TRAVELLING FORELANDS AND THE SHORE LINE 

 PROCESSES ASSOCIATED WITH THEM 



by 



Francis F. Escoffier 



Office, "District Engineer, 



Mobile District, Corps of Engineers 



The shore formation here under consideration is that classified 

 by Johnson(l) as a truncated cuspate foreland. It is a V-shaped area 

 of lowland that projects from a shore and travels along it consistent- 

 ly in one direction. A generalized sketch of a travelling foreland 

 is shown in Figure 1. The front and back of the foreland are both 

 gently curved, the front making a steeper angle with the shore than 

 the back. The foreland may terminate in either a sharp or a blunt 

 point. Back of the front and parallel thereto there lies a series of 

 beach ridges, each of which represents a former front of the foreland. 



The foreland travels because wave action is continually removing 

 beach material from the back and depositing it in the form of successive 

 beach ridges on the front. To understand how wave action produces 

 these results it is necessary to consider the part played by the angle 

 of incidence of the waves in determining their drift-producing power. 



In Figure 2 there is shown a wave crest ab approaching a shore 

 line cd with an angle of incidence i. On inspection it is seen that 

 unless i = the length of shore cd upon whl&h the wave breaks is longer 

 than the crest length ab. Hence the energy expended on each foot of 

 shore line must be correspondingly less than the available energy per 

 foot of wave crest. In fact, it is reasonable to suppose that, for 

 a wave of given size, the energy expended per foot of shore line varies 

 with the cosine of the angle of incidence, since that function represents 

 the ratio of the lengths ab and cd. 



Now, for the wave to move beach material alongshore it should 

 have an alongshore component. Thus, if the angle of incidence were 

 zero, there would be no shore drift although the expenditure of wave 

 energy per foot of shore line would in that case be a maximum. Since 

 the alongshore component of a wave of given size varies with the sine 

 of the angie of incidence and since the energy it expends upon the 

 shore varies with the cosine of the same angle, it follows' that the 

 drift-producing power of the wave varies with the product of these two 

 functions, i.e., with sin i cos i. But sin i cos i reaches its maximum 

 value when i z h5°. Hence waves of a given size will produce the 

 heaviest shore drift when they strike the shore with an angle of 

 incidence of U5°. These results should, of course, be accepted as 

 approximate only. 



(1) Johnson, D. W. - Shore Processes and Shore Line Development, John 

 Wiley and Sons, 1919. 



