Cross Section Subdivision for Conveyance Calculations

The determination of total conveyance and the velocity coefficient for a cross section requires that flow be subdivided into units for which the velocity is uniformly distributed. The approach used in HECRAS is to subdivide flow in the overbank areas using the input cross section n-value break points (locations where n-values change) as the basis for subdivision (see figure below). Conveyance is calculated within each subdivision from the following form of Manning's equation (based on English units):

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

2)	$\begin{array}{l}\displaystyle K=\frac{1.486}{n}AR^{ 2/3}\end{array}$

Symbol	Description	Units
$\begin{array}{l}K\end{array}$	conveyance for subdivision
$\begin{array}{l}n\end{array}$	Manning's roughness coefficient for subdivision
$\begin{array}{l}A\end{array}$	flow area for subdivision
$\begin{array}{l}R\end{array}$	hydraulic radius for subdivision (area / wetted perimeter)
$\begin{array}{l}S_f\end{array}$	slope of the energy gradeline

The program sums up all the incremental conveyances in the overbanks to obtain a conveyance for the left overbank and the right overbank. The main channel conveyance is normally computed as a single conveyance element. The total conveyance for the cross section is obtained by summing the three subdivision conveyances (left, channel, and right).
HEC-RAS Default Conveyance Subdivision Method

An alternative method available in HEC-RAS is to calculate conveyance between every coordinate point in the overbanks (see figure below). The conveyance is then summed to get the total left overbank and right overbank values. This method is used in the Corps HEC-2 program. The method has been retained as an option within HEC-RAS in order to reproduce studies that were originally developed with HEC-2.
Alternative Conveyance Subdivision Method (HEC-2 style)

The two methods for computing conveyance will produce different answers whenever portions on the overbank have ground sections with significant vertical slopes. In general, the HEC-RAS default approach will provide a lower total conveyance for the same water surface elevation.

In order to test the significance of the two ways of computing conveyance, comparisons were performed using 97 data sets from the HEC profile accuracy study (HEC, 1986). Water surface profiles were computed for the 1% chance event using the two methods for computing conveyance in HEC-RAS. The results of the study showed that the HEC-RAS default approach will generally produce a higher computed water surface elevation. Out of the 2048 cross section locations, 47.5% had computed water surface elevations within 0.10 ft. (30.48 mm), 71% within 0.20 ft. (60.96 mm), 94.4% within 0.4 ft. (121.92 mm), 99.4% within 1.0 ft. (304.8 mm), and one cross section had a difference of 2.75 ft. (0.84 m). Because the differences tend to be in the same direction, some effects can be attributed to propagation of downstream differences.

The results from the conveyance comparisons do not show which method is more accurate, they only show differences. In general, it is felt that the HEC-RAS default method is more commensurate with the Manning equation and the concept of separate flow elements. Further research, with observed water surface profiles, will be needed to make any conclusions about the accuracy of the two methods.