FLOW OF WATER IN DREDGED DRAINAGE DITCHES. 51 
10 #) There were some roots and dead vegetation in the channel 
during the winter months. Silting had occurred along the course, and 
there was a mud island 3 feet wide by 10 feet long tying lengthwise of 
the channel a short distance above the lower end of the slope course. 
During the summer months the island was covered with water grass. 
There was also a growth of grass along the edge of low-water stages 
and two patches of grass in the channel along the course. 
The highest stages for which values of n were determined were 
about half-bankful. Referring to Table 5, it can be seen that the 
values of n obtained during the summer months are higher than those 
obtained during the winter months, owing to the growth of grass in 
the channel; also that the values for the summer months increase as 
the stage decreases, due no doubt to the fact that for the lower stages 
the grass filled a much greater proportion of the cross-sectional area 
of the channel. Plate XVIII, figure 1,is a view of the channel, and 
figure 9 E’ shows the average cross section. 
DISCUSSION OF NORTH CAROLINA EXPERIMENTS, 
Attention is especially called to the fact that in these experiments 
values of n were not obtained for bankful stages, and this should 
be kept in mind when comparing the views and descriptions of the 
channels with the values of n obtained. A value of n for a near- 
bankful stage was obtained on the Back Swamp channel, but in the 
other channels the highest stages for which values of n were deter- 
mined were about half, and in some instances considerably less than 
half, of a bankful stage. 
The chief value of these experiments lies in the fact that values of 
jm were determined before and after the growth of grass in three of 
the channels, from which a general idea can be obtained as to the re- 
tarding effect of grass upon the flow in water courses. 
With the exception of Back Swamp the discharges of the channels 
were quite small, and for this reason it is not believed that the results 
are as accurate as were obtained for the other experiments herein 
discussed, since the effect of errors in making small discharge measure- 
ments is usually greater than in the measurement of large discharges. 
EXPERIMENTS IN FLORIDA. 
These experiments were made on the main channels of the Fells- 
mere, Vero, and Fort Pierce Drainage Districts, and on two courses 
of Lateral No. 2 of the Fort Pierce Drainage District. They were 
conducted by F. E. Staebner, drainage engineer of this bureau, 
during 1916 and 1917. 
