17_ 



and some of this loss would have been diminished by additional deposition had outside 

 sediment advected to the site. Therefore, to determine site erosion potential, only the 

 largest events (and not entire years) need be considered. To this end, the WIS hindcast data 

 for the Atlantic coast (Brooks and Brandon, 1995; Tracy and Cialone, 1996) were analyzed 

 and the five largest storms based on significant wave height for the period simulated (1976- 

 1994) were chosen to determine the erosion potential from the PDS. The station selected to 

 reflect the deep water wave height used as LTFATE input was WIS station 100. This 

 station is located in 100 m of water at 43.50 degrees north latimde and 69.75 degrees west 

 longimde, approximately 42 km directly east of the PDS (Figure lb) and is exposed to the 

 same fetch as the PDS. The model shoals the deep water waves to reflect what the wave 

 heights would be at the PDS, although at this site (with 40-70 m water depths) shoaling 

 does not significantly reduce deep water wave heights. These shoaled wave values, which 

 vary within the PDS depending on water depth at each location (cell), are then used in the 

 simulations to determine bottom orbital velocities. The five storms chosen for simulation 

 are detailed in Table 2. The maximum deep water wave height for any storm was 14.8 m 

 (reduced by shoaling less than 15%). An event with this magnitude of wave height will 

 clearly have a significant impact on the sediment bed at 60 m. The current conditions and 

 tidal elevations necessary for LTFATE inputs for each of these storms were developed 

 using the ADCIRC circulation model described previously in the text. Each event was 

 simulated with time varying wave and hydrodynamic forcings described above. 



Table 2 includes a summary of the findings for the LTFATE modeling of the 

 storms. January and February 1978 storms were combined into one 34 day period (with 

 two peaks in the wave height) because the events were so close that they can reasonably be 

 modeled as one large event (i.e., sediment layers were not reset between the two events). 

 These two storms had the largest estimated wave heights for the entire WIS hindcast, 

 measuring 14.8 m and 12.1 m respectively for each of the storms. As can be seen, the 

 maximum depth of erosion from any of these events is approximately 0. 1 1 m during the 

 1978 storms. This is a significant amount of erosion, but reasonable considering the 

 magnimde of storm. The total volimie of erosion from this storm was 1.4x10^ m^. The 

 other events, with maximum wave heights less than 11m resulted in 0.05-0.08 m 

 maximum depth of erosion and between 6.9x10'* and 9.6x10'* m^ of erosion from the site. 

 Figure 6a is a contour plot of erosion depths at the LTFATE PDS for the 1978 storms and 

 Figure 6b is the results for the more modest 1979 storm. It should be stated that this is, for 

 the most part, the gross erosion. The net erosion would likely be less because of the influx 

 and re-deposition of sediments eroded from areas outside the PDS. The assumed zero TSS 

 concentration at inflow boundaries does not permit as thick a new layer of sediment to 

 replenish the eroded bed compared to the case where a non-zero TSS concentration is 

 specified. The model does, however, simulate redeposition of sediments eroded at the 

 PDS. This does have an impact on the total volume of erosion. For example, total erosion 



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



