60 



Tidal harmonic analysis of the current velocity data revealed that near-bottom tidal 

 currents at this location are very weak and predominantly driven by the M 2 semi-diurnal 

 constituent having a period of 12.42 hrs. The amplitude of the M 2 current was 

 approximately 3 cms" 1 , compared to less than 0.5 cms" 1 for all other constituents. At this 

 location, the M 2 tidal current rotates in a counter-clockwise direction around an ellipse 

 having a major axis oriented roughly east-west. 



Analysis of residual currents (after the mean current and the tidal currents had been 

 removed from the observed records) revealed that storms had almost no effect on the hourly 

 averaged near-bottom currents at PDS. This result is consistent with prior studies in the Gulf 

 of Maine which showed that currents are not coherent with local winds or coastal pressure 

 fluctuations during the passage of storms. 



Near-Bottom Turbidity 



Time-series measurements of turbidity during the 78-day measurement period were acquired 

 using optical sensors at levels of 33 and 81 cm above the seafloor. Both sensors provided 

 excellent quality data with no appreciable biofouling such that turbidity fluctuations above a 

 consistently low (less than 2 mgT 1 ) background level could be distinguished. 



Storm Effects on Near-Bottom Currents and Turbidity 



The near-bottom turbidity data acquired during the nine storm events having significant wave 

 heights greater than 2 m (from late February through mid-May 1996) revealed that sediment 

 resuspension was substantial during two storms, both of which had significant wave heights 

 in excess of 3 m. Analysis of the near-bottom current data during these storms revealed that 

 high-frequency oscillatory currents were induced by the storm waves such that instantaneous 

 (1-sec) current speeds reached approximately Vi knot (25 cms" 1 ) as the large waves passed 

 over the measurement site. The burst sampling scheme of the internally recording current 

 meter was insufficient to allow spectral analysis of wave-induced current fluctuations, but the 

 standard deviation of current speed (SDCS) within 1-min sampling periods (during which 30 

 samples were acquired at 0.5 Hz intervals) revealed that intensified currents occurred 

 simultaneously with observations of high near-bottom turbidity during the most intense 

 storms. 



Although the available data acquired during the moored instrument program and from 

 NOAA buoy 44007 are insufficient to derive a quantitative relationship between wave 

 characteristics (height, period, duration, and incident direction), near-bottom currents and 

 bottom stress, and sediment resuspension at PDS, the results presented herein are very useful 

 for 1) validation of hindcast models of storm-induced sediment resuspension (Gailani 1997), 

 and 2) design of more ambitious measurement and analysis programs to evaluate wave- 

 induced bottom stress and sediment resuspension under a variety of wave conditions. 

 Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



