SCALE EFFECT IN SUCH A MODEL STUDY 



The question arises as to whether the ratio between the littoral 

 transport along a model submerged beach at the foot of a littoral barrier, 

 and that of a normal beach holds for the prototype. To successfully 

 ascertain this, one must first determine how the sediment motion in these 

 two cases is affected by flow turbulence, and the extent of scale effect 

 on the generation, diffusion, and decay of turbulence. Owing to the 

 complicated nature of the problem, the discussion is necessarily 

 qualitative and even speculative. 



When waves approach shoaling water, the wave crests tend to become 

 steeper and narrower, while the troughs become flatter. Close to the 

 shore line, waves begin to deform rapidly by increasing the front slope 

 and reducing the rear slope, leading, in the limiting case of a breaking 

 wave, to a front slope which goes beyond the vertical. The location of 

 the breaking zone often coincides with that of the longshore bar. The 

 tumbling over of wave crest-s creates local turbulence of high intensity 

 which keeps much sediment in suspension to be carried downcoast by the 

 littoral current. The suspension of sediment particles takes place in 

 the zone where the turbulence is generated, and it is not associated 

 with the turbulence which is in the process of diffusion and decay. 



On the other hand, when a wave reaches a cliff part of it is re- 

 flected. The remaining part of the energy is converted into the kinetic 

 energy of turbulent flow at the face of the cliff through impact. The 

 turbulence thus generated gradually diffuses toward the sea, and in the 

 meantime, decays through the action of fluid viscosity, until finally 

 the two balance each other and the flow becomes irrotational again. In 

 the process of expanding seaward, some of the turbulence reaches the 

 neighborhood of the submerged beach at the base of the cliff, thereby 

 keeping the local sediment in suspension if the strength of turbulence 

 is large enough. The littoral transport in this case depends to a large 

 degree on the extent of turbulent diffusion, and on the intensity of the 

 diffused turbulent flow which is that left-over from the original 

 turbulent energy after continuously being dissipated into heat. 



Realizing that the suspension of sediment takes place in the tur- 

 bulence-generation zone on a normal sloping beach, and in the turbulence 

 diffusion zone at the foot of a cliff, one now turns to the question 

 of what is the effect of the difference in scale in model and prototype 

 on the different stages of turbulence development. In a flow problem 

 which involves surface waves, the Froude law is used in conducting model 

 studies. If I. is the ratio of length scales and V r is the ratio of 

 velocity scales, then the Froude criterion states that 



V = I * CD 



r r 



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