Appendix 

 WAVES METERING SHALLOW WATER: BREAKERS AND SURF. 



A manual on forecasting breakers and surf Is In ^reparation. 

 For temporary guidance the transformations of waves that enter 

 shallow v/ater are briefly discussed here. 



Consider a wave which approaches a straight coast off which 

 the depth to the bottom Increases regularly and slowly, and as- 

 sume that In deep water the wave crest Is parallel to the coast 

 line. At a distance from a coast at which the depth to the bottom, 

 d_, Is about 1/2 the wave length transformation from a deep-water 

 wave to a shallow-water wave begins to be perceptible. The veloc- 

 ity of progress decreases but the period remains unaltered so that 

 the decrease In velocity appears as a decrease in wave length. If 



the wave lengths in deep water, L^, and In shallow water , L , are 



o ^ 



known, the depth to the bottom is obtained from the equation: 



tanh 2ir ^ = _s 

 s o 



Where the depth is less than 1/25 of L^ the equation is reduced to 



L 2 

 d = A rs 

 27r L^ 



These equations have been used to determine the bottom topography 

 from aerial photographs of waves. 



The wave height reruains constant until a depth is reached 

 which equals about 1/25 of the wave length in deep water. This is 

 explained by the fact that if the effect of friction is disregarded 



58 



