PREDICTION OF IRREGULAR WAVE RUNUP 



by 

 John Ahrens 



I . INTRODUCTION 



This report provides a technique for predicting the runup of irregu- 

 lar wind-generated waves of the type observed in most coastal waters. 

 No guidance for prediction of irregular wave runup is currently available 

 in the Shore Protection Manual (SPM) (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1975) or other Corps publications. 

 Therefore, this interim guidance is provided until results of the Coastal 

 Engineering Research Center (CERC) laboratory and field studies of the 

 runup of irregular waves are available. 



The approach is consistent with and parallels the SPM development of 

 runup prediction for regular waves of constant period and amplitude. 

 Although quite simple, this approach gives a reliable estimate of the 

 extreme values of irregular wave runup. Because of the method of formu- 

 lation, the runup estimates are conservative in that they yield somewhat 

 higher extreme values than would be predicted by more complex prediction 

 techniques; e.g., Saville, McClendon, and Cochran (1962). A conservative 

 approach is desirable because there are little field or laboratory data to 

 substantiate any prediction technique for irregular wave runup. 



II. PREDICTION OF THE DESIGN SIGNIFICANT WAVE RUNUP 



The technique for regular wave runup prediction in the SPM (Sec. 7.21) 

 is used to predict the runup for the design significant wave. That value 

 is dependent on slope roughness and porosity and an easier method to 

 determine this influence is given in this report (Sec. III). This adjusted 

 significant wave runup is then used to predict the irregular wave runup 

 distribution (see Sec. III). 



The first step in calculating the design significant wave runup is to 

 select the design wave or waves. Selection is based on many factors 

 (discussed in the SPM, especially Sec. 7.12), but ultimately the problem 

 can be reduced to selection of a design significant wave height and period. 

 The runup predicted by SPM for a wave with the design significant height 

 and period will be referred to as the design significant runup, R s . 



Using the equivalent deepwater significant height and period for the 

 design wave, the runup for this wave on a smooth slope can be calculated 

 as shown in SPM (Sec. 7.21). When the runup is corrected for scale effects 

 (see Fig. 7-13 of the SPM), it is the R s for a smooth, impermeable 

 structure. 



If the structure being designed is not smooth, roughness and porosity 

 may be accounted for by using the SPM method (Sec. 7.21) or by applying a 



