J6 



LTFATE. The probable reason for the significantly lower LTFATE predicted 

 concentrations compared to the SAIC data is due to the measurement by the turbidity meter 

 of sediments transported to the site from other locations that are not simulated by 

 LTFATE. The suspended solids boundary concentrations for the LTFATE simulations 

 were assumed to be zero. In reality some sediment would be advected from other sites near 

 the PDS and would settle at the PDS after the first two events occurred (creating a new 

 surficial layer for the third event). Because no data were available and because the rocky 

 outcrops may block some sediments from reaching the PDS (depending on the flow 

 direction), all boundary concentrations at inflow locations for the model were assumed 

 zero. Therefore, once the resuspended plume left the PDS, no new sediment was available 

 for deposition. When the second event occurred, there was no surficial layer at the 

 LTFATE PDS and erosion rates for the now exposed lower layers of sediment were 

 considerably less. This resulted in artificially low LTFATE predicted TSS concentrations 

 from the second event. It is interesting to note that despite the larger wave height (and thus 

 higher bottom stresses) during the second event in die calibration period, that the turbidity 

 data indicate lower TSS concentrations than during the first event. This is also the case for 

 TSS concentrations during the third and largest event (storm 7) when compared to the first 

 event (storm 5). This would indicate that despite re-deposition, the surficial layer has not 

 yet fully redeveloped from the first event. 



The maximum calculated erosion depth at any location on the PDS for the April 6- 

 18 calibration period was 1.1 cm and the total volume of sediment eroded from the site 

 was 850 m^. Figure 5 is a contour plot of the erosion from the calibration period. The 

 values of Xcr, Ao, and m were estimated from the above calibrations and are reasonable first 

 estimates. As previously stated, the lower layers of sediment were given characteristics 

 comparable to other well consolidated ocean sediments. When applied to other events 

 during the SAIC data collection period, the LTFATE simulations using the inputs 

 described above produced reasonable results. These simulations indicated erosion during 

 the events where the field data indicated increased turbidity, although the wave height and 

 thus the erosion amounts were considerably less than the events during the April 1996 

 calibration period. 



2.3 Results for Storm Calculations 



LTFATE has the ability to simulate either long (year) or short (several days) 

 periods of time. The long term simulations are used when significant erosion occurs 

 frequently. As the SAIC data suggest, normal non-storm conditions do not produce erosion 

 at the PDS (i.e., mrbidity levels converted to TSS suggest near zero concentrations and 

 therefore do not support significant erosion). Even events such as the April, 1996 storms 

 resulted in minimal computed erosion with most locations experiencing less than 1 cm loss, 



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



