53_ 



significant wave heights dropped below roughly 2 m, turbidity levels again approached 

 background levels, suggesting that the suspended material had resettled on the seafloor. We 

 suspect that the short-term events of mildly elevated turbidity levels observed during the two 

 days following storm 3 were due to horizontal advection of slightly turbid water past the 

 moored sensor, rather than isolated events of local sediment resuspension. 



Next, we shall investigate the mechanism by which the storm 3 and/or its surface 

 waves caused the apparent resuspension of bottom sediments. Earlier in this section, it was 

 shown that the observed storms had no significant effect on the hourly averaged near-bottom 

 currents in PDS. To further illustrate this point, the middle tier in Figure 3-1 lc presents the 

 observed near-bottom current speed during storm 3, but for this presentation, the average 

 current speed for each 1-min sampling interval (acquired six times per hour) is shown. 

 Although 1-min averaged current speeds varied from roughly to 20 cms" 1 during the four- 

 day record, the major fluctuations occurred at periods of the semi-diurnal tide (12.42 hrs). 

 Minor, high-frequency speed fluctuations occurring early on March 20 may have been due to 

 storm effects, but, overall, storm 3 had no significant effect on currents averaged over 

 durations of 1 min or longer. 



Assessment of the potential effects of surface waves on near-bottom currents requires 

 analysis of currents on time scales equivalent to the wave periods (e.g., 5 to 10 sec). 

 Consequently, we must assess the variability in currents within each 1-min sampling interval. 

 Although the 30 current measurements acquired during each sampling interval are 

 insufficient to conduct a spectral analysis of the high-frequency current fluctuations to 

 identify the predominant periods of the current variability, a simple computation of the 

 standard deviation of current speed during each 1-min sampling interval does shed light on 

 the temporal variability in near-bottom current fluctuations that may be driven by surface 

 waves. For example, the third tier from the bottom in Figure 3-1 lc presents the time series 

 of the standard deviation of near-bottom current speed (SDCS) within each 1-min sampling 

 interval during the 4-day period bracketing storm 3. These results illustrate that SDCS 

 values ranged from background levels near 1 cms" 1 prior to the arrival of the storm, to 

 maximum values of approximately 10 cms" 1 near the time of maximum near-bottom 

 turbidity. SDCS levels after the storm returned to about 3 cms" with slightly more high 

 frequency variability than before the storm. It is also worth noting that the SDCS values 

 were comparable to the magnitude of the 1-min averaged current speeds (i.e., 10-15 cms" 1 ) 

 during the passage of the storm, such that the maximum instantaneous current speed would 

 be roughly l h knot (=25 cms" 1 ): the sum of the mean current (=10-15 cms" ) and the 

 standard deviation (SDCS =15 cms" 1 ). This analysis of SDCS illustrates that the wave- 

 generated, oscillatory, near-bottom currents were responsible for the elevated near-bottom 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



