17_ 



spacing (Figure 1-4). The postcap survey was conducted at higher spatial resolution (25 ft 

 lane spacing), but over a smaller area. The October bathymetric survey collected by SAIC 

 was conducted using a 15 m lane spacing (approximately 45 ft), and over both east- west 

 and north-south lanes. Because of the different survey parameters, calculating differences 

 in depth by the normal procedure of subtracting equal grids over replicate areas was not 

 reliable, so therefore, qualitative differences between the surveys are discussed below. In 

 addition, comparing the depths collected around the cell (that presumably did not change) 

 showed variation in depth of +2 ft among all of the surveys. This variability is due to 

 different survey parameters, different vessels (with potentially different depths of the 

 transducer), and potential error due to sea state. These errors would be much reduced, and 

 allow for electronic survey comparison, if the surveys were conducted using exact replicate 

 survey parameters and equipment. All three surveys have been corrected to MLW for 

 comparison purposes. 



The predisposal baseline bathymetric survey showed ambient water depths ranging 

 from 5 to 40 ft in the area around where the cell was excavated (Figure 1-4). Water 

 depths in the cell itself prior to disposal ranged from 56-64 ft MLW. The cell walls were 

 fairly irregular, showing scalloping along the edges remnant from the clamshell 

 excavation. The deepest part of the cell was in the southwestern corner, and there were 

 several topographically higher areas along the edges and in the center of the cell. 



The postcap survey conducted after completion of the project (Figure 3 -2a) showed 

 steep, almost vertical, western and eastern cell walls (lanes were not extended to the 

 southern and northern ends of the cell). Depths ranged between 46-50 ft, with the 

 shallowest depths in the center of the cell consistent with the areas of dredge-induced sand 

 waves shown by the side-scan sonar data. The southern end of the cell was relatively flat 

 (49-50 ft). The average thicknesses of dredged and cap material, therefore, was 

 approximately 10 ft, with a range of thicknesses over the variable topography of the cell 

 floor of approximately 6-14 ft. 



- The survey conducted in October by SAIC resulted in somewhat less steep cell 

 walls, and a more irregular topography on the floor of the cell (Figure 3-2b). Overall, the 

 bottom of the cell appeared to increase in depth by 1-4 ft (i.e., the bottom of the cell was 

 deeper) over most of the cell during the period between the July and October postcap 

 surveys. Some areas north and south of the sandy peaks from the final postdisposal survey 

 appeared to remain relatively constant between the two surveys (49-51 ft, blue areas). 

 Considering the approximate 1-2 ft of error between the two surveys, the maximum 

 consolidation that could be confidently estimated was at least 2 ft in the far southwestern 

 corner of the cell and in the center of the cell below the thick sandy peaks. 

 Conservatively, this resulted in an overall consolidation of 10-20% since the final postcap 

 survey, concentrated in the dredged material as sand is relatively incompressible. 

 Including the consolidation prior to capping, the total consolidation of the dredged material 

 ranged from 20-40% during the four months past the initial capping. This value was 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



