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 nvalue break points (locations where nvalues 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):
2) 
K=\frac{1.486}{n}AR^{ 2/3} 
Symbol  Description  Units 

 conveyance for subdivision 

 Manning's roughness coefficient for subdivision 

 flow area for subdivision 

 hydraulic radius for subdivision (area / wetted perimeter) 

 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).
An alternative method available in HECRAS 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 HEC2 program. The method has been retained as an option within HECRAS in order to reproduce studies that were originally developed with HEC2.
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 HECRAS 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 HECRAS. The results of the study showed that the HECRAS 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 HECRAS 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.