57 



factor, approximately 16,280 m 3 of dredged material would be expected to be measured by 

 the depth difference analysis. 



The 1992 survey extended 200 m further south than the 1991 survey. Bathy metric 

 volume calculations between the two surveys could only be based on the area common to 

 both surveys, in this case the area of the 1991 survey. However, it is unlikely that any 

 significant amount of material was present outside the 1200 x 800 m survey area, since 

 REMOTS® photographs did not indicate dredged sediments beyond Station F100S. In 

 addition, the barge disposal records show that most disposal took place north of the WLIS 

 "F" buoy (Figure 1-3). 



Prior to the depth difference analysis and the subsequent assessment of changes in 

 mound height, the depths recorded during the 1992 survey were standardized to the 1991 

 survey. This procedure minimized variability in recorded depths resulting from differences 

 in the actual, observed tidal parameters and the predicted tidal parameters used for data 

 analysis. The depth standardization process could be applied only to the 1200 x 800 m area 

 common to both the 1991 and 1992 surveys. The additional 1200 x 200 m area (to the 

 south) covered by the 1992 survey could not be standardized to the 1990 survey. Depths 

 presented in Figure 3-4 (the contour plot of the entire 1200 x 1000 m 1992 survey) have not 

 been adjusted, whereas depths presented in Figure 3-5 (the 1200 x 800 m section of the 1992 

 survey) reflect changes due to the standardization process (approximately 60 cm). 



Depth contours recorded for the WLIS mounds showed that the minimum water 

 depths of the "B," "C," "D," and "E" mounds remained unchanged since the 1990 

 survey. Closer analysis of the bathymetric depth matrix showed an approximate 0.25 m 

 decrease in the height of the WLIS "E" mound. Sediments comprising the WLIS "E" 

 mound were deposited during the 1990-1991 disposal season. Some settling and 

 consolidation of the WLIS "E" sediments was likely still in progress at the time of the 1991 

 survey. Surface shell lag observed at Station F300N (located on the "E" mound) suggested 

 that some winnowing of the silt/clay sediment component had occurred. Similar surface 

 shell lag has been observed at the peaks of the other WLIS mounds. Once this shell layer is 

 exposed, it serves to protect the mound from further winnowing or erosion and adds to the 

 stability of the mound. 



Depths recorded at the WLIS "A" mound showed an apparent decrease in mound 

 height of approximately 0.5 m. The "A" mound is the oldest of the disposal mounds in 

 WLIS; therefore, depth changes of 0.5 m are unlikely to result from the continued 

 consolidation of sediments. Examination of the survey navigation data showed that the 

 horizontal position of the fathometer in the 1992 survey was shifted by approximately 7.3 m 

 (relative to 1991) at the point of transit across the "A" mound. In areas of rough or uneven 

 topography the exact position of the vessel within a survey lane can have a significant effect 

 on the depths recorded (Williams 1993). This type of error is particularly significant within 



Monitoring Cruise at the Western Long Island Sound Disposal Site, July 1992 



