58 



volumes of dredged material to the northwest of CLIS 95 will provide a large lateral 

 containment cell that utilizes the historic NHAV 74, SP, and NORWALK mounds as well 

 as the southeastern ridge of NHAV 93 (Figure 4-6). The CLIS 94 mound to the northeast 

 of the NHAV 93 mound complex begins to close another basin at CLIS that will utilize the 

 slopes of STNH-N, NHAV 74, SP, and CLIS 91 (Figure 4-6). Future disposal activity 

 should be directed to a point northeast of the NHAV 74 mound to complete that 

 containment cell. 



The wealth of time series data collected over the NHAV 93 and CLIS 94 mounds 

 has provided significant insight into the process of disposal mound consolidation at CLIS. 

 After a period of rapid settlement documented by the multiple bathymetric and REMOTS® 

 sediment-profile photography surveys conducted during the 1993-94 disposal season, 

 changes in the NHAV 93 mound morphology appear to have slowed (Morris et al. 1996). 

 At 2.5 years after the completion of capping operations, precision bathymetry documents 

 the continued, slow consolidation of the NHAV 93 mound on the CLIS seafloor, with a 

 maximum loss in height of 0.5 to 0.75 m. These results concur with the technical studies 

 performed in the late- 1980s by the US Army Corps of Engineers, Waterways Experiment 

 Station (WES), as well as the geotechnical analysis of sediments deposited at various 

 capped mounds at CLIS for the DAMOS Program (Poindexter-Rollings 1990; Silva et al. 

 1994). 



The findings of the September 1995 and July 1996 surveys suggest the behavior of 

 the CLIS 94 mound appears to be following the same pattern. A period of rapid 

 consolidation during the deposition of CDM was documented through the use of repetitive 

 bathymetric surveys of this bottom feature (Morris 1997). The moderate consolidation 

 represented in Figure 3-10 is expected to continue at a slow rate for the next five to ten 

 years with little change in overall width or shape. Continued bathymetric monitoring of 

 this capped mound is not a necessity; however, occasional monitoring will provide 

 additional insight into the longer term behavior of silt/clay disposal mounds. 



Repetitive bathymetric surveys over established disposal mounds are the primary 

 tool used to quantify settlement by measuring apparent loss in mound height. The images 

 obtained from the REMOTS® surveys are also helpful in consolidation studies by ruling 

 out reduction in mound height due to erosion of the surficial sediment layers. The 

 displacement of both ambient and deposited sediments can be generated by particle 

 resuspension due to passage of storm events, or through transport by tidally derived bottom 

 currents passing over dredged material deposits. The occurrence and severity of an 

 erosional event can be documented by observing distinct changes in physical appearance 

 within the top 20 cm of the sediment. Significant coarsening of sediment grains within the 

 top 5 cm of the benthos (winnowing), high boundary roughness values (>3.0 cm; scour). 



Monitoring Cruise at the Central Long Island Sound Disposal Site, July 1996 



