EXECUTIVE SUMMARY 



A numerical model, used to predict the long term fate of sediments (LTFATE), was 

 applied to assess the potential stability of sediment caps at the Portland Disposal Site, 

 Maine. The modeling was performed by the Coastal and Hydraulics Laboratory of the 

 U.S. Army Corps of Engineers, Waterways Experiment Station for the New England 

 District. 



The results showed that a cap of 50-100 cm thick, composed of sediments similar to 

 those used in the model, would provide protection for the capped sediments even under 

 extreme wave conditions of 14.8 m (48 ft). Using conservative parameters, the model 

 predicted erosion from such waves might remove 11-22 cm of a cap. Thus, capped 

 sediments (imder a 50-100 cm cap) are not likely to be at risk of erosion and would remain 

 within the disposal mound. 



Depending on the characteristics of the sediments chosen as cap material, actual site 

 losses could be significantly lower than model estimates. Therefore, once a sediment is 

 chosen for cap material, laboratory and field experiments should be performed to 

 determine the erosion potential for these sediments. In addition, further monitoring of the 

 PDS to measure on- and off-site sediment concentrations, bottom roughness, and near 

 bottom hydrodynamics would increase model accuracy. Other factors not accounted for in 

 this modeling effort, such as estimates of sediments transported to and possibly deposited 

 at the PDS, would tend to further reduce the estimate of actual erosion. 



Model calibration was accomplished using data provided from a field sampling 

 array that was deployed during events with waves ranging up to 5.4 m (17.7 ft) in height. 

 Severe historical wave conditions were determined through the use of the Wave 

 Information Study (WIS) hindcast for the Atlantic Coast and the ADCIRC ocean 

 circulation model. This included custom refinement of the ADCIRC model grid in the 

 New England region to provide more accurate predictions.. 



