532 



INMAN AND BAGNOLD 



[chap. 21 



Permeability also affects the slope of a swash beach of coarse material in 

 another way. A loss of swash energy occurs over the iqiper part of the beach by 

 a mass loss of water l)y downward gravity percolation. As a consequence the 

 vahie of c in relation (18) decreases progressively towards the beach crest where 

 it may actually become zero, there being here no return flow at all over the 

 beach surface. Hence, on model pebble beaches the angle /S increases pro- 

 gressively upwards, and may approach the angle of repose, just as it is found 

 to do in nature. It has been shown (Bagnold, 1940) that the increase in beach 

 slope towards the beach crest can be prevented in the model by the insertion of 

 an impermeable plate just below the beach surface. 



Fig. 7a shows the profile of a typical model beach of small pebbles (diameter 



5 4 



DISTANCE- 



3 

 FEET 



+2 



+ I 





5 4 



DISTANCE- 



FEET 



Fip. 7. Comparison of model beach profiles formed by waves during: (a) a stationary 

 water level ; and (b) a falling tide. The model beach consisted of pebbles with a mean 

 diameter of 7 mm and a natural angle of repose of 33|° ; breaker height at the plunge 

 point was approximately 29 cm. (After Bagnold, 1940.) 



7 mm). The profile and its slope reproduce those formed in nature very well. 

 When sand is introduced below the beach the well known abrupt discontinuity 

 in angle between sand and pebble is also reproduced. For waves of a given 

 height, the profiles are but little affected by changes in the wavelength of the 

 attacking waves. The effect of a slowly falling tide on the same pebble beach is 

 to produce a nearly plane beach face with a uniform slope of 19°, as shown in 

 Fig. 7b. This draw-down of the beach face by falling tides is similar to that 

 observed to occur in nature where the tide range is large compared with the 

 wave height (Inman and Filloux, 1960). 



