BULLETIN 832, U. S. DEPARTMENT OF AGRICULTURE. 60 



(7) That ordinarily a dredged channel quickly deteriorates in 

 hydraulic efficiency unless systematically maintained (see results for 

 Old Town Creek in Table 1). 



(8) That abrupt variations in cross section play an important part 

 in reducing the hydraulic efficiency of a channel (compare figs. 2, 

 4, 6 3 8, 10, and 12 with values of n in the tables). 



(9) That generally, in designing a proposed dredged channel, a 

 value of n of 0.030 should be used if the channel is to be smoothly 

 dredged, and of 0.035 if roughly dredged. If the above values are 

 used the channels should be carefully maintained, and if not to be so 

 maintained a value of n should be selected in accordance with the 

 worst anticipated conditions for the channel. However, if it is 

 known that such conditions will obtain as were found for some of 

 the channels in western Iowa, a somewhat lower value of n may be 

 used, depending upon the anticipated conditions of the channel. 



(10) That in computing the capacity of an existing channel the 

 value of n chosen should whenever possible be based upon a compar- 

 ison of the conditions in the existing channel with the conditions of 

 channels for which values of n have been determined ; such compar- 

 ison being made, if not by actual inspection, at least with the aid of 

 careful descriptions and views of the investigated channels. 



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