16 



4.0 DISCUSSION 



The bathymetric survey measured depth changes since the August 1992 survey. 

 The X-Star subbottom survey, run concurrently, was conducted to map the extent or 

 thickness of the fine-grained dredged material deposit observed in REMOTS® sediment- 

 profile photographs in May and August 1992. The subbottom survey did show the location 

 of the fine-grained dredged material deposit (in agreement with its location as detected in 

 the bathymetric survey), as well as surface sediment features. It also showed a mound that 

 was 1 m thick (height above the ambient bottom reflector). 



Pre- and postdisposal bathymetric measurements in 1991 and in 1992 calculated the 

 disposal mound height as 71 cm. The resolution of the bathymetric system can be up to 

 ±25 cm. The results of the 1994 bathymetric survey showed a 25 cm increase in mound 

 height. Again, this measured increase is at the outer limits for the resolution of the 

 system. Comparing a 71 cm (+25 cm) disposal mound from the bathymetry results to a 

 1 m deposit on the subbottom record leads to two possibilities. The height of the mound 

 may not have changed if the mound height at 71 cm and the thickness at 1 m are within the 

 error range. If the 1 m thick deposit does represent an increase over the 71 cm measured 

 in 1992, active bed transport may have transported ambient material to the peak of the 

 mound. 



The subbottom record also recorded the presence of ripples and waves. The form 

 of these features, and the lack of any subbottom layering, indicate that they are most likely 

 sand structures. The location of the sand ripples (<50 cm in height) is concentrated west 

 of the disposal mound, corresponding to the valley seen in the bathymetry. This is in 

 contrast to the 1992 REMOTS® sediment-profile survey where the ripples were observed 

 in stations south of 41° 12.750' N (Wiley 1996a). The shifting nature of these sand ripples 

 is consistent with active bed transport documented in the area. The larger sediment waves 

 (50 cm or more in height) were seen in the previous bathymetric studies of the area. The 

 dynamic nature of these features is supported by their correspondence to areas of sediment 

 accumulation (Figure 3-3). 



Bathymetric and Subbottom Survey at the Cornfield Shoals Disposal Site, July 8, 1994 



