three slopes. The small tank is provided with a glass wall allowing 

 observation of the experimental beach. 



The large tank was subdivided into three parallel compartments 

 with beach slopes of 1 on 30, 1 on 50, and 1 on 70, respectively. The 

 experimental beach of slope 1 on 70 is shown in figure 2B. The sand 

 of the experimental beach and of a part of the approach segment is 

 placed on a platform. The remaining major part of the approach 

 segment, consisting of a wooden platform, is firmly attached to the 

 concrete bottom of the tank. In the sketch the experimental beach is 

 to the left of mark 0, and the approach segment of the right of 0. 

 In the three compartments the approach conditions were alike. 



The sand used for the experiments was commercial Potomac River 

 sand. Sieve analysis for the distribution of grain size gave the follow- 

 ing percentage values: 



Orain size Percent 



(millimeter) larger than 



2.38 1.01 



1.168 6.68 



0.589 27.08 



0.297 72.93 



0.149 95.66 



0.074 99.30 



The defining parameters of the experimental sand in Krumbein's 

 notation are, doM=0.4:2 millimeter and o-^=0.96. (See fig. 3.) 



The quantities observed during a test are shown in figure 4, which 

 is representative of the basic data. The envelopes of the wave crest 

 and trough, the undisturbed water surface, and the initial beach 

 profile were marked on the glass tank wall, then transferred to a 

 graph. The point of impending wave break, point Si in figure 4, was 

 believed to be significant and was noted. At the point of impending 

 wave break the wave front in the immediate vicinity of the crest is 

 almost vertical and the water particles on the crest are moving with 

 a velocity slightly below the velocity of wave propagation. As motion 

 continues the crests deform rapidly, then break. It will be shown 

 later that the transport of sand at the point of impending wave break 

 is a maximum. 



In the tests made with the small tank, the gradual formation and 

 stabilizing of the bar was observed through the glass wall. Roughly 

 speaking, stability was reached in one hour with a slope of 1 on 15, 

 in two hours with a slope of 1 on 30, and in 4 hours with a slope 1 on 

 70, the time intervals being the duration of the respective tests made 

 in the small tank. It was not possible to observe the formation of 

 the bars during the tests made in the larger tank; therefore, the time 

 intervals for those tests were determined from the values obtamed for 

 tests in the smaller tank by comparing wave lengths and using a 

 transference equation based on the Froude number. The minimmn 



