4.0 DISCUSSION 



4.1 Areal Extent, Topography, and Stability of Dredged Material 



One objective of the REMOTS® sediment-profile and 800 x 800 m bathymetric 

 surveys was to delineate the extent and topography of the dredged material 

 deposited during the past two years' disposal activities. The results indicated a 

 significant accumulation of dredged material at the designated disposal point. The 

 main body of the disposal mound was a circular deposit up to 5.3 m thick with 

 dredged material layers (ca. 5.3 - 0.1 m thick) extending approximately 225 m from 

 the disposal buoy. Although a difference in grain size did not exist between the 

 ambient sediments and dredged material, a relic and/ or mottled RPD layer observed 

 in conjunction with acoustically-determined changes in depth indicated the presence 

 of dredged material. Relic oxidized layers form when a rapidly-developing redox 

 boundary near the sediment surface is buried during subsequent disposal activities. 

 Consistent with the results of the 1987 and 1988 REMOTS® and bathymetric 

 surveys, several stations in the vicinity of the WLIS "A", "B", and "C" mounds had 

 relic dredged material layers, noted principally as discontinuous sand layers at depth 

 or as relic RPD layers. These sediments resulted from disposal operations prior to 

 the 1989 - 1990 disposal season. 



A discrepancy of 83,650 m 3 existed between the barge log estimate of 185,000 

 m 3 and the volume calculation of 101,350 m 3 (confidence limits of 98,410 and 

 104,280 m 3 ). The barge log volume is assumed to be an overestimate due to the 

 significant amount of interstitial water associated with the dredged material in the 

 barges and the self-compaction of the material after it is disposed. In a mass balance 

 study in the New York Bight, Tavolaro (1984) determined an apparent decrease in 

 volume of approximately 41% when comparing the barge log volume and the volume 

 estimated by comparing pre- and postdisposal bathymetric surveys. Applying this 

 factor to the present data results in a corrected barge log estimate of approximately 

 109,150 m 3 of dredged material, which is slightly greater than the upper confidence 

 limit determined for the bathymetric volume calculations. 



The height of the WLIS "A" and "C mounds decreased 0.25 m, while the 

 height of the "B" mound decreased approximately 1.0 m (from a minimum water 

 depth of 29.75 m in July, 1988 to 30.75 m in July, 1990). Several factors could have 

 contributed to the observed changes in depth. Although erosional forces can 

 decrease mound height, previous studies have not provided evidence of significant 

 mound erosion at WLIS (SAIC, 1990a,b), and the 1990 REMOTS® photographs did 

 not reveal winnowing of sediments at the WLIS "B" and "C" mounds. The WLIS 

 disposal mounds are deeper than the dynamic equilibrium between deposition and 

 erosion forces shown to exist at water depths of 20 m (McCall, 1978). In addition, 

 dredged material disposal at the "A" and "C" mounds has not occurred in the past 

 five years; therefore, one would expect that the sediments are compacted, and the 

 "A" and "C mounds are relatively armored. Poindexter-Rollins (1990) showed that 



Monitoring Cruise at the Western Long Island Sound Disposal Site 



