which had a maximum thickness of 2 meters, occurred slightly east 

 of the buoy at a depth of approximately 16.4 meters. Based on 

 changes in depth, the radius of the mound was determined to be 

 about 250 meters. 



The exact boundary of the area covered by recently- 

 deposited dredged material could not be determined based on the 

 REMOTS® photographs because the coverage provided by the REMOTS® 

 station grid was limited. The REMOTS® results did indicate that 

 the new mound had a north-south radius of about 350 to 400 meters 

 and an east-west radius greater than 600 meters, which is larger 

 than the radius indicated by the bathymetry results (Figure 3-76) . 

 This is mainly because of the camera's ability to detect thin 

 layers on the flanks of the deposit. Such layers were below the 

 limits of detection by precision bathymetry and were therefore 

 unaccounted for in the volume difference calculation. As a 

 conservative estimate, it was calculated that the mound flank 

 occupied an area of approximately 527,200 m 2 . Assuming an average 

 dredged material thickness of 10 cm in this area, which is again 

 a conservative estimate, results in a volume of 52,720 m of 

 material on the mound flanks not accounted for in the bathymetric 

 depth difference calculation. Adding this to the depth difference 

 volume of 79,200 m 3 results in a final total of 131,920 m of 

 dredged material detected on the bottom using the combined 

 technigues. 



The final total volume estimate of 131,920 more closely 

 approaches, but remains less than, the scow log volume estimate of 

 164,045 m 3 of disposed material. There are several reasons for 

 the discrepancy. First, the volume estimate of material on the 

 mound flank based on REMOTS® was very conservative, in terms of 

 both the areal extent of the material and its thickness. It is 

 likely that additional material which was not accounted for by 

 either REMOTS® or bathymetry occurred beyond the area covered by 

 the REMOTS® station grid. Second, the scow log estimate was 

 derived from the drafts of the loaded scows, which typically hold 

 a large volume of water collected with the dredged material. This 

 leads to an overestimation of the total amount of material. In 

 addition, the effects of the loss of interstitial water from the 

 dredged material during descent and compaction of the material on 

 the bottom will cause the depth difference to be less than the scow 

 log volume estimate. 



Another objective of the bathymetric surveys at CLIS was 

 to assess the stability of past disposal mounds. Direct comparison 

 of the topography at each of the past disposal mounds surveyed did 

 not detect any significant changes in depth that could be 

 attributed to any particular process (erosion, consolidation, 

 etc.). This was not unexpected since surveys conducted after 

 Hurricane Gloria in October 1985 revealed only local redistribution 

 of sediment, and no storms of the same magnitude occurred since the 

 surveys . 



33 



