J2 



values and change in values with depth of Xcr, Ao, and m in equation 21. These erosion 

 potential data determined from cores can also be used to quantify variation in sediment 

 parameters between the placed and natural sediments for the area. 



From the above discussion, it can be seen that the TSS data collected by SAIC are 

 an excellent indicator of surficial layer critical shear stresses in the region near the PDS, 

 but do not necessarily result in an accurate indication of the erosion rates of placed or 

 namral sediments at the PDS. Therefore these data were used only to produce critical shear 

 stresses for the surficial sediments and to serve as a general indicator of the upper bounds 

 of erosion firom the surficial sediment layer at the site. Calibration of the model using these 

 data was further complicated by the fact that LTFATE is a two-dimensional, vertically 

 integrated model and the data were near-bottom concentrations. Additional TSS data 

 further up in the water column from the bed would have produced a total water column 

 load which could be converted to vertically averaged concentration estimates for calibration 

 of the two-dimensional model. It is, however, unlikely that TSS concentrations at mid- 

 depth in the PDS would have been elevated during the weak to moderate storms of the 

 SAIC data collection period. When comparing the near bottom TSS field data to model 

 estimates of vertically averaged TSS, the model results are expected to be lower in light of 

 this difference. 



2.1 Model Forcing/Calibration 



Hydrodynamic boundary conditions input to LTFATE include vertically averaged 

 currents at the boundary, wave height, wave period, and tidal elevation (although in these 

 water depths tidal elevation contributions will be negligible). The period of April 6-18, 

 1996, was chosen as the furst period for model calibration. This period included three 

 events (as determined by wave heights at the buoy). These events are defmed as storm 

 numbers 5, 6 and 7 by McDowell and Pace (1997). The first event, on April 8 (storm 5) 

 included maximum significant wave heights of 3.5 m. This event was followed closely by 

 another on April 10 (storm 6), which included maximum significant wave heights of 3.9 

 m. The third event, on April 16 (storm 7), was the largest with maximum significant wave 

 heights of 5.4 m (Figure 4). Other events during the SAIC data collection period were also 

 simulated, but the events were not as large as the April calibration period. Two LTFATE 

 hydrodynamic input data sets were developed for this period. Both used as input the wave 

 height and period from the buoy. Two current velocity data sets were available for 

 LTFATE boundary condition development. The first is the data from the SAIC near 

 bottom current meters. These data were used as the vertically averaged velocity for the 

 first data calibration period LTFATE input data set. The second input data set was 

 developed using results of the ADCIRC simulation of tidal currents for the same period. 

 The two velocity inputs were similar in magnimde, generally under 15 cm/s (current meter 



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



