To describe the present status of research in shore line protection, 

 the subject is broken down into three categories: 



1. Wave action, i.e., the erosive force. 



2. The reaction of the shore to the wave. 



3. Improvement works to modify the reaction of the 

 shore to the wave. 



WAVE ACTION 



Wave Generation . The generation, transmission, and decay of waves 

 is a rather complex phenomenon. Classical wave theory (Airy, Gerstner) 

 by itself does little to help the practicing engineer understand the 

 wave action he encounters around the seacoast. Empirical formulae 

 available before 1940 left much to be desired. Present practice finds 

 two basic methods developed in this country for forecasting the genera- 

 tion and decay of waves. The first was developed during World War II 

 to meet the need for accurate wave prediction in connection with 

 amphibious landings; the method uses the concept of "significant waves" 

 and is known as the Sverdrup-Munk method^ ^^ after its developers. The 

 method was subsequently modified by Bret Schneider ^2^. 



More recently, a newer method of forecasting the generation and 

 decay of wave trains has been developed which considers the entire 

 component spectrum of the wave train in contrast to the Sverdrup-Munk 

 definition of a single wave height and wave period as the significant 

 wave. The more recent method was developed by Dr. Pierson and Dr. 

 Neumann of the College of Engineering of New York University (3, 3a). 



Dr. Darbyshire of England has developed a method^^''' which also 

 results in the prediction of a wave spectrum. 



The Sverdrup-Munk method has seen widespread use over the past 10 

 years. The Pier son-Neumann method, being fairly recent and more complex 

 in application, has not yet seen as extensive use. The Beach Erosion 

 Board has had made a S-year "hindcast" from Synoptic weather charts 

 using both methods^^*^^ . A comparison of the results of these two 

 hindcast studies shows a greater frequency of higher waves by the 

 Sverdrup-Munk method; this difference may be attributed not only to 

 differences in method and theory but also to the actual application of 

 the methods (such as weather map analysis). Unfortunately there are not 

 sufficient wave observations to check the relative accuracy of the two 

 methods. Actually, both methods are completely dependent on accurate 

 meteorological estimates of the velocity, extent, and duration of the 

 winds which develop a given wave train. Further improvement in wave 

 forecasting may accordingly have to await further developments in 

 meteorology. Certainly, further improvements will have to await the 

 accumulation of sufficient wave measurements to establish the accuracy 

 of the prediction methods developed. 



♦Numbers in parentheses refer to list of references on pages 23 to 27, 



18 



