FLOW OF WATER IN DREDGED DRAINAGE DITCHES. \J 



WHEN CONSTRUCTED. 



The age of the channel at the time the experiments were made may 

 be obtained by referring to this heading and to column 2; the former 

 shows approximately when the channel was constructed and the latter 

 the date when the experiments were made. The time elapsed since 

 the construction of a dredged channel is to some extent a measure of 

 the condition of the channel as regards growth and other obstructions 

 to flow, except where the channel has been carefully maintained. It 

 is also a measure of the probable extent of erosion or silting that has 

 occurred in the channel since construction. The growth of vegeta- 

 tion in a channel checks erosion and promotes silting, but on the 

 other hand active erosion in a channel checks and often entirety pre- 

 vents the growth of vegetation. 



Low values of n are generally found for properly finished, newly 

 dredged channels of uniform cross section, as was the case for Mud 

 Creek (Table 1) and South Forked Deer River, near Roberts, Tenn. 

 (Table 3). The effect of erosion on such a channel is to make it more 

 irregular and thereby to increase the value of n. However, after a 

 certain amount of erosion has taken place it does not necessarily fol- 

 low that further erosion will tend to increase the irregularity of the 

 channel, as may be seen from the results obtained for the North 

 Forked Deer River near Trenton, Tenn. (Table 3). 



ACCURACY OF RESULTS. 



With few exceptions there are no reasons to suspect inaccuracies 

 in the result of these experiments. The gauging sections and slope 

 courses were so selected as to remove as far as possible the probability 

 of large errors in the field measurements. In some instances, which 

 will be mentioned under the separate discussion of each channel, fa- 

 vorable sites for gauging stations and slope courses were not obtained. 

 Where no mention is made as to the reliability of the results, it may 

 be inferred that the results obtained are entirely reliable. Compara- 

 tively speaking, the most accurate results were obtained for the high 

 stages in the various channels, since there was a greater probability 

 of error in the gaugings and cross-sectional measurements for the lower 

 stages. 



DESCRIPTION OF EXPERIMENTS. 



A description of the six sets of experiments and a discussion of the 

 results obtained are given in the following pages. 



EXPERIMENTS IN LEE COUNTY, MISS. 



Experiments were conducted on five dredged channels in Lee 

 County, Miss., namely: Old Town Creek, Mud Creek, Twenty Mile 

 Creek, Connewah Creek, and Chawappah Creeks (see Table 1). 

 146908°— 20— Bull. 832 2 



