35_ 



From the above analysis of near-bottom water temperature and flow direction, it 

 seems plausible that the striking differences in mean flow direction among the three 

 deployment periods are real. There are, however, three possible scenarios that could have 

 contributed to this abrupt change in current characteristics between deployments: 



1 . The near-bottom currents actually changed abruptly during the two brief servicing 

 events. 



2. The tripod containing the current meter was placed at different locations for each 

 deployment, and because of the extremely rough topography (rock ledges and isolated 

 rocks) the tripod may, for each deployment, have resided very close to different 

 small-scale topographic features rising a few meters above the bottom. If so, it is 

 likely that the currents 46 cm above the seafloor were greatly affected by the local 

 topography and obstructions to flow. 



3. The S4 electromagnetic current meter was, for some reason unknown to SAIC or the 

 manufacturer, adversely affected by the magnetic characteristics of the seafloor at the 

 measurement site. 



While the current records suggest that mean near-bottom current directions differed 

 significantly during each of the three deployments, we wish to investigate whether the 

 fluctuating component of the currents (due to tides and high-frequency processes) also differs 

 among the three records. As a first step, the mean current vector for each of the three 

 deployment periods was determined: 





Mean 



Mean 





Speed 



Direction 





(cms 1 ) 



(°T) 



Deployment 1: 



7.4 



154 



Deployment 2: 



6.6 



196 



Deployment 3: 



7.0 



312 



Although the mean current directions were different among the three deployments, the 

 mean speed was very consistent (7.0 ± 0.4 cms" 1 ) among the deployments. As seen in the 

 lower tier of Figure 3-6, the fluctuations in currents superimposed on the mean flow during 

 each of the three deployments are relatively small compared to the means. To determine 

 whether tidal processes are responsible for the majority of these temporal fluctuations, tidal 

 harmonic analyses were conducted on the current records from both the first and third 

 deployment periods, which had 37- and 20-day durations, respectively. For these analyses, 

 the amplitude and phase of the tidal current for each of the tidal harmonic constituents was 

 determined from the time series record; then the results from this constituent analysis were 

 used to predict the magnitude and direction of the oscillatory tidal vectors for the time period 

 of the measurements. Note that in a single current record there can be a wide variety of 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



