archiving can cost at least as much as the deployment and maintenance of the 

 gages. It is essential that these analysis costs be factored into the project 

 budget. Typical sampling schemes used at CERC projects are listed in 

 Table 11. 



Table 1 1 



Wave Data Sampling Intervals, Typical CERC Projects 



Instrument 



Location 



Sample interval, hr 



Sea Data self-contained wave gage 



Ocean coastlines 



4 or 6 



Sea Data self-contained wave gage 



Great Lakes 



2 or 3 



CERC Directional Wave Gage (DWG) 



Ocean coastlines 



1 



NOAA wave and meteorology buoys 



Oceans and lakes 



1 



Quality control of wave data 



One aspect of wave analysis, which is absolutely critical to the validity of 

 the overall results, is the quality control procedures used to ensure that the 

 raw data collected by the gages are truly representative of the wave climate at 

 the site. Wave gages are subject to mechanical and electrical failures. The 

 pressure sensors may be plugged or may be covered with growths while 

 underwater. Nevertheless, even while malfunctioning, gages may continue to 

 collect data which, on cursory examination, may appear to be reasonable. As 

 an example, Figure 31 shows pressure records from two instruments mounted 

 on the same tripod off the mouth of Mobile Bay, Alabama. The upper record 

 in the figure is from a gage with a plugged pressure orifice. The curve 

 reflects the overall change in water level caused by the tide, but high- 

 frequency fluctuations caused by the passing of waves have been severely 

 damped. The damping is more obvious when a single wave burst of 1,024 

 points is plotted (Figure 32). Without the record from the second gage, 

 would an analyst have been able to conclude that the first instrument was not 

 performing properly? This type of determination can be especially 

 problematic in a low-energy environment like the Gulf of Mexico, where calm 

 weather can occur for long periods. 



Another difficult condition to diagnose occurs when the wave energy 

 fluctuates rapidly. Many computerized analysis procedures contain user- 

 specified thresholds to reject records that contain too many noise spikes. 

 Occasionally, however, violent increases in energy do occur over a short time, 

 and it is important that the analysis procedures do not reject these records. As 

 an example, one of two gages in Long Beach Harbor (the lower curve in 

 Figure 29) may have malfunctioned and written many noise spikes on the 

 tape. In reality, the gage recorded unusual energy events within the harbor. 

 Another example, from Burns Harbor, Indiana, is shown in Figure 33. When 

 wave height was plotted against time, numerous spikes appeared. In this case, 

 the rapid increase in energy was genuine, and the spikey appearance was 

 caused by the plotting of many weeks of data on one plot. An examination 

 of the individual pressure records (Figure 34) reveals how rapidly the energy 



Chapter 5 Analysis and Interpretation of Coastal Data 



83 



