39_ 



observed currents (first and fourth tiers from the top in Figure 3-8) are presented after 

 removal of the mean for each deployment. The predicted tidal currents (second and fifth 

 tiers from the top) illustrate the amplitude of the predicted tidal current in each vector 

 component. The residual (observed minus predicted) currents therefore represent all 

 remaining current fluctuations, regardless of frequency. These residual currents are clearly 

 weak (generally less than 5 cms" for each vector component) and composed primarily of 

 high-frequency fluctuations having periods of hours. There is a conspicuous absence of 

 significant events having periods of 2 to 4 days, which normally dominate coastal current 

 records in response to atmospheric forcing during the passage of storms. 



Upon first assessment of these residual current records, we were surprised that the 

 storms which generated surface waves as high as 3 to 5 m had almost no effect on the hourly 

 averaged, near-bottom currents at PDS. This result is, however, consistent with results from 

 past current measurements in the western Gulf of Maine which showed that currents in the 

 upper 100 m of the water column are not coherent with local winds and storm events 

 (Vermersch, Beardsley, and Brown 1979). It is believed that currents associated with 

 relatively energetic, baroclinic features are not coherent with local winds. The unusually 

 rough bottom topography in the western Gulf of Maine may interact with the coastal flow to 

 produce baroclinic eddies having a wide variety of vertical and horizontal length scales due to 

 the irregular characteristics of the seafloor topography. If so, these eddies would contribute 

 considerable high-frequency energy to the moored current records at PDS. 



The absence of low-frequency energy and the frequent occurrence of high-frequency 

 fluctuations in the near-bottom currents records from PDS support the hypotheses outlined 

 above regarding the processes governing the currents in the vicinity of PDS. 



As a final illustration of the lack of coherence between storms and near-bottom 

 currents at PDS, Figure 3-9 presents hourly current speed and direction data (both including 

 the mean and after the mean had been removed), and surface wave characteristics as 

 measured by NOAA buoy 44007. The significant wave height record is a good indicator of 

 the date, duration, and intensity of the numerous storm events that affected the region during 

 March to May 1996. Comparison of the current vectors and the significant wave heights 

 (fifth and sixth tiers from the top in Figure 3-9, respectively) illustrates that hourly averaged 

 near-bottom currents were not intensified during the storm events. 



3.3.2 Analysis of Near-Bottom Turbidity 



As described in Section 2.2, the relative turbidity of the near-bottom water at PDS 

 was monitored by two optical turbidity sensors mounted in close proximity to the current 

 meter on the tripod. The turbidity sensors were situated at heights of 33 and 81 cm above 



Oceano graphic Measurements at the Portland Disposal Site during Spring of 1996 



