54 



turbidity levels during storm 3. As the oscillatory currents subsided, the majority of the 

 suspended material apparently resettled to the seafloor (Figure 3-1 lc). 



Similar observations of wave-generated sediment resuspension correlated with high 

 SDCS values are presented in Figures 3-1 le, f, and g for storms 5, 6, and 7, respectively. 

 Storm 7 had the largest significant wave heights (5.8 m) of all storms monitored during 

 spring of 1996 (Table 3-4). During this storm (Figure 3-1 lg) the SDCS reached maximum 

 values of roughly 16 cms" 1 shortly after the highest waves were encountered, and near- 

 bottom turbidities approached 40 mg-1" 1 at both sensor levels following the time of most 

 intense SDCS. Wave periods rose to a maximum of roughly 10 s during the time of 

 maximum waves and persisted for approximately 12 hours. 



The highest (reliable) near-bottom turbidity levels ("65 mg l" 1 ) during the spring 

 measurement program were encountered during storm 5 although maximum significant wave 

 heights only reached 3.5 m (Figure 3-1 le). Wave periods during this storm reached 11.1s 

 (see Table 3-4) and persisted for more than 24 hours. The duration of elevated SDCS values 

 was much longer during storm 5 than during storm 6, and somewhat longer than during 

 storm 7. We suspect this was due to the relatively long duration of 1 1 s waves associated 

 with storm 7. 



As an additional comment on the apparent differences in sediment resuspension 

 among the relatively strong storms, it is noteworthy that sediment resuspension during storm 

 6 (maximum of 8 mgT 1 ) was much less than during storm 5 (65 mg l" 1 ) which had 

 comparable maximum significant wave heights (Table 3-5). We suspect that the extended 

 duration of long-period waves of storm 5 (Table 3-4) would cause greater sediment 

 resuspension, but the magnitude of the SDCS values were similar for the two storms, 

 suggesting that the short-term energy was similar during the peak of each storm. It is 

 possible that the relatively lower near-bottom turbidity during storm 6 (Table 3-5) was 

 partially due to a lack of fine-grained sediment available on the seafloor prior to storm 6, 

 which occurred only two days after storm 5. We suspect that, as the time period between 

 storms increases, the amount of fine-grained sediment on the seafloor also will increase due 

 to a combination of normal sediment deposition and aperiodic contributions from lateral 

 transport of dredged material from other locations within PDS. The consistently low, near- 

 bottom, background turbidity levels during the spring measurement program do, however, 

 suggest that little if any dredged material was transported laterally from PDS disposal 

 locations to the site of the moored instrument array. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



