

59_ 



Analysis of the annual wave statistics revealed that wave characteristics were very 

 similar for the 3.4 years of wave records, and significant wave heights less than 2 m 

 occurred from 90 to 95 % of the time during each year. The maximum significant wave 

 height observed in each of the four measurement years ranged from 5.6 to 7.3 m. Overall, 

 wave characteristics during the first half of 1996 were typical of other recent years at this 

 location. 



Analysis of the seasonal variability in wave conditions at PDS revealed that mean 

 significant wave heights were 1.2 m in winter (December through February), approximately 

 1 m in spring and fall, and 0.7 m in summer (June through August). Mean wave periods 

 were approximately 8 sec during all seasons, with a standard deviation of approximately 



3 sec. Maximum significant wave heights were roughly 3 m in summer, compared to 6 m in 

 winter, and 7 m in spring and fall. Significant wave heights greater than 3 m were observed 

 over 4% of the time in winter, compared to 2% in spring and fall, and 0% in summer. 



The duration of large waves during the passage of major storms is relatively brief, 

 often persisting for only a fraction of a day. Quantitative analysis of storm waves revealed 

 72 events during the 3.4-year analysis period that had significant wave heights between 3 and 



4 m; average durations for these events were only 6 to 8 hrs. Wave heights in the range of 5 

 to 6 m were observed only 22 times; average durations were only 1 to 3 hrs, for a total of 59 

 cumulative hours over the 3.4 years. 



The wave records from March through May 1996 exhibited nine storm events 

 attaining significant wave heights greater than 2 m, with one reaching 5.8 m. This storm 

 activity was similar to that during other recent years, and sufficient for analysis of storm- 

 generated currents as they affect bottom sediment resuspension. 



Near-Bottom Currents 



The three consecutive deployments of moored instrumentation at PDS yielded nearly 

 complete records of near-bottom currents, water temperature, pressure, and relative turbidity 

 over the period from late February to mid-May 1996. Hourly averaged near-bottom current 

 speeds during the measurement period ranged from approximately to 20 cms" 1 , with the 

 majority of the variability occurring at periods of approximately 12 hr in association with the 

 semi-diurnal tide. 



The mean current speed for each of the three deployments was very consistent (7.0 + 

 0.4 cms" 1 ), but the mean direction varied greatly among the deployments, presumably due to 

 rough topography (e.g., boulders and rock ledges) in close proximity to the moored 

 instrumentation. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



