3.0 RESULTS 



3.1 Bathymetry 



The July 1994 survey grid extended approximately 300 m west of the boundary of 

 the surveys conducted in 1991 and 1992. Extending the boundary allowed the bathymetry 

 to show a continuation of the slope in the southern half of the survey and a depression 

 trending northwest in the northwest corner of the survey (Figure 3-1). Decreased depth at 

 the disposal location, first observed in May 1992 (Wiley 1996a), was visible during the 

 July 1994 survey as well. 



To compare the July 1994 data with the 1992 data, all results were corrected to 

 areas in the May 1992 survey unaffected by disposal (an accepted method for normalizing 

 to a benchmark survey). Comparing the May 1992, August 1992, and July 1994 surveys 

 shows that the general bathymetry at the site has changed very little (Figure 3-2). The 

 shallowest point is at the north border of the grid; the disposal location is marked by 

 perturbations to the contours on the slope; and the deepest area is 55 m at the southern 

 border. 



Between July 1994 and May 1992, the greatest changes in depth were 25 cm 

 (Figure 3-3). Material appears to have accumulated along the slopes in the northwest and 

 southeast corners of the survey and at the disposal location. The accumulation at the 

 disposal location occurred where the July 1994 contours broadened compared to the May 

 1992 survey. Areas of apparent loss between May 1992 and July 1994 are mostly very 

 small areas associated with surveying across a steep slope (SAIC 1993). 



3.2 Subbottom Survey 



The subbottom records from the X-Star survey at CSDS clearly showed the dredged 

 material deposit (dredged material reflector above the ambient bottom reflector) at the 

 disposal location. The X-Star records also provided a detailed record of surface features 

 such as the disposal mound and sand waves that are below the resolution of the bathymetric 

 survey. Because the survey was run with the tidal currents (west to east) on the odd 

 numbered lanes and against the current on the even numbered lanes, records were clearer 

 on the even numbered lanes where data was collected on a finer scale. 



The ambient surface reflector was seen clearly below the dredged material reflector 

 in lanes 7, 8, and 9 (Figure 3-4). The maximum height of the disposal mound, based on 

 the depth to ambient reflector, was 1 m. North and south of the mound, where the deposit 

 was less than approximately 1 m in height (lanes 4, 6, 10, and 12), the X-Star did not 



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



