FLOW OF WATER IN DREDGED DRAINAGE DITCHES. 59 



Consequently it is impossible to formulate definite rules to govern 

 one in choosing the proper value of n for any particular channel. It 

 is believed that views, together with complete descriptions of chan- 

 nels for which the values of n have been determined, afford the best 

 means of arriving at the proper value of n to employ for any partic- 

 ular channel. This applies especially to existing channels where it 

 is desired to ascertain the capacity. 



In order to determine the capacity of a proposed dredged channel 

 it is necessary to assume anticipated conditions of channel. As is 

 readily seen from the results of these experiments, values of n for 

 dredged channels vary greatly, depending principally upon irregu- 

 larities of side slopes and cross section due to erosion, caving banks, or 

 faulty construction; upon obstructions and growth in the channel 

 due to a lack of maintenance; and, under certain conditions, upon 

 the effect of a lining of silt in the channel. In most cases where 

 erosion takes place in a newly-constructed and well-finished dredged 

 channel, the roughness coefficient increases, but the capacity of the 

 channel as a rule also increases, since the enlarged cross section more 

 than offsets the effect of the increased roughness coefficient. In 

 some instances practically no difference in capacity in a newly- 

 dredged channel may result due to erosion, after a certain amount of 

 erosion has taken place, as was found to be the case for the experi- 

 ments conducted at Trenton, Tenn. (see Table 3). 



CONCLUSIONS, 



A careful study of the results of these experiments suggests the 

 following conclusions : 



(1) That a deposit of slick, slimy silt on the sides and bottom of a 

 channel greatly reduces frictional resistance to flow (see results 

 for Allen and Willow Creeks in Table 4). 



(2) That the clearing of perennial growth from a channel will 

 greatly increase its capacity (see results for Old Town Creek in Table 1 ) . 



(3) That the growth of grass and weeds in a channel during the 

 summer greatly decreases its capacity (see results of experiments for 

 North Carolina in Table 5). 



(4) That the accumulation of drift, trees, logs and other obstruc- 

 tions in a channel greatly decreases its capacity (see results for South 

 Forked Deer River Channel at Campbell's levee in Table 3). 



(5) That after a certain amount of erosion has taken place in 

 a channel, further erosion does not necessarily increase the roughness 

 of the perimeter (see results for North Forked Deer River in Table 3). 



(6) That the roughness coefficient n is appreciably higher for a 

 roughly dredged channel than for a smoothly dredged one (see results 

 for the South Forked Deer River at Jackson and Roberts, in Table 3). 



