43 



The depth profile plot of lane 16 indicated a maximum change in height of 3.0 m at 

 the NL-TR mound compared to the maximum change in height of 2.4 m as calculated by the 

 depth difference (Figure 3-5). These results may originate from the limitations of 

 bathymetric analyses on steep slopes and the resulting variability of recorded depths in 

 consecutive bathymetric surveys. Smooth topographies minimize the effect of the ship's 

 position within a survey lane on the recorded depth. Examination of the bathymetric contour 

 plot (Figure 3-3) revealed a 1.0 to 1.25 m change in depth within a 25 m distance (equivalent 

 to the width of one bathymetric survey lane) along some regions of the NL-TR mound. 

 Examination of the raw navigation data records at the peak of the NL-TR mound revealed an 

 11m separation between the positioning of the fathometer's transducers during the 1990 and 

 1991 surveys, representing a possible 0.50 m variation in recorded depth. Depth profile 

 plots represent raw bathymetric data while depth difference results present data that has been 

 filled, smoothed, and corrected to the previous year's surveys. 



Disposal logs showed an estimated disposed volume of 31,475 m 3 for 1990-1991. 

 Volume calculations based on the depth difference between the 1990-1991 surveys indicated 

 that 10,500 m 3 ± 320 m 3 (95% C.I.) of sediment accumulated in the vicinity of NL-TR. 

 Tavolaro (1984) showed that volume estimates, based on disposal logs, overestimate the 

 amount of dredged material because of the significant amount of interstitial pore water and 

 sediment " fluffing " associated with material placed into the barges. In addition, 

 discrepancies occur due to compaction of dredged material on the bottom and the significant 

 volume of material deposited on the flanks of the mound in layers too thin to be detected 

 acoustically. Tavolaro calculated that the depth difference volume estimates based on 

 successive bathymetric surveys will be as much as 41 % less than the disposal log volume 

 estimates. Applying the 41% factor to the disposal log estimate of 31,475 m 3 for dredged 

 material released in the vicinity of NL-TR resulted in a corrected volume of 18,541 m 3 of 

 dredged material. The bathymetric survey was able to detect 57% of this adjusted volume. 



The dimensions of recently deposited dredged material in the vicinity of NL-TR as 

 detected by REMOTS® photographs (Figure 4-2) extended 200 m north, 400 m west, and 

 300 m south of the disposal site center. Recently deposited dredged material was also 

 present at Station 400E. The depth difference plot based on bathymetry also showed small 

 deposits of material distributed to the north, south, and west of the disposal buoy. The 

 REMOTS® survey grid did not encompass the center of the NL-TR mound, but photographs 

 did show evidence of the sand, the last component of cap material, east of the disposal center 

 (Figure 3-11). The presence of dredged material in a large number of REMOTS® stations 

 away from the disposal buoy indicates that thin dredged material layers, less than 20 cm 

 thick, were present. This volume of material would likely contribute significantly to the 

 remaining 43% of the corrected disposal log volume. A plot of the barge release points over 

 the 1990-1991 disposal season indicated that the majority of barges were releasing near the 

 designated Buoy 91 location (Figure 4-3). 



Monitoring Cruise at the New London Disposal Site, June 1991 



