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3.0 CONCLUSIONS 



Despite the deep water at the PDS, moderate erosion due to wave action is possible 

 and should be factored into cap design. Modest events, such as the ones experienced at the 

 site during the 1996 data collection period apparently pose little threat to mound stability. 

 Model predictions of approximately 1 cm maximum erosion depth would be less with 

 redeposition of sediment advected to the site from other locations. Greater erosion is 

 confined to the few episodic events with return periods of several years. WIS hindcasts 

 indicate that the region is susceptible to very large waves. Orbital velocities produced by 

 these waves result in substantial shear stress at the PDS. It would appear that the erosion 

 even from deep water waves on the order of 14.8 m would not penetrate more than 0.22 m 

 using conservative values for the erosion parameters, and only 0.11 m using the bottom 

 layer parameter values determined for San Francisco Bay sediments. These erosion depths 

 would be even lower if re-deposition of sediments transported from other sites were 

 included in the simulations. 



As previously stated, the values of Ao can vary over orders of magnitude, but the 

 higher values are reserved for freshwater lakes and rivers and values deemed reasonable 

 for open ocean sediments were used for this modeling project. There is still considerable 

 room for variability in the value of Ao and this variation would significantly alter the model 

 erosion estimates. An order of magnitude less erosion than the model predictions is a 

 possibility. The possibility of an order of magnimde more erosion, however, would be 

 almost impossible. This rate of erosion would create enormous near bottom sediment 

 concentrations and, as previously mentioned, the near bottom currents at the PDS are 

 small. Therefore, although there is sufficient energy to suspend large amounts of sediment, 

 there is little current to transport it off site. At these concentrations the majority of 

 sediment (especially the coarse grained fraction) would quickly settle back onto the bed 

 thus reducing the net erosion significantly. 



These initial calculations indicate that, except for the surficial sediments (top 0.2 

 m), disposal mounds are safe from erosion during high intensity, low frequency events. A 

 moderately (0.6-1.0 m) thick cap with fairly strong resistance to erosion (sediment 

 characteristics similar to the erosion parameters simulated in this modeling effort) could 

 completely isolate the placed material during rare strong storms. However, initially it 

 would be necessary to examine the cap after large wave events (for example, events with 

 wave magnimde comparable to the January 1978 event) to assess cap damage by 

 determining thicknesses of erosion and to establish if cap replenishment is necessary. 



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



