BULLETIN 832, U. S. DEPARTMENT OF AGRICULTURE. 60 
(7) That crdinarily 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, 8, 10, and 12 with values of 7 in the tables). 
(9) That generally, in designing a proposed dredged channel, a 
value of 2 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 m computing the capacity of an existing channel the 
value of » 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 
eareful descriptions and views of the investigated channels. 
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