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B.2 IIIIIIIIIIII Selection of basic n value. This step requires the selec- tion~ n value for a straight, uniform, smooth channel in the natural materials involved, The selection involves consideration of what may be regarded as a hypothetical channel. The conditions of straight alignment, uniform cross section, and smooth side ,and bottom surfaces with- out vegetation should be kept in mind, Thus the basic n will be visualized as varying only with the materials forming the sides and bottom of the channel. The minimum values of n shown by reported test results for the . best channels in earth are in the range from 0.016 to 0.018. Practical limitations associated with maintaining smooth and uniform channels in earth for' any appreciable period indicate that 0,02 is a realistic basic n. The .basic n, as it is intended for use in this procedure, for natural or excavated channels, ~y be selected from the table below. Where the oottom' .and sides of a channel are of different materials this fact may be recog- nized in' selecting the basic n, Character of channel Basic n Channels in earth Channels cut into rock Channels in fine gravel Channels in coarse gravel 0,02 0.025 0.024 0,028 __election of modifying value for surface irregularity. The sel~e based on the degree of roughness or irregularity of the surfaces of channel sides and bottom. Consider the actual surface irregulari- ty; first} in relation to the degree of surface smoothness obtainable with the natural materials involved, and second, in relation to the depths of flow under. consideration. Actual surface irregularity comparable to the best sur- face to be expected of the natural materials involved c~s for a modifying value of zero. Higher degrees of irregularity. induce turbulence and call for increased modifying values, The table below may be used as a guide to the sel.ection. DeRree of irreRUlarity Smooth Surfaces comparable to The best obtainable for the materials involved, ModifyinR value 0,000 Minor Good dredged channels; slightly eroded or scoured side slopes of canals or drainage channels, 0.005 Moderate Fair to poor dredged channels; moderately sloughed or eroded side slopes of canals or drainage channels. 0,010 Severe Badly sloughed banks of natural channels; badly eroded or sloughed sides of canals or drainage channels; unshaped, jagged and irregular sur- faces of channels excavated in rock. 0.020 ~. ., ",-,.