Hydraulic Parameters

The hydraulic parameters used to compute sediment transport capacity are taken from the output of steady or unsteady flow runs. The user is required only to indicate for which profile the sediment transport computations will be made for each sediment reach. HEC-RAS automatically retrieves the required hydraulic input parameters, depending on which sediment transport function has been selected. Therefore, steady, or unsteady flow computations must be run before sediment capacity computations can be performed. The hydraulic parameters are retrieved from the steady output computations for the left overbank, main channel, and right overbank, as defined by the sediment bank stations. The total sediment transport for the cross section is then the sum of the three sub-sections.

Because different sediment transport functions were developed differently with a wide range of independent variables, HEC-RAS gives the user the option to select how depth and width are to be computed. The HEC-6 method converts everything to an effective depth and width by the following equations:

1)	$\begin{array}{l}\displaystyle \displaystyle EFD= \frac{\sum_{i=1}^n D_{avg}a_iD^{2/3}_{avg}}{\sum_{i=1}^n a_iD^{2/3}_{avg}}\end{array}$

2)	$\begin{array}{l}\displaystyle \displaystyle EFW= \frac{\sum_{i=1}^n a_iD^{2/3}_{avg}}{EFD^{5/3}}\end{array}$

Symbol	Description	Units
$\begin{array}{l}EFD\end{array}$	Effective depth
$\begin{array}{l}EFW\end{array}$	Effective width
$\begin{array}{l}a_i\end{array}$	Area of subsection i
$\begin{array}{l}D_{avg}\end{array}$	Average depth of sub section i
$\begin{array}{l}n\end{array}$	Number of subsections

However, many of the sediment transport functions were developed using hydraulic radius and top width, or an average depth and top width. For this reason, HEC-RAS allows the user to designate which depth/width method to use. If the default selection is chosen, then the method consistent with the development of the chosen function will be used. For irregular cross section shapes, RAS uses the effective depth/effective width or hydraulic radius/top width as the default. Also available for use is the hydraulic depth, which is used to represent the average depth and is simply the total area of the section divided by the top width. RAS computes these depth/width parameters for the left overbank, main channel, and right overbank, as designated by the bed load stations.