FLOW OF WATER IN DREDGED DRAINAGE DITCHES. - 9 
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 entirely 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 
wil 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. 
PERIMENTS IN LEE CCUNTY, MISS. 
Experiments were conducted on five dredged channels in Lee 
County, Miss., namely: Old Town Creek, Mud Creek, Twenty Mile 
Creek, Connewah Creek, and eoreppan Creeks (see Table 1). 
146S08°—20—Bull. 832-2 
