Air Entrainment in High Velocity Streams

For channels that have high flow velocity, the water surface may be slightly higher than otherwise expected due to the entrainment of air. While air entrainment is not important for most rivers, it can be significant for highly supercritical flows (Froude numbers greater than 1.6). HEC-RAS now takes this into account with the following two equations (EM 1110-2-1601, plate B-50):

For Froude numbers less than or equal to 8.2,

1)	$\begin{array}{l}\displaystyle D_a=0.906D\left( e \right)^{0.061F}\end{array}$

For Froude numbers greater than 8.2,

2)	$\begin{array}{l}\displaystyle D_a=0.620D\left( e \right)^{0.1051F}\end{array}$

Symbol	Description	Units
$\begin{array}{l}D_a\end{array}$	water depth with air entrainment
$\begin{array}{l}D\end{array}$	water depth without air entrainment
$\begin{array}{l}e\end{array}$	numerical constant, equal to 2.718282
$\begin{array}{l}F\end{array}$	Froude number

A water surface with air entrainment is computed and displayed separately in the HEC-RAS tabular output. In order to display the water surface with air entrainment, the user must create their own profile table and include the variable "WS Air Entr." within that table. This variable is not automatically displayed in any of the standard HEC-RAS tables.