Composite Manning's n for the Main Channel

Flow in the main channel is not subdivided, except when the roughness coefficient is changed within the channel area. HEC‑RAS tests the applicability of subdivision of roughness within the main channel portion of a cross section, and if it is not applicable, the program will compute a single composite n value for the entire main channel. The program determines if the main channel portion of the cross section can be subdivided or if a composite main channel n value will be utilized based on the following criterion: if a main channel side slope is steeper than 5H:1V and the main channel has more than one n-value, a composite roughness $\begin{array}{l}n_c\end{array}$ will be computed [Equation 6-17, Chow, 1959]. The channel side slope used by HEC-RAS is defined as the horizontal distance between adjacent n-value stations within the main channel over the difference in elevation of these two stations (See SL and SR of the figure below).

For the determination of $\begin{array}{l}n_c\end{array}$ , the main channel is divided into $\begin{array}{l}N\end{array}$ parts, each with a known wetted perimeter $\begin{array}{l}P_i\end{array}$ and roughness coefficient $\begin{array}{l}n_i\end{array}$ .

1)	$\begin{array}{l}\displaystyle n_c =\left[ \frac{\displaystyle \sum_{i=1}^N \left(P_i n_i^{1.5} \right)}{P} \right]^{2/3}\end{array}$

Symbol	Description	Units
$\begin{array}{l}n_c\end{array}$	composite or equivalent coefficient of roughness
$\begin{array}{l}P\end{array}$	wetted perimeter of entire main channel
$\begin{array}{l}P_i\end{array}$	wetted perimeter of subdivision I
$\begin{array}{l}n_i\end{array}$	coefficient of roughness for subdivision

The computed composite $\begin{array}{l}n_c\end{array}$ should be checked for reasonableness. The computed value is the composite main channel n value in the output and summary tables.