3 . RESULTS 



3 . 1 Bathymetry 



The minimum depths for the three disposal mounds "A" , 

 "B", and "C" (Figure 3-1) are 29.25, 32.5, and 27.75 meters, 

 respectively. These minimum depths were found during the October 

 1985 survey (Figure 3-2) to be 29.25, 32.5, and 28.5 meters, 

 respectively. This change in bottom topography at mound "C" was 

 the result of disposal operations occurring there between 3 

 October 1985 and 8 August 1986. Tabulation of scow logs for this 

 period indicate that approximately 73,23 cubic meters (95,730 

 cubic yards) of dredged material were deposited at or near the 

 buoy location. The scow logs also indicate that individual scow 

 loads were dumped up to 2 00 meters from the buoy in all 

 directions although the average distance to the buoy was closer 

 to 50 meters. Comparison of Figures 3-1 and 3-2 reveals a 

 significant decrease in depth at mound "C" as well as the west 

 flank of "C" and the northeast flank of mound "A". These areas 

 are well within the scope of the recent disposal operations. 



Volume difference calculations conducted for the 

 October 1985 and August 1986 surveys estimated the amount of 

 dredged material deposited to be approximately 35,700 cubic 

 meters. Much of the large difference in this volume estimate 

 from the scow log estimate (7 3,230 m 3 ) can be attributed to the 

 methods of estimation. The scow log estimates are derived from 

 the draft of the loaded scow. The scow typically holds a large 

 volume of water collected with the dredged material. This leads 

 to an overestimate of the total amount of material. The volume 

 difference calculations are based on acoustic measurements that 

 can reliably detect differences in depth of approximately 10-15 

 cm. The amount of material at the flanks of the mound in layers 

 less than this can be significant and therefore causes an 

 underestimate of dredged material deposited. This effect can be 

 more pronounced when environmental factors, such as weather, 

 reduce the positioning capabilities of the disposal scows. In 

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

 dredged material during descent and compaction once on the bottom 

 will contribute to the difference in volume estimates (see 

 Section 4.1 for further discussion). 



To determine the significance of this estimated deposit 

 of material, the statistical error of this estimate was 

 calculated for the WLIS survey area. A detailed description of 

 the calculations reguired to determine this error and the 95% 

 confidence limits around it can be found in DAMOS Contribution 

 #60 (SAIC, 1986a) . For the present volume difference 

 calculations of the 800 x 800 m survey area, the standard error 

 was determined to be 6720 m 3 . To insure the reliability of this 

 estimated volume difference, 95% confidence limits were 

 calculated and resulted in a range of 22,530 to 48,870 m 3 . This 



