40 



WLIS typically consist of mixtures of fine sand and silt/clay and provide no unique 

 anomalies to distinguish one dredging project from another. As a result, SAIC employed 

 both bathymetric and spatial gradients of several REMOTS® parameters to delimit the 

 perimeter of fresh dredged material. 



A second objective of the June 1991 monitoring cruise was to assess the stability of 

 the inactive "A", "B", "C", and "D" disposal moimds (formed prior to the 1990-1991 

 disposal season). The 1991 bathymetric data showed that the heights of the "A", "B", and 

 "C" mounds remained relatively unchanged since the previous July 1990 bathymetric survey; 

 therefore, these mounds are stable. At the time of the July 1990 survey, a 1.0 m decrease in 

 the WLIS "B" mound height was observed when compared to the prior 1988 bathymetric 

 survey. Poindexter-RoUins (1990) showed that mound consolidation can continue several 

 months after disposal, resulting in significant decreases (approximately 1.5 m) in mound 

 height (Figure 4-1). Disposal on the "B" mound (the active disposal point during the 

 1987-1988 disposal season) occurred within two months prior to the 1988 survey; therefore, 

 mound consolidation was likely still in progress. Given that no dredged material has been 

 disposed at these mounds ("A", "B", and "C") during the past three years, one might expect 

 that these sediments are consolidated and that these moimds are relatively well-armored and 

 stable. The depth profile plot of lane 18 (the bathymetric survey lane transecting the apex of 

 the WLIS "B" mound) shows the changes in mound height observed during the 1988, 1990, 

 and 1991 surveys (Figure 4-2). 



The 1991 bathymetric data revealed an apparent 0.5 m decrease in the height of the 

 WLIS "D" mound (the active disposal point for the 1989-1990 disposal season). WUS "D" 

 received dredged materials within two months before the July 1990 survey; therefore, we 

 infer that mound consolidation continued during this period. Previous studies have not 

 provided evidence of significant mound erosion at WLIS (SAIC 1990b, 1990c), nor did the 

 REMOTS® photographs from WLIS during this survey. The WLIS mounds are deeper than 

 the dynamic equilibrium between depositional and erosion forces previously shown to exist at 

 water depths of 20 m in Long Island Sound (McCall 1978). However, surface scour can 

 occur at the top of dredged material mounds as the flow field accelerates over and aroimd 

 these topographic feamres. As a result, some fine-grained materials could be washed away 

 from the mound apex and transported to deeper regions. The surface shell lag deposit 

 observed in REMOTS® photographs from the apex of the WLIS "D" mound provided further 

 evidence of the possible washing away of fine-grained (>4 phi) sediments. It is likely that a 

 combination of mound consolidation and scouring accounts for the observed changes in the 

 WLIS disposal mounds. We infer that, because dredged material was deposited on the WLIS 

 "E" mound two weeks prior to the 1991 survey, consolidation of this material and scouring 

 processes will result in an apparent decrease in mound height in a subsequent bathymetric 

 survey of the WLIS Disposal Site. 



Monitoring Cruise at the Western Long Island Sound Disposal Site, June 1991 



