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()- \~./1 O.t~~\C11 \lI~\o" B.6 tet t = the straight length of the reach under consideration, s tm = the meander length of the channel in the reach, Compute modifying value for meandering in accordB9ce with the following table Ratio tm/ ts Degree of. Modifying meande.rinp:: value Minor Appreciable Severe 0.000 0,15 ns 0.30 ns 1.0 to 1.2 1.2 to 1.5 1.5 and greater Where lengths for computing the appro~imate value of tm/ts are not readily obtainable the degree of meandering can usually be judged reasonably well, 7th step, Computation of n for ~he reach. The value of n for the reach is obtained by adding the values determined in Steps 1 through 6. An illus- tration of the estimation of n is given in ExBlllple 1, Dealing with cases where both channel and flood plain flow occurs. f'\.",:,~ Work with natural streams and flpod~s often requires consideration of a wide range of discharges, At the higher stages both channel and overbank or flood plain flow are involved. Us~ally the conditions are such that the channel and flood plain will have dif1erent degrees of retardance and, there- fore, different n values, In such cases the hydraulic computations will be improved by dividing the cross sections into parts or subdivisions having different n values. . . The reason for and effect of subdividing cross sections is to permit the composite n for the reach to vary with stage above the bankfull stage, This effect is illustrated by Example; 2. The usual practice is to divide the cross section into two parts; one: subdivision being the channel portion and the other the flood plain, More than two subdivisions may be made if conditions indicate wide variations of n. However, in view of the practical aspects of the problem, more than three subdivisions would not normally be justified, In estimating n for the channel SUbdivision, all of the factors dis- cussed above and all of the procedural steps would be considered, Although conditions might indicate some variation of n with stage in the channel, it is recommended that an average value ~f n be selected for use in the hy- draulic computations for all stages, In the case of flood plain subdiYisions, the estimate of n would con- sider all factors except meandering. . That is, the estimate would employ all of the procedural steps except Step 6., Flood plain n values will normally be somewhat greater than the, channel values,j[Agricultural flood plai.n conditions are not likely to indicate an n less than 0,05 to 0,06, Many cases will justify values in the! 0,07 to 0,09 range and cases calling for values as high as 0.15 to 0,20 may be encOuntered, These higher values apply primarily because of the relatively shallow depths of flow, The two ,.