n 



SAIC collected an extensive data set including near bottom ( < 1 m) current 

 velocity, wave height, wave period and turbidity (converted to TSS concentrations) from a 

 bottom resting tripod at one site at the southwest comer of the PDS for the period February 

 27 to May 14, 1996 (McDowell and Pace, 1996). The turbidity data were collected hourly 

 at 33 and 81 cm above the bottom and the current velocities were collected at 0.5 Hz 

 intervals for a one minute period every ten minutes using an InterOcean S-4DW electro- 

 magnetic current meter (McDowell and Pace, 1996). Wave height and period data were 

 also collected at NOAA buoy 44007 in deeper water approximately 6.4 km southwest of 

 the PDS (Figure 2). The data collection period included nine moderate storm events, with 

 significant wave heights ranging from 2-5 m. The data indicate elevated turbidity during 

 seven of nine storms, with highest suspended solids concentrations occurring during the 

 two storms with significant wave heights in excess of 3m. The near bottom average current 

 velocities were not affected by the onset of events, but were rather consistent in magnitude 

 for the entire period of measurement. Therefore the increased turbidity associated with 

 events can be attributed to either the increased bottom orbital velocities at the site or 

 sediment suspended in shallower water and advected to the site. The data indicate that 

 peaks in concentration were associated with peak orbital velocities. This lack of a lag 

 factor between peak orbital velocities and concentrations indicates that the increased 

 turbidity is due to local suspension of sediments at or near the PDS (McDowell and Pace, 

 1996). This information is an excellent indicator for estimating the critical shear stress for 

 the surficial sediments at and surrounding the PDS. 



However, these data do not give a complete picture of surficial sediment erodibility 

 at the PDS. Further data would be necessary for this. Because LTFATE is a localized 

 model, calculations of TSS concentrations at the site, in this case the PDS, are heavily 

 dependent on boimdary concentrations used as model input. These data are generally 

 acquired from a second turbidity measurement some distance (and preferably updrift) from 

 the site. In addition, placed sediments are probably physically and chemically different 

 from the surrounding sediments. Therefore, the two sediments would erode at different 

 rates. The fraction of the TSS measured by the turbidity meters from placed verses natural 

 sediments is unknown, and would be difficult to determine. It is also well known that 

 critical shear stress for initiation of suspension varies with depth. As previously mentioned, 

 the SAIC data set provided data for estimating the critical stresses for the uppermost layer. 

 However, the data do not provide information concerning material below this surficial 

 layer. A more accurate picture of the varying rates can be acquired by collecting 

 undismrbed core samples of the sediments of interest and performing various laboratory 

 experiments to estimate the resistance to erosion and the rates of erosion as a function of 

 depth of burial. These data, although expensive to collect, are often used to calibrate 

 cohesive sediment transport models like LTFATE, i.e., they are used to determine the 



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



