' FLOW OF WATER IN DREDGED DRAINAGE DITCHES, 45 
DISCUSSION OF WESTERN IOWA EXPERIMENTS. 
The low values of n obtained for the Allen and Willow Creek 
Ditches (0.0128 to 0.0143) are. so far as the writer knows, without 
precedent for dredged channels. From a close examination of the 
views of these channels one would expect low values of n for channels 
with such regular and smooth side slopes and uniform cross sections. 
However, the fact that the values of n are so low can not be attributed 
alone to these conditions, since a low value of 2 was obtained for 
a bankful stage of the Boyer River channel, which does not fulfill 
these ideal conditions of regularity and uniformity. The low values 
are therefore due mostly, no doubt, to the coating of slick, silty mud 
with which the perimeters of the channel were covered, the tendency 
being to smooth up irregularities. It is not conclusively apparent 
why the values of n for Pigeon Creek are so much higher than for 
Willow Creek since the conditions of the ditches are somewhat similar. 
It is possible that the slight difference in the condition of the channel 
and the greater fall rendered the mud coating less effective in reducing 
frictional resistance. 
In view of the resuits obtained for Pigeon Creek, the Monona- 
Havrison Ditch, and the lower part of the Boyer River channel, it is 
possible that the effectiveness of this coating of mud in reducing 
friction is affected by vegetation, roughness of channel, angle of side 
slopes, irregularity of cross section, and slope of channel. It is not 
recommended that such low values of n as obtained for Allen and 
Willow Creeks be employed in the design of proposed dredged drainage 
channels unless there is no question but that all the conditions of the 
Allen and Willow Creek channels wili be duplicated. It should also 
be remembered that these ditches are comparatively new, and unless 
the proposed ditches are to be carefully maintained to keep the 
channels free of growth and obstructions, and unless there is good 
reason to believe that they will retain their original smooth slopes 
and uniform cross section, a low value of n should not be used, even 
though the silty conditions as described above could be expected to 
prevail. 
EXPERIMENTS IN NORTH CARGLINA. 
Values of n were determined for five courses of channels in Back 
Swamp and Jacob Swamp, North Carolina, namely: Turkey Branch, 
Back Swamp, Jacob Swamp at Lovett Road, Little Jacob Swamp, 
and Jacob Swamp at the Nashville & Chattanooga Railroad. These 
experiments were made by A. D. Morehouse, formerly a drainage 
engineer of this bureau. 
