CTR (Table 4-6). Mapping of the surficial sediment type and 

 thickness indicated the CDM apron spread to a diameter > 600 m within the northwest- 

 southeast trending trough (Figure 4-22). Multiple sediment strata (CDM over pseudo- 

 UDM, or CDM over pseudo-UDM over ambient) with buried RPDs were often visible 

 within the REMOTS® photographs obtained from the outer stations (Figure 4-23). 



The coarse sand fraction of the outer chaimel was present at many of the 33 

 REMOTS® stations, often marking the CDM/pseudo-UDM interface (Figure 4-24). 

 However, stations near the center of the survey grid tended to display layers of the finer 

 grained, dark silt at the surface, representative of the last stage of CDM from the middle 

 reach (Figure 4-25). Furthermore, thicker layers of homogeneous silt, that most likely 

 obscured the presence of a sand layer, were concentrated to the south and east of the 

 survey grid where bathymetric changes were most pronounced. 



Chunks of natural wood (limbs, branches, and bark) were visible on the seafloor 

 and in the sediment, possibly remnants of the logging or ship building industries once 

 prevalent on the Royal River. We also observed shell fragments within the sediments 

 photographed by the REMOTS® camera. A large section of common razor clam (Ensis 

 directus) shells provided further evidence of the esmarine origin of the recently deposited 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



