c. While suspension events associated with individual waves are evident 

 (sharp spikes), the largest concentrations are at low frequencies and 

 are associated with wave groups. 



d. Suspension events associated with wave groups persist in the water 

 column for a considerably longer time than those associated with 

 individual waves. 



e. The low-frequency modulation of sediment re-suspension is present at 

 all cross-shore locations, but is most evident in the inner surf zone. 



Summary of data characteristics 



Table Fl summarizes the output of the fixed OBS for all runs collected. 

 The two columns on the right indicate the mean and standard deviation of 

 sensor output (g/l) for all runs. These mean and standard deviation values 

 may be used to identify data sets that have larger dynamic signals and hence, 

 may be of more interest for study. 



Identification of marginal data sets 



The overall OBS data set is considered to be of good quality, and, while 

 there are often malfunctioning sensors in each run, all of the data runs have 

 usable data. It is suggested that sensors near the sea surface be filtered using 

 data from the resistance wave gauge to eliminate data when the sensors are out 

 of the water. Data sets in which known difficulties were encountered are 

 listed in Table 6-2. 



The sheer volume of data collected during SUPERTANK has precluded a 

 rigorous treatment to eliminate spurious signals which may be present. Spuri- 

 ous signals are most often caused by floating debris (in this case, wood chips) 

 or rapid transition from water to air and back into water, rather than elec- 

 tronic noise. It is suggested that all data be plotted as time series prior to use 

 in numerical calculations to avoid inclusion of contaminated signals. Elimina- 

 tion of outliers, such as discussed in Chapter 7, using robust techniques is rec- 

 ommended along with the following observations. Generally, concentration 

 consistently decreases with elevation, along with the dynamic range seen at a 

 particular level. Hence, concentration profiles that have inversions or sensors 

 that fluctuate widely immediately above a rather quiescent sensor should be 

 viewed as suspect, for there is no reasonable physical explanation for these 

 observations. Sensors with these characteristics should be investigated prior 

 to their inclusion in any analysis. 



Chapter 6 Suspended Sediment Concentration 



117 



