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Section V. FIELD OBSERVATIONS 



A. Form and dimensions of natural bars. — The bars of the Pomeran- 

 ian coast have been sounded by Otto and Hartnack (references 2 and 4), 

 and the latter's paper includes records of soundings by Lohmann and 

 others. Evans (reference 5) measured the bars of Lake Michigan. 

 The data from these investigations will be examined here to ascertain 

 whether any similarity exists between the form of natural bars and 

 those developed in laboratory tests. 



Consider the profiles of Lake Michigan bars shown in figure 11, from 

 which two groups of bars are selected; one consists of the bars in a 

 first zone of the four profiles, i. e., the bars closest to the shore and 

 the other, those in a second zone the next seaward. The bar base 

 depth Hb of each bar was determined, and the distance y and z of pro- 

 file points of a given bar obtained, from which the space parameters 

 y/Hs and z/Hb are formed. The averaged and smoothed values of 

 these parameters for the bars of the two groups are shown in figure 12. 

 The values are plotted as rectangles for the first zone and circles for 

 the second zone. Since the distribution of points for the two zones is 

 approximately the same, a mean curve can be drawn as shown in the 

 figure. The curve may be representative of the bars of Lake Michigan, 

 In preparing the Pomeranian coast data, the average for the given 

 locality was taken irrespective of the zones of bars. These data are 

 represented by dots, triangles and crosses and the mean curve through 

 the points is representative of the bars of the Pomeranian coast. The 

 lowest curve represents the average value for the bar determined by 

 the experiments based on the data shown in figure 9. 



The form of the experimental bars varies considerably from that of 

 the natural bars. The natural bars are flatter and longer than the 

 experimental bars; significantly, however, the depth of the crests ex- 

 pressed as a fraction of the depths of bar base have practically the 

 same value for the natural and the experimental bars. 



No adequate explanation can be offered now as to the cause of the 

 difference in the forms of the natural and experimental bars, except 

 to suggest that the waves producing the natural bars are of a mixed 

 type, and the order and extent of turbulence is of a different scale 

 resulting in a difference in the suspension and motion of particles. 

 It is believed that such factors would tend to produce flatter bars, 



A more detailed comparison of the crest and trough depths of the 

 natural bars is given in table 1. The sequence of bars defines the 

 zones in which the bars occur, the bar nearest shore lying in the first 

 zone. 



