25_ 



DAMOS Capping Model predicted the formation of a conical pseudo-UDM deposit 

 approximately 1.2 m high with flanks extending up to 250 m from the central point of 

 disposal on a flat bottom. The CDM to pseudo-UDM ratio of 0.56:1.0 was low relative to 

 standard capping operations. Typically, a sediment cap of >0.5 m in thickness is required 

 over the UDM deposits to provide a sufficient buffer against possible storm-related 

 resuspension and the burrowing of benthic organisms. For this demonstration project, the 

 thickness of capping material covering the pseudo-UDM mound was forecasted to be 20 

 cm deep. 



The accuracy of mapping the pseudo-UDM and CDM layers on the complex 

 topography of the floor of PDS was expected to be limited, because comparisons between 

 sequential bathy metric surveys are reliable only for detecting changes greater than 20 cm. 

 Single-beam bathymetric surveys conducted over the majority of DAMOS disposal sites 

 yield reliable datasets that provide strong depth difference comparisons. However, these 

 sites are located in areas of flat or gently sloping seafloor. The data collected on the 

 irregular bottom topography of the disposal sites of Maine (CADS, RDS, and PDS) tend to 

 reduce the effectiveness of the standard bathymetric data collection and processing 

 techniques. The complex topography of the Maine sites, a product of glacial scour, yields 

 depth difference plots with a significant number of survey artifacts (Sugden and John 

 1990). Survey artifacts are false indications of changes in depth due to differences in 

 average grid values resulting from variation in survey vessel track between surveys (Figure 

 2-10). Because the targeted disposal location had a flat ambient depth relative to the 

 region, survey artifacts were expected to be more limited immediately around the disposal 

 buoy, increasing the success of using single-beam bathymetry at the site.. 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



