types of structures which would function both as a barrier and as a wave 

 absorber. The final technique adopted was to develop a model cliff in 

 the wave basin by the scouring action of waves. While the overhanging 

 cliff wall gradually takes its shape, a small submerged sand beach is 

 developed at the foot of the cliff. The sediment transport along the 

 beach is then measured, and the rate compared with that on a natural 

 beach without the cliff. Finally, the scale effect of the model study 

 is discussed in a qualitative manner, and a possible description of 

 sediment motion around the littoral barrier is given. 



**************** 



SEDIMENT MOTION AIDNG A CLIFF 



Field observations indicate that many of the natural littoral 

 barriers have a rugged steep rock face, sometimes overhanging the sea, 

 with a small submerged beach developed at the base of the cliff. Inasmuch 

 as an artificial littoral barrier developed in a preceding series of tests 

 did not function sufficiently well as a wave absorber, it appeared con- 

 ceivable that a model cliff could be developed in a model basin by the 

 action of waves themselves. This would call for cliff material which 

 could be moved and re-shaped by the waves, but at the same time should 

 have sufficient strength to stand up vertically against the action of the 

 sea. After many trials it was found that a mixture of fine sand (median 

 diameter 0.275 mm) and wet and swelled bentonite seemed to serve the 

 purpose. Part of the bentonite was left in the mixture in small lumps 

 which, after being exposed to the wave action, were more resistant against 

 erosion than other parts of the mixture, thus giving an irregular surface 

 to the exposed area. 



The mixture was first molded into a thick vertical wall which was 

 placed in a 1-foot by 6- foot by 12 -foot wave basin at an angle of 20 

 degrees with the approaching waves (see Figure 1 (a)). A wave machine, 

 driven by an A-C electric motor, was located at the opposite end of the 

 basin. The speed of the wave machine, and consequently the period of the 

 waves, could be adjusted by a Vickers speed reducer. The amplitude of the 

 wave was controlled by an adjustable eccentric. A sand trap was installed 

 at the downcoast end of the vertical barrier, and any sediment which 

 settled at the bottom of the hopper was pumped continuously by a jet 

 pump back to the upcoast end of the barrier through a return pipe (Chien, 

 1952). The jet flow was supplied by a sump pump which drew water from 

 the basin near the wave machine. Throughout the experiment, a constant 

 water depth of 0.53 foot was maintained in the basin, and the still-water 

 level was 0.22 foot below the top of the barrier. A 0.52-second wave with 

 an amplitude of 0.042 foot was chosen for the experiment. 



When waves first hit the vertical wall most of the incident wave 

 energy was reflected. In time the front face of the wall began to cave 

 in. Clay particles were washed away and sand was left behind and 

 accumulated at the base of the wall. Afterwards the sand piles were 



22 



