equivalency is usually considered valid in deep and intermediate water but 

 may not be satisfactory in shallow water (Horikawa 1988). 



Directional wave statistics are also subject to misinterpretations depending 

 upon the computation method. At sea, very rarely do the waves come from 

 only one direction. More typically, swell, generated by distant storms, may 

 approach from one or more directions, while the local wind waves may have a 

 totally different orientation. Researchers need to distinguish how the wave 

 energy is distributed with respect to both direction and period (i.e., the 

 directional spectral density, S(f,Q)). The directional distribution of wave 

 energy is often computed by a method developed by Longuet-Higgins, 

 Cartwright, and Smith (1963) for use with floating buoys in deep water. 

 Other distribution functions have been proposed and used by various 

 researchers since the 1970's (Horikawa 1988). Although the various methods 

 do not produce the same directional wave statistics under some circumstances, 

 it is not possible to state that one method is superior to another. 



The user of environmental data must be aware of the convention used to 

 report directions. Table 13 lists the definitions used at CERC; other 

 institutions may not conform to these standards. 



Table 13 



Reporting Conventions for Directional Environmental Measurements 



Type 



Convention 



Example 



Wind 



FROM WHICH wind is blowing 



North wind blows from deg 



Waves 



FROM WHICH waves come 



West waves come from 270 deg 



Unidirectional 

 currents 



TO WHICH currents are flowing 



East current flowing to 90 deg 



Some oceanographic instruments are sold with software that performs semi- 

 automatic processing of the data, often in the field on PC computers. In some 

 instruments, the raw data are discarded and only the Fourier coefficients saved 

 and recorded. The user of these instruments is urged to obtain as much 

 information as possible on the mathematical algorithms used by the gage's 

 manufacturer. If these procedures are not the same as those used to analyze 

 other data sets from the area, the summary statistics may not be directly 

 comparable. Even more serious, this author (Morang) has encountered 

 commercial processing software that was seriously flawed with respect to the 

 calculation of directional spectra. In one field experiment, because the 

 original raw data had not been archived in the gage, the data could not be 

 reprocessed or the errors corrected. As a result, the multi-month gage 

 deployment was rendered useless. 



88 



Chapter 5 Analysis and Interpretation of Coastal Data 



