Weir Flow Coefficient

Weir flow over a roadway is computed in the culvert routines using exactly the same methods used in the HECRAS bridge routines. The standard weir equation is used:

1)	$\begin{array}{l}\displaystyle Q=CLH^{3/2}\end{array}$

Symbol	Description	Units
$\begin{array}{l}Q\end{array}$	flow rate
$\begin{array}{l}C\end{array}$	weir flow coefficient
$\begin{array}{l}L\end{array}$	weir length
$\begin{array}{l}H\end{array}$	weir energy head

For flow over a typical bridge deck, a weir coefficient of 2.6 is recommended. A weir coefficient of 3.0 is recommended for flow over elevated roadway approach embankments. More detailed information on weir discharge coefficients and how weirs are modeled in HEC-RAS may be found in "Modeling Bridges" of this manual. Also, information on how to enter a bridge deck and weir coefficients can be found in "Modeling Culverts" of the HEC-RAS User's Manual, "Editing and Entering Geometric Data."