3.13.5.1Relative Resistance Factor (R)
The relative resistance factor (R) is a multiplication factor applied to the primary Manning’s n value of the channel. Wherever the R value changes across the cross-section, a new parallel sub-channel is created. The total conveyance for the whole cross-section is determined by carrying out a parallel channel analysis of all the sub-channels. This approach allows the variation in bed resistance across a cross-section to be accounted for, and to force a parallel channel analyses so that conveyance does not decrease with height when the wetted perimeter suddenly increases (eg. when overbank areas just become wet).
If using effective area (see Section 3.13.6), an R of 1.0 must occur at some point in the profile to indicate the primary sub-channel. If a value of 1.0 is not found an ERROR occurs, as grossly incorrect channel velocities can occur when using effective area. The Manning’s n value of the primary sub-channel is that specified in the 1d_nwk layer for the channel. The primary sub-channel does not have to be the lowest part of the cross-section.
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