remolded by the combined action of waves, littoral current and gravity, 

 and a small submerged beach developed. By then the wave reflection 

 was noticeably reduced, as more and more wave energy was absorbed by 

 the sand beach and by the rugged surface of the overhanging wall. The 

 run was stopped and the water drained. Figure 1 (b) shows a general 

 view of the cliff model thus developed, which represents a small reach 

 of a natural littoral barrier. 



The motion of sediment along the submerged beach at the base of 

 the cliff was next measured. In order to keep the size of the beach 

 unchanged, the overhanging part of the cliff was first protected against 

 further erosion by the waves. This was accomplished by pasting a layer 

 of Hydrostone over the surface of the vertical wall (Figure 1 (c)). 

 The wave basin was re-filled with" water to its original level and the 

 wave action resumed. The rate of. littoral transport was determined by 

 diverting the sediment-water mixture flow from the upcoast end of the 

 return pipe into a container, and by measuring the sediment thus collected 

 during a certain time interval. The measurement was repeated every 15 

 minutes until the rate of transport became constant. Figure 2 shows the 

 profile of the cliff with a small submerged beach at its foot which is at 

 equilibrium. The submerged beach starts at an elevation 0.32 foot from 

 the still-water level. It has an overall slope of 2.2:10, except at 

 the very end, where it inclined more or less at the angle of repose of 

 the beach material. The width of the beach was about 0.63 foot. The 

 littoral transport along such a submerged beach at the foot of a cliff 

 was 1.60 lbs. per hour, and this is to be compared with the transport 

 rate along the part of a natural sand beach which is submerged by the 

 same depth of water. 



Next the cliff model was replaced by a sand beach at a constant slope 

 of 2.2:10 and oriented at 20 degrees with the approaching wave (Figure 4 (a)). 

 The same sand which was mixed with the clay in the first part of the 

 experiment was used. The run was repeated with the same characteristics of 

 waves and the same depth of water in the basin. Figure 3 shows a profile 

 of the beach when it finally reached equilibrium, and Figure 4 (b) is a 

 general view of the beach. Along such a sand beach the littoral transport 

 was 25.5 lbs. per hour. Again the upper part of the beach was stabilized 

 down to an elevation which was 0.32 foot below the still-water surface, 

 leaving the lower end of the beach open for further wave action (Figure 4 (c)), 

 The sediment motion along the submerged sand beach was observed. In this 

 condition the transport rate was reduced to a trivial 0.018 lb. per hour, 

 indicating that there is practically no sediment movement along such a 

 submerged beach when the cliff is not present. These two sets of ex- 

 periments clearly demonstrate that the presence of a cliff considerably 

 increases the littoral transport at larger depths, which, however, is still 

 a small percentage of the total transport that can move along a normal 

 sand beach. 



23 



