10 



2.0 APPLICATION OF LTFATE TO THE PDS 



The PDS lies in 40-70 m of water 13 km off the coast from Portland, ME (Figure 

 2), and encompasses approximately 3.5 km^. The site is described by McDowell and Pace 

 (1996) as having "rocky, irregular relief composed of a complex network of bedrock 

 outcrops, shear ridges caused by glacial scour, and small topographic depressions 

 containing namral sedimentary deposits of gravel, sand, silt and clay". Three bathymetric 

 surveys of the site, performed in 1976, 1989 and 1996 were available for this analysis. 

 Each survey covered different portions of the PDS. The 1996 survey covered 

 approximately the southern 45% of the site as well as small area surrounding the perimeter 

 of the site. The 1976 and 1989 siureys were at nearly identical locations close to the center 

 of the site. Data fi-om the 1989 and 1996 surveys were combined to create the site 

 configuration for the LTFATE simulations. These surveyed regions combined cover 

 approximately 60% of the total PDS as well as a narrow band (<200 m wide) around the 

 southern half of the PDS and form an inverted 'T' shaped configuration. For LTFATE 

 simulations, this was defined as the LTFATE PDS region of interest (Figure 3a). Although 

 this region does not cover die entire PDS, the lack of bathymetry data for the remaining 

 portions made simulation of the entire PDS unpractical. From a mound stability 

 standpoint, the area not included is probably unimportant. The stability of a mound in the 

 LTFATE PDS region would be only minimally effected by bathymetry variation in other 

 regions of the PDS. It is significant to note that all areas of the PDS that have previously 

 been used for sediment placement fall within the LTFATE PDS region. 



Surrotmding the LTFATE PDS region, a 125 m wide band was developed as a 

 transition zone and the depths in this band were gradually varied from the adjacent 

 LTFATE PDS depth to 60 m. All areas surrounding the LTFATE PDS mound and this 

 band were assumed to be 60 m in depth. No sediment was permitted to erode from the 

 band or 60 m depth regions. The grid for the LTFATE sunulations including die PDS and 

 surrounding regions is 129x118 cells. Each cell is 25x25 m^. Figure 3a is a depth contour 

 plot of the LTFATE PDS and surrounding regions. 



The fact that the site contains many large rocky outcrops indicates that sediments 

 can not erode from all areas of the PDS. Rather, most sand, silt and clays are confined to 

 the deeper channel regions between these outcrops. Therefore, for all LTFATE 

 simulations, areas within the PDS that are shallower than 55 m were assumed to be solid 

 rock and experienced no sediment erosion (Figure 3b). A 55-60 m limit for sand, silt and 

 clay is probably a reasonable estimate given known characteristics of the site. From this 

 analysis, approximately 57% of the LTFATE PDS is covered with erodible material. 



A Predictive Model for Sediment Transport at the Portland Disposal Site, Maine 



