ESTIMATING NEARSHORE CONDITIONS FOR IRREGULAR WAVES 



by 

 William N. Seelig and John P. Ahrens 



I . INTRODUCTION 



Waves are often irregular in height, period, and direction with some or 

 most of the waves breaking or near the point of breaking during extreme wave 

 conditions. Procedures for predicting design wave conditions for irregular 

 waves are not discussed in the Shore Protection Manual (SPM) (U.S. Army, Corps 

 of Engineers, Coastal Engineering Research Center, 1977). Much of the infor- 

 mation in the SPM is for uniform unidirectional waves. This report supplements 

 SPM Sections 2.3, 3.85, and 7.12 with state-of-the-art irregular wave prediction 

 techniques developed by Goda (1975a, 1975b) and suggested by Goda, Takayama, 

 and Suzuki (1978). Easy-to-use methods for estimating nearshore wave height, 

 angle of approach, and resulting water level setup for irregular waves are pre- 

 sented. The methods are intended for open sections of the coast with contin- 

 uously shallowing depth contours. Wave setup due to narrow band frequency 

 spectra and surf beat is considered but other forms of wave-wave and wave- 

 current interaction and spectral shape factors have been neglected. Design 

 curves and examples of estimating the nearshore significant wave height are 

 also available in Seelig (1979) . 



The transformation and attenuation of waves propagating from deep water to 

 a beach is a problem of considerable difficulty because of a lack of field data 

 and a poor theoretical understanding of the complex wave deformation process. 

 The methods presented in this report are empirical in nature with the physics 

 of the actual problem only partially understood. The results, based on labora- 

 tory and limited field data, are considered promising enough to recommend their 

 application in selected field calculations. The results of calculations should 

 be carefully examined to assure that the basic assumptions of the method have 

 not been violated and that reasonable answers result. 



In Section II the directional spreading of a wave energy model suggested by 

 Goda, Takayama, and Suzuki (1978) is used to predict the refraction coefficient 

 and refracted wave height for the nearshore point of interest. The height is 

 then used, in Section III, as input to the surf zone wave height distribution 

 model developed by Goda (1975a, 1975b) to estimate the nearshore wave conditions 

 and setup. 



The input information necessary for the application of these prediction 

 techniques includes the deepwater significant wave height, wave period of peak 

 energy density and dominant wave direction, the directional spreading of wave 

 energy parameter, S*, beach slope, and water depth at the point of interest. 

 In any given design situation, some of these parameters may be more reliable 

 than others. Analyses in Section IV show the sensitivity of predicted nearshore 

 wave heights to these input parameters. 



Section V applies the techniques presented in Sections II and III to predict 

 wave transformation along the 580-meter-long pier at the Coastal Engineering 

 Research Center's (CERC) Field Research Facility (FRF) at Duck, North Carolina, 



