Uniform Flow Computations

For preliminary channel sizing and analysis for a given cross section, a uniform flow editor is available in HEC-RAS. The uniform flow editor solves the steady-state, Manning's equation for uniform flow. The five parameters that make up the Manning's equation are channel depth, width, slope, discharge, and roughness.

1)	$\begin{array}{l}\displaystyle Q=f(A, R, S, n)\end{array}$

Symbol	Description	Units
$\begin{array}{l}Q\end{array}$	Discharge
$\begin{array}{l}A\end{array}$	Cross sectional area
$\begin{array}{l}R\end{array}$	Hydraulic radius
$\begin{array}{l}S\end{array}$	Energy slope
$\begin{array}{l}n\end{array}$	Manning's n value

When an irregularly shaped cross section is subdivided into a number of subareas, a unique solution for depth can be found. And further, when a regular trapezoidal shaped section is used, a unique solution for the bottom width of the channel can be found if the channel side slopes are provided. The dependent variables A, and R, can then be expressed in the Manning equation in terms of depth, width and side slope as follows:

2)	$\begin{array}{l}\displaystyle Q=f(Y,W,z,S,n)\end{array}$

Symbol	Description	Units
$\begin{array}{l}Y\end{array}$	Depth
$\begin{array}{l}W\end{array}$	Bottom width
$\begin{array}{l}z\end{array}$	Channel side slope

By providing four of the five parameters, HEC-RAS will solve the fifth for a given cross section. When solving for width, some normalization must be applied to a cross section to obtain a unique solution, therefore a trapezoidal or compound trapezoidal section with up to three templates must be used for this situation.