41_ 



the seafloor to acquire data from two levels and provide basic redundancy in the event that 

 one sensor malfunctioned or became fouled or obstructed. 



Time series results from both turbidity sensors are presented in Figure 3-10, in 

 addition to records of significant wave height, near-bottom water temperature, and near- 

 bottom current speed (after the mean had been removed for each deployment period). Both 

 turbidity records illustrate low background turbidity levels throughout the three deployments, 

 with the majority of the measurements lying below 5 rng-1 . The maximum observed 

 turbidity during the measurement period was roughly 160 mgf 1 , as measured by the lower 

 sensor on April 5. It is, however, likely that this measurement was erroneous due to a 

 temporary physical obstruction to the optical sensor pathway (e.g., seaweed caught on the 

 sensor or supporting bracket) rather than gradual biofouling of the sensor, as the high values 

 were achieved suddenly and with no concurrent elevation in turbidity at the other sensor 

 situated only 48 cm above. 



Two other major events, characterized by elevated turbidity levels at both sensor 

 levels, are apparent on April 9 and 17 (Figure 3-10). As discussed in further detail below, 

 these data truly represented elevated turbidity levels in the lower water column that resulted 

 from oscillatory currents generated by storm waves. Additional events having slight 

 elevations in near-bottom turbidity occurred during the measurement program, but are not 

 evident in Figure 3-10 due to the coarse scale of the turbidity axis. 



3.3.3 Analysis of Near-Bottom Currents and Turbidity during Storm Events 



Previous sections demonstrated that significant storms did pass over PDS during the 

 period from February 27 to May 14, 1996, but the hourly averaged currents exhibited no 

 intensification due to the storms. The simultaneous occurrence of large surface waves and 

 elevated near-bottom currents during at least a few major storms raises the question of what 

 physical mechanism is responsible for the high turbidity levels. Could the energy of the 

 storm-generated waves penetrate to the seafloor within PDS causing significant, oscillatory 

 near-bottom currents that resuspended ambient sediment? Or could the elevated near-bottom 

 turbidity be a result of horizontal transport of material that had been resuspended elsewhere? 

 To investigate these hypothetical processes and their frequency of occurrence at the 

 measurement site, we have identified nine separate storm events during the February to May 

 1996 deployment period which had maximum significant wave heights of at least 2 m (Table 

 3-4). Time series plots presenting surface wave and near-bottom current, water temperature, 

 and turbidity data during each storm event are presented in Figures 3-1 la through 3-1 li to 

 illustrate short-term fluctuations in each parameter as well as their potential coherence. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



