Because any estimating method involves the calculation of a series of hydraulic characteristics of the cross section, arbitrary water-surface elevations are applied to the cross section. The computation of flow or conveyance for each water-surface application requires a hydraulic radius, as seen in Figure 6‑4. The hydraulic radius is intended as an average depth of a conveyance. A hydraulic radius and subsequent conveyance is calculated under each arbitrary water surface elevation. If there is significant irregularity in the depth across the section, the hydraulic radius may not accurately represent the flow conditions. Divide the cross section into sufficient subsections so that realistic hydraulic radii are derived.

Subsections may be described with boundaries at changes in geometric characteristics and changes in roughness elements (see Figure 6‑5). Note that the vertical length between adjacent subsections is not included in the wetted perimeter. The wetted perimeter is considered only along the solid boundaries of the cross sections, not along the water interfaces between subsections.

Adjacent subsections may have identical n-values. However, the calculation of the subsection hydraulic radius will show a more consistent pattern as the tabulation of hydraulic characteristics of the cross section is developed.

Subdivide cross sections primarily at major breaks in geometry. Additionally, major changes in roughness may call for additional subdivisions. You need not subdivide basic shapes that are approximately rectangular, trapezoidal, semicircular, or triangular.

Subdivisions for major breaks in geometry or for major changes in roughness should maintain these approximate basic shapes so that the distribution of flow or conveyance is nearly uniform in a subsection.