1D Steady Flow Program Limitations

The following assumptions are implicit in the analytical expressions used in the current version of the program:

1. Flow is steady.
2. Flow is gradually varied. (Except at hydraulic structures such as: bridges; culverts; and weirs. At these locations, where the flow can be rapidly varied, the momentum equation or other empirical equations are used.)
3. Flow is one dimensional (i.e., velocity components in directions other than the direction of flow are not accounted for).
4. River channels have "small" slopes, say less than 1:10.

Flow is assumed to be steady because time dependent terms are not included in the energy equation (.Equations for Basic Profile Calculations v6.0:Energy Equation). Flow is assumed to be gradually varied because (.Equations for Basic Profile Calculations v6.0:Energy Equation) is based on the premise that a hydrostatic pressure distribution exists at each cross section. At locations where the flow is rapidly varied, the program switches to the momentum equation or other empirical equations. Flow is assumed to be one-dimensional because (.Critical Depth Determination v6.0:1) is based on the premise that the total energy head is the same for all points in a cross section.

The limit on slope as being less than 1:10 is based on the fact that the true derivation of the energy equation computes the vertical pressure head as:

For a channel bottom slope of 1:10 (5.71 degrees) or less, the cos($//<![CDATA[ \begin{array}{l}\theta\end{array} //]]>$) is 0.995. So instead of using d cos($//<![CDATA[ \begin{array}{l}\theta\end{array} //]]>$) , the vertical pressure head is approximated as d and is used as the vertical depth of water. As you can see for a slope of 1:10 or less, this is a very small error in estimating the vertical depth (.5 %).

If HEC-RAS is used on steeper slopes, you must be aware of the error in the depth computation introduced by the magnitude of the slope. Below is a table of slopes and the cos($//<![CDATA[ \begin{array}{l}\theta\end{array} //]]>$):

If you use HEC-RAS to perform the computations on slopes steeper than 1:10, you would need to divide the computed depth of water by the cos($//<![CDATA[ \begin{array}{l}\theta\end{array} //]]>$) in order to get the correct depth of water. Also, be aware that very steep slopes can introduce air entrainment into the flow, as well as other possible factors that may not be taken into account within HEC-RAS.