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 n 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 results obtained for Pigeon Creek, the Monona- 

 Harrison 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 will 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 CAROLINA. 



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. 



