PART VI: IRREGULAR WAVE TESTS 



171. This section of the report examines profile evolution caused by irregular waves and compares the 

 results with corresponding regular wave cases. All of the irregular wave tests were conducted with the 

 sloping revetment as illustrated in Figure 6. 



Background 



172. Much of the established design guidance for sediment transport has been derived in part from 

 laboratory tests conducted with movable-bed models using uniform, regular wave trains. For engineering 

 design based on this guidance, the irregular wave condition which exists in nature is commonly represented 

 by a single statistical wave height parameter that is taken as being equivalent to the regular wave height in 

 the design formulae. 



173. With the advent of irregular wave- generating capabilities in the laboratory, the means are 

 available to systematically examine differences between regular and irregular waves and their effects on the 

 process being modeled. The objective of such studies is to determine which irregular wave parameter best 

 matches the regular wave parameter used to establish the design guidance. This is most important for 

 projects that are constrained to using design criteria developed from regular-wave tests. Eventually, older 

 design criteria will be superseded by new criteria developed from field data and/or laboratory tests 

 incorporating irregular waves. 



174. Shallow-water, irregular waves typically can be represented either by a statistical wave height 

 parameter or by an energy-based parameter. Statistical wave height parameters are averages of the time 

 series of waves taken over time whereas the wave process is assumed stationary. Typical parameters include 

 mean wave height (average of all waves), Hrms (RMS square wave height), and Hij^ (average of highest 1/3 

 waves). The primary energy-based wave height is Hmo, which is directly related to the energy contained in 

 the wave spectrum and approximately equal to //1/3 under the narrow-banded Gaussian assumption. 



175. Although Hmo and i/1/3 are approximately equivalent in deep water, the two parameters can be 

 distinctly different as the waves shoal (Thompson and Vincent 1984, 1985; Hughes and Borgman 1987). 

 Therefore, in selecting an irregular wave parameter to provide equivalence to regular waves, it will be 

 necessary to determine whether a statistical parameter or an energy-based parameter is more appropriate. 



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