30_ 



70° 01. 614 'W in the southeast quadrant of PDS (Figure 3-1). The Royal River Project 

 Area bathymetric survey grid consisted of 33 lanes oriented east-west at 25 m lane spacing. 

 The project area was first occupied during the August 1995 baseline survey, as well as the 

 November 1996 pseudo-UDM and February 1997 CDM bathymetric surveys. A larger 

 1950 m X 1000 m area encompassing the southern half of the disposal site was occupied as 

 part of the second baseline bathymetric and side-scan sonar survey (Figure 3-1). Detailed 

 bathymetric charts of the southern regions of PDS were generated and compared during the 

 various stages of capped mound development. 



3.3 Bathymetry 



Precision bathymetry entails the collection of depth soundings along predetermined 

 survey lanes to map seafloor topography, providing information on bottom slopes as well 

 as geological and sedimentological features. Sequential bathymetric surveys that occupy 

 the same area of seafloor are valuable in detecting and quantifying changes in bottom 

 topography over time. By calculating the changes in depth between two individual 

 bathymetric surveys (depth differencing), accumulation of disposed dredged material or the 

 reduction in mound height due to consolidation or erosion can be measured. 



The DAMOS Program generally uses single-beam bathymetry, which provides 

 precise depth data ( + 0.05% of overall depth) for the seafloor directly under the survey 

 vessel (Murray and Selvitelli 1996). For the PDS surveys, the individual soundings were 

 averaged and gridded within 12.5 m x 25 m cells, yielding a digital depth matrix. 

 Contour charts and three-dimensional representations of the bottom were then produced 

 through interpolation between gridded values. Depth difference plots are generated by 

 comparing corresponding gridded values to detect changes on the seafloor. 



Efforts to minimize the development of survey artifacts formed by differences in 

 survey vessel track or configuration within the sequential bathymetric surveys were made. 

 One research vessel, with identical survey configurations, was used to complete the data 

 collection efforts during the February 1996 baseline survey, as well as the precap and 

 postcap surveys. In addition, all four bathymetric surveys were performed with the same 

 navigation, data collection, and data processing software. 



3.3.1 Bathymetric Data Collection 



An ODOM DF3200 Echotrac® Survey Fathometer with a narrow beam, 208 kHz 

 transducer measured individual depths to a resolution of 3.0 cm (0.1 ft) as described in the 

 DAMOS Navigation and Bathymetry Reference Report (Murray and Selvitelli 1996). 

 Depth values transmitted to PINSS were adjusted for transducer depth. The acoustic 

 remms of the fathometer can reliably detect changes in depth of 20 cm or greater due to 



TJie Portland Disposal Site Capping Demonstration Project, 1995-1997 



