Irregular waves sometimes coalesce in the surf zone and travel up the 

 slope as a single entity. Runup crests are also lost when the runup- 

 rundown cycle for one wave is not completed before the arrival of the next 

 wave near the Stillwater line. Waves arriving before the completed rundown 

 cycle retard and confuse the return flow but do not necessarily cause a 

 distinct runup crest. This piling up of water around the Stillwater line 

 is a component of the wave setup commonly observed in the surf zone. The 

 phenomenon where runup crests are lost should logically depend on the slope 

 and roughness of the runup board as well as the wave conditions. The 

 observation that many waves caused no runup maximum for some wave condi- 

 tions confirms the complexity of wave action in the surf zone and merits 

 further investigation. A similar loss of runup crests was noted by Webber 

 and Bullock (1968). 



Table 3 shows that the difference between the maximum runup, R(max), 

 and R becomes greater as the waves becomes more complex. There is also 

 an increase in the dimensionless extreme runup, R (max) /V rms , as the wave 

 conditions become more complex. The average value of R(max)/D rms increases 



from 2.85 to 3.71 to 4.46 for one, two, and three component waves, respec- 

 tively. These findings are consistent with those of van Oorschot and 

 d'Angremond (1968) who noted an increase in normalized extreme runup values 

 associated with an increase in the width of the wave spectrum. 



The irregular arrival of waves of different amplitudes into the surf 

 zone and their interaction with the return flow of previous waves create 

 opportunities for unusually high runup which do not occur for monochro- 

 matic waves. Another reason for high extreme values of runup for irregular 

 waves is the growth of the highest wave crests as waves progress from 

 monochromatic to complex. Table 3 shows the growth of the highest crest, 

 D(max), relative to ^> vms as the wave conditions become more complex. 

 Visual observations indicated that runup was influenced more by crest 

 height than wave height. 



Runs 7 and 8 were intended to be replicates; however, the R(max)'s 

 differ significantly. A record length of 40 seconds is probably too small 

 to establish reproducible extreme values for irregular waves and their 

 runup. A zero value for e for runs 5 and 6 also appears to be due to 

 the short-record length since the two component waves would be expected to 

 produce some crests below the Stillwater level (negative crests) which 

 would cause e to be nonzero. One negative crest due to reflection caused 

 e to be 0.26 for run 3. Epsilon would normally be zero for a monochromatic 

 wave with no reflection. The record for run 1 was truncated such that 

 there was one less zero crossing (equivalent to one-half a zero up- 

 crossing by Draper's (1966) method) than wave crest, causing that mono- 

 chromatic wave to also have a nonzero value for e. The short-record 

 length and wave reflection have caused the monochromatic waves to appear 

 more irregular than the two-component waves when judged by e in Table 3. 



Pressure records were also collected for the wave conditions shown in 

 Table 3 and digitized at a rate of 10 per second. Four pressure transducers 



19 



