44 



dredged material (e.g., void ratios and 

 water content). However, one of the 

 initial conditions of the model was that the 

 mounds were formed of slurried sediment. 

 This initial condition may not be valid in 

 many circumstances, thereby exaggerating 

 the rate of consolidation. 



Silva et al. (1991) collected core 

 material for geotechnical analyses and also 

 modeled his results. Due to limitations of 

 the MOUND model (it is not as accurate 

 for multilayer situations), they compared 

 the results using both the MOUND model 

 and the CONSOL model (Wong and 

 Duncan 1984). They calculated 0.74 and 

 2.5 m of consolidation at STNH-N and 

 STNH-S, respectively. Compared to 

 "actual" bathymetric changes near the 

 center of each capped mound, these 

 estimates of consolidation account for 

 approximately 75 % of the observed 

 change. More importantly, consolidation 

 using the CONSOL model showed a 

 substantial consolidation of the basement, 

 or ambient bottom, as well as the mound 

 material. 



Both studies concluded that a large 

 percentage of the apparent loss of material 

 at STNH-S after Hurricane David could be 

 due to geotechnical compression. Both 

 predicted an increased consolidation at 

 STNH-S, in part due to the higher water 

 content of the silt cap material. In 

 addition, both models predicted that 

 consolidation should occur rapidly after the 

 initial formation of a capped mound. 



Over the past 14 years, monitoring 

 surveys have been conducted often, with 



additional cruises after selected storm 

 events (Appendix C). The results of these 

 surveys indicated that, despite the passage 

 of several significant storms with 

 characteristic energy levels equivalent to 

 or in excess of Hurricane David (e.g., 

 Hurricane Gloria in 1985), contours at 

 both STNH-N and STNH-S remained 

 essentially similar to those observed in 

 1980 (e.g., SAIC 1989, 1990a). 



Given the resolution of the acoustic 

 systems, the similarity in sequential 

 bathymetric contour plots suggested that 

 total transport to date has resulted in less 

 than 10 to 20 cm of erosion from the caps. 

 Higher resolution observations provided by 

 REMOTS® analysis supported this 

 conclusion, showing that sediment erosion 

 during Hurricane Gloria was limited to 

 between 0.2 and 2 cm (SAIC 1989, 

 Fredette et al. 1988). The data indicated 

 that the capped mounds were quite stable 

 over a period of years. This longer term 

 record supported the suggestion that the 

 initial change observed at STNH-S was a 

 function of the age of the deposit and 

 possibly the associated degree of 

 consolidation (Fredette et al. 1988). 



3.1.4 Physical Monitoring: Conclusions 



Acoustic and direct and indirect visual 

 methods have shown that 



• Contaminated sediments have been 

 effectively covered by cap 

 sediments, with no evidence of 

 appreciable mixing or displacement. 



Sediment Capping of Subaqueous Dredged Material Disposal Mounds 



