Friction Loss Evaluation

Friction loss is evaluated in HEC-RAS as the product of $\begin{array}{l}\overline{S}_f\end{array}$ and $\begin{array}{l}L\end{array}$ (.Equations for Basic Profile Calculations v6.4:Energy Head Loss), where $\begin{array}{l}\overline{S}_f\end{array}$ is the representative friction slope for a reach, and L is defined by (.Equations for Basic Profile Calculations v6.4:3). The friction slope (slope of the energy gradeline) at each cross section is computed from Manning's equation as follows:

1)	$\begin{array}{l}\displaystyle S_f=\left( \frac{Q}{K} \right)^2\end{array}$

Alternative expressions for the representative reach friction slope $\begin{array}{l}S_f\end{array}$ in HECRAS are as follows:

Average Conveyance Equation

2)	$\begin{array}{l}\displaystyle \overline{S}_f=\left( \frac{Q_1+Q_2}{K_1+K_2} \right)^2\end{array}$

Average Friction Slope Equation

3)	$\begin{array}{l}\displaystyle \overline{S}_f=\frac{S_{f1}+S_{f2}}{2}\end{array}$

Geometric Mean Friction Slope Equation

4)	$\begin{array}{l}\displaystyle \overline{S}_f=\sqrt{S_{f1}\times S_{f2}}\end{array}$

Harmonic Mean Friction Slope Equation

5)	$\begin{array}{l}\displaystyle \overline{S}_f=\frac{2\ \left(S_{f1}\times S_{f2}\ \right)}{S_{f1}+S_{f2}}\end{array}$

The Average Conveyance method (2) is the "default" equation used by the program; that is, it is used automatically unless a different equation is selected by the user. The program also contains an option to select equations, depending on flow regime and profile type (e.g., S1, M1, etc.). Further discussion of the alternative methods for evaluating friction loss is contained in "Overview of Optional Capabilities."