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Section VI. FACTORS AFFECTING MECHANISM OF BAR FORMATION 



AND MOVEMENT 



As stated in the introduction, certain observations of the mecha- 

 nism of bar formation and movement were made dm-ing the com-se of 

 the experiments. While these observations are somewhat limited 

 it is believed that they are of sufficient value to justify some discussion. 



A. SaTid transportation on smooth beaches. — A qualitative measure 

 of the bed motion of sand in the initial and the final stages of bar 

 formation was obtained from an experiment in the small tank with 

 a 1 on 70 slope, wave height ol 0.32 foot, and wave period of 1.4 

 seconds. The undisturbed water depth Hi, at the point where waves 

 began to deform was 0.58 feet. In figure 13 are shown the initial 

 condition of the beach, and the envelopes of the wave crests and 

 troughs. The breaker was of the spilling type and was completed 

 at station 4, In figm-e 14 are shown the last stages of the bar formation 

 and the corresponding envelopes of crests and troughs. Very marked- 

 changes are noticeable both in the beach surface and the water surface^ 

 Ripple marks in the area in front of the bar are in evidence. The 

 general slope of this area is almost horizontal. The breaker, now of the 

 plunging types, has moved toward the shore. It is important to note 

 that the bar was formed, not where the breaker was initially present,, 

 but appreciably nearer the shore. 



Determinations of the rate of transportation of sand were made for 

 the initial and the final conditions of the beach. Initially measure- 

 ments were made with galvanized rectangular traps, one-half inch 

 wide, one inch deep, and extending across the tank. Four of these 

 traps were set level with the sand surface at one-foot intervals to 

 collect the sand in motion along the beach. Samples were collected 

 for selected intervals and the locations of the traps were changed 

 periodically so as to obtain the rate of sand transportation over a. 

 stretch of 13 feet between the stations and 13. The data thus 

 obtained are shown graphically in figure 15 on which the rate of sand 

 transportation, Q, is plotted against the station distances. The rate 

 is expressed as pounds per hour per foot of width. On the graph are 

 shown the location of the breaker as it initially occurred and the final 

 position of the bar. 



The maximum rate of transportation of sand on the initial smooth 

 beach in the area investigated occurred at the point of impending 

 wave break. Considerable movement of sand was observed along 

 the beach surface where the waves were breaking. The breaker was 

 of the spilling type, and the distinctive discontinuity of the plunging 

 type of breaker was lacking. The bar is eventually formed at a 

 point where the rate of sand transportation is nearly a minimum. 

 Movement of sand occurs on the beach up to the limit of wave action. 



