54 



6.0 CONCLUSIONS 



Based on acoustically detected changes in depth at the NHAV buoy location, 

 disposal and capping operations formed a CAD mound with a diameter of 800 m and 

 height of 2.5 m. Depth difference calculations between the interim disposal and precap 

 surveys detected a 100 m wide pocket of consolidation over the mound apex. It was 

 determined that the majority of the material shifted to the northeast, forming a 150 m wide 

 plateau at the top of the UDM mound. The primary factor causing the structural failure of 

 the apex was likely to be the initial placement of CDM over the northwest quadrant of the 

 NHAV 93 mound, building the apex beyond the critical angle of repose, causing 

 redistribution of material downslope. A contributing factor to the collapse of the mound 

 apex could have been the subsurface consolidation of the UDM deposit due to de- watering. 



A question had existed concerning the coverage of UDM in the northwestern 

 quadrant of the NHAV 93 mound due to conflicts in the schedule of capping and survey 

 operations. However, DAMOS disposal logs indicate an estimated barge volume of 

 76,000 m 3 was released over cap placement points A, F, and J before the completion of the 

 precap survey. In addition, subbottom and geotechnical core data collected over the 

 northern portion of the NHAV 93 mound in July 1994 indicate that 0.5 to 0.75 m of cap 

 material is present northwest of the buoy location. Recolonization over the entire surface 

 of the new CAD mounds is expected to progress at a rate typical of open-water dredged 

 material disposal sites. 



This capping project demonstrated the successful execution of a long-term 

 management strategy at the most active disposal site in New England. The strategy 

 included the thoughtful placement of small to moderate volumes of dredged material in 

 order to support the containment of large volumes of UDM and effectively isolate it from 

 further interaction with the marine environment. Also, the continued use of this 

 management approach will concentrate disposal into the formation and subsequent filling of 

 containment cells, maximizing the finite capacity of the 6.85 km 2 disposal site. Although 

 all primary indications suggest the attainment of all of NED's goals, monitoring at the 

 NHAV 93 mound should continue for the next several years to assess biological recovery 

 and long-term cap integrity (Morris 1994; Germano et al. 1994). 



The wealth of data generated by the repetitive survey operations during CAD 

 mound construction and annual monitoring are providing a great deal of insight into the 

 processes that continue to affect this and other dredged material mounds. The inspection 

 of cap integrity and quantification of overall consolidation could lead to answers pertaining 

 to dredged material mass balance, consolidation rates, material slumping, material de- 

 watering, and physical changes in basement material. 



Monitoring Surveys of the New Haven Capping Project, 1993-1994 



