FLOW OF WATER IN DREDGED DRAINAGE DITCHES. 7 
The increase in the value of n, due to the growth of grass in channels, 
is shown by the results of the experiments made in North Carolina 
(Table 5). These experiments were made during the winter and the 
following summer, before and after the growth of grass in the channels. 
DESCRIPTION OF CHANNELS. 
At the right-hand side of each of the Tables 1 to 6 is given a complete 
description of the drainage channel. These detailed descriptions, 
supplemented by the views of the channels shown in the plates, are 
intended to assist the engineer in the proper choice of n for his partic- 
ular use. Under the description of channels are included the factors 
er conditions that influence the flow of water m a channel. These 
factors are described under the following headings: Course, cross 
section, side slopes, bottom, soil, and condition. Also the approxi- 
mate date when the channel was constructed is given, from which can 
be determined the age of the channel at the time the experiments 
were made. 
COURSE. 
The length and alignment of the course of channel are discussed 
under this heading. Where the length of the course is unduly short, 
the probability of error in the slope measurements is somewhat 
greater than where it is comparatively long. It was impossible in 
many instances, other governing factors being duly considered, to 
foliow the general rule adopted, namely, that the length of the dibpe 
course should be at least 10 times the top width of the channel. 
With the exception of two courses of channels, the alignment: of 
the slope courses was practically straight. These exceptions consisted 
of a bend in the Sugar Creek a and of several irregular crooks 
in the old river channel at Campbell’s levee, near Jackson, Tenn. 
(Table 3). 
CROSS SECTION. 
Under this heading variations in the shape of the cross section 
—— the courses are noted, and the reader is referred to figures 2, 4, 
6, 8, 10, and 12, which show the per cent variation from the average 
ee orion area for all cross sections along the slope courses. In 
most cases this per cent variation is shown for low, medium, and 
high stages. From these figures an idea can be obtained as to the 
progressive changes in size of the channel along the courses and 
whether these changes are gradual or abrupt. For instance, in figure 
2 A,it is seen that the per cent variations are much larger and the 
chasse in size of the channel along the course are much more 
abrupt for the low than for the high stage, which accounts largely 
for the higher values of n obtained for the low stages as recorded in 
Table 1. 
