11 



3.0 RESULTS 



3.1 Bathymetry 



The 1200 x 1200 m bathymetric survey conducted at COS in 1991 covered the area 

 previously surveyed in July 1990 (Figure 3-1). After the July 1990 bathymetric survey, the 

 disposal buoy was redeployed in September 1990 southeast of the CLIS-89 location (CLIS- 

 90). The release of 59,000 m 3 of dredged material at the buoy formed a mound with a 

 diameter of approximately 100 m and a minim um water depth of 16.6 m (Figure 3-2). The 

 CS-90-1 capped mound was originally formed in the fall of 1989 and capped in the spring of 

 1990. It received an additional 8,730 m 3 of capping material during the 1990/1991 disposal 

 season. In the 1991 bathymetric survey, the CS-90-1 mound had a diameter of 100 m and 

 was at least 0.60 m in height (18.2 m minimum water depth) (Figure 3-2). Inactive mounds 

 CLIS-88, CLIS-87, and CLIS-89 appeared to be unchanged between the 1990 and 1991 

 bathymetric surveys. 



A visual comparison of the 1990 and 1991 surveys (Figures 3-1 and 3-2) shows a 

 decrease in water depth from 18.8 m to 16.6 m at the CLIS-90 buoy location. At the CS-90- 

 1 mound, minimum water depth increased from 18.0 m in 1990 to 18.2 m in 1991. The 

 depth difference map comparing the two depth matrices indicated increased dredged material 

 thickness of 2.4 m at the CLIS-90 mound and decreases of 0.2 m at CS-90-1 (Figure 3-3). 

 The diameter of the CLIS-90 mound detected in the isopach map was approximately 200 m 

 for thicknesses greater than 20 cm. Small deposits of material < 40 cm were scattered to 

 the west and south of this mound. Larger decreases in dredged material thickness on the 

 southeast side of the CLIS-87/88 mounds may be a result of inaccuracies in comparing 

 surveys conducted over a steeply sloping bottom. The seafloor in this area showed changes 

 of up to 2 m over 25 m (Figure 3-4). A shift of approximately one boat width (10 m) in the 

 lane navigation for compared surveys could result in apparent depth differences of 0.5 to 

 1.0 m. A volume calculation based on depth difference between the 1990 and 1991 survey 

 indicated an increase of 39,503 m 3 of dredged material (95% confidence limits; 18,270 m 3 to 

 60,736 m 3 ). 



3.2 REMOTS® Sediment-Profile Photography 

 3.2.1 Sediment Grain Size 



3.2.1.1 Active Mounds 



Within the main survey grid, there existed a patchy mosaic of silt-clay, very fine 

 sand, fine sand, and medium sand (Figure 3-5). A large part of the surveyed area, 

 particularly over the most recent disposal mounds, showed very fine or fine sand, reflecting 

 the effects of disposal. The presence of silt-clay mud patches within this area apparently 



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



