If you improperly subdivide the cross section, the mathematics of Manning’s Equation may cause a switchback. A switchback results when the calculated discharge decreases with an associated increase in elevation or depth (see Manning’s Equation for Uniform Flow in Chapter 6, and Figure 7‑13). A small increase in depth can result in a small increase in cross-sectional area and large increase in wetted perimeter and a net decrease in the hydraulic radius. The discharge computed using the smaller hydraulic radius and the slightly larger cross-sectional area is lower than the previous discharge for which the water depth was lower. Use more subdivisions within such cross sections in order to avoid the switchback.

A switchback can occur in any type of conveyance computation. Computer logic can be seriously confused if a switchback occurs in any cross section being used in a program. For this reason, always subdivide the cross section with respect to both roughness and geometric changes. Note that the actual n-value may be the same in adjacent subsections. However, too many subdivisions can result in problems, too. (See Chapter 6 for more information.)