69_ 



bedrock outcrops, large boulders, and sediment-laden valleys on the seafloor (Figure 4- 

 10). Combined with the February 1996 bathymetric data, the side-scan sonar data depicted 

 the geologic and sedimentological features of the disposal site (Figure 4-11). The data 

 provided supporting evidence that the large basin feamre identified in the bathymetric data 

 would be the optimum area for receiving dredged material. 



4.4 Royal River Project Area Precap Survey 



Excavation of the anchorage area and upper chaimel reaches was conducted from 1 

 October 1996 to 14 November 1996 (Section 2.1). Disposal logs indicated an estimated 

 barge volume of 39,500 m^ of pseudo-UDM was deposited to the south and southeast of 

 the PDA buoy. The precap (pseudo-UDM) survey, consisting of precision bathymetry, 

 REMOTS® sediment-profile photography, and sediment grab sampling, was completed in 

 mid-November to determine the height, size, and shape of the pseudo-UDM deposit, as 

 well as track the distribution of dredged material within the project area. 



4.4.1 Bathymetry 



Following the deposition of pseudo-UDM, SAIC performed a third bathymetric 

 survey over the 800 x 800 m Royal River Project Area on November 20, 1996. Although 

 no significant changes were clearly evident in the standard bathymetric chart generated for 

 the project area, depth difference comparisons between the February 1996 baseline and the 

 November 1996 precap survey did detect an accumulation of material to the south and 

 southeast of the PDA buoy position (Figure 4-12). As predicted, survey artifacts occurred 

 in the depth difference plots in regions with a highly irregular topography, corresponding 

 to the strong slopes of bedrock outcrops to the south and west documented by the side-scan 

 sonar results. Because in many areas the apparent thickness due to survey artifacts was 

 greater than the accumulation around the PDA buoy (> 1 m), a definitive description of the 

 UDM material footprint required integrating both the bathymetric and sediment-profile 

 camera data (Section 4.4.2). 



The accumulation of pseudo-UDM tended to follow the confines of the PDS bottom 

 topography with the detectable footprint of the mound remaining within the namrally 

 occurring basin feamre (Figure 4-13). The bathymetric profile of the UDM deposit 

 showed two east-west oriented lobes of material approximately 1.25 m high. These 

 apparent lobes, however, may have been compromised by survey artifacts. After analysis 

 of sediment-profile data (Section 4.4.2), the eastern lobe, present over a pre-existing 

 topographic feamre, was likely a result of survey artifact. 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



