35 



sloughing. Because the walls remained unsupported (unfilled cell) at the time of this 

 survey, however, these erosion processes may continue, especially on the eastern side of 

 the pit because of the impact of vessel propeller wash. 



For Phase 2 of the BHNIP, the cells will be filled up to ambient seafloor depth 

 (channel depth), so that this result does not affect the dredging and monitoring plans. For 

 the Phase 1 CAD cell, the interesting implication of the monitoring data is that material 

 will continue to settle into the cell, from both weakened cell walls and entrapped sediment 

 and debris. During Phase 2 surrounding areas will be deepened by 5 to 7 feet decreasing 

 unsupported cell wall height. This suggests a sedimentation rate that is more rapid than the 

 surrounding Boston Harbor seafloor, making the estimation of overall sand cap thickness 

 more difficult to assess as time progresses. Ultimately, dependent upon the sedimentation 

 rate and the rapidity of erosion of the remaining cell wall, the dredged material will be 

 covered to ambient depth with sediment. This very thick layer of sediment, be it sand, 

 BBC, or ambient fine-grained sediment, will provide ample containment for the dredged 

 material placed at the bottom of the cell. 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



