58 BULLETIN 832, V, S. DEPARTMENT OE AGRICULTURE. 



eter of the channel, as may be seen by referring to Plate XX, figure 

 2. The values of n obtained are very high, due no doubt principally 

 to the presence of vegetation. 



Unusual care was exercised in making the slope measurements 

 and gaugings for both courses of channel. The slope for course A 

 was very small and required precise measurements to secure reli- 

 able results. 



DISCUSSION OF FLORIDA EXPERIMENTS. 



Owing to the fact that the slopes for channels in this section of the 

 country are quite small, the velocities are necessarily low. As a 

 result, the channels are subject to silting and to the rapid growth of 

 vegetation. Unless channels in this section are to be carefully main- 

 tained, it can be seen from the results of the experiments as given 

 in Table 6 that a comparatively high value of n should be used in 

 their design. 



VARIATION OF n WITH STAGS IN CHANNEL. 



The results of these experiments show that generally the value of 

 n decreases as the stage in the channel increases. This was pointed 

 out in an article in the Engineering News, February 24, 1916, hy 

 E„ E. Horton, as being true for natural river channels and was 

 attributed to the fact that: 



In determining the coefficients of roughness in natural river channels the entire 

 cross section as determined by soundings is taken into account, whereas if the cross 

 section is irregular there may be pockets or holes in the bed of the stream containing 

 slack water at lower stages, so that the full area of section is not effective. Similarly at 

 low stages of the stream there is likely to be more or less fall that is not effective as 

 slope and that does not contribute to producing velocity. 



While the above is also generally true for dredged channels, yet 

 these experiments show a great many exceptions, one of which is that 

 of Mud Creek, where the values of n obtained were practically the 

 same for all stages in the channel, owing to the fact that there was 

 no appreciable amount of ineffective slope or cross-section and practi- 

 cally no difference in the degree of roughness of the channel for 

 different stages (see Plate III, fig. 1 and Table 1). Another exception 

 which shows the very reverse of the general statement above is that 

 of Bogue Phalia, where n increased for an increase of stage. This 

 was due to the excellent condition of the lower part of the channel 

 as compared with that of the upper part (see Plate V, fig. 1, and 

 Table 2). 



APPLICATION OF RESULTS. 



It is impossible to describe with absolute accuracy the conditions 

 existing in a channel; such as degree of regularity, amount and nature 

 of obstructions and vegetation, and uniformity of cross section. 



