FORECASTING BREAKERS AND SURF ON A STRAIGHT 

 BEACH OF INFINITE LENGTH 



GENERALIZED DIAGRAMS FOR FORECASTING BREAKERS AND SURF 

 ON A STRAIGHT BEACH OF INFINITE LENGTH 



The following memoranda were first published in limited 

 issue as Technical Reports HE-116-13 and HE-116-6?, 

 Fluid Mechanics Laboratory, University of California „ 

 They are reproduced here to bring the wave forecasting 

 methods therein to the attention of research workers and 

 other persons interested in wave forecasting,, The 

 memoranda were prepared by the staff of the Department 

 of Engineering, University of California, February 194-7 « 



In forecasting the characteristics of breakers and surf at a particular 

 beach, the process is simplified with the aid of diagrams „ From a deep 

 water forecast of wave height (H ), period (T), and wave direction, a fore- 

 cast also may be made of breaker height (eJ) } depth at breaker (d^), angle 

 of breaker with the bottom contours (ocb J s an< ^ distance from edge of still 

 water to breaker (X^)o The surf forecasts are made simple with the aid of 

 diagrams presented in •> Breakers and Surf, Principles in Forecasting", H.O, 

 NOo 234, a nd supplementary data 5 however, the forecast can be simplified 

 further by means of other diagrams obtained by repiotting the curves in 

 Manual No c 234 ° Figure 1 shows such a plot in which all forecast computa- 

 tions are made graphically from diagrams appearing on a single page„ This 

 figure applied only to Las Pulgas Beach near Oceanside, California! however, 

 many of the diagrams are general in character and can be used at any 

 locality by merely shifting scales . Figure 2 shows the basic diagrams which 

 can be adapted to any beach„ The discussion to follow describes the method 

 of plotting each of the individual diagrams as well as explaining the steps 

 necessary in adapting the data to any beach such as Las Pulgas Beach (Fig- 

 ure 1 ) 



Breaker Height 



When waves with a particular steepness in deep water (H /L_) approach 

 a straight shore line of infinite length with wave crests parallel to the 

 bottom contours, the height of the waves changes according to the ourve 

 of H/H' as a function of d/L shown in Plate I, of H»0o Manual No 234<> 

 The value of d/L at which the waves break depends on the initial steepness 

 (H /L )<, If, however, the waves in deep water make an angle oc with the 

 shore, as they move into shallow water, refraction occurs and wave heights 

 are reduced „ The angle with which the waves make with the bottom contours 

 depends on the value of d/L and the deep water angle oc „ This re- 

 lationship is shown in Plate II (H.O. „No„ 234) . Also shown in this plate 

 are corresponding values of k, the correction factor to wave height because 

 of refraction» 



By definition, the following relationships may be expressed; k = H 'p (1) 



H o 

 23 



