The largest computational differences will be found when comparing the HEC-2 special bridge routines to the equivalent HEC-RAS bridge methodologies. The following is a list of what is different between the two programs:
The HEC-2 special bridge routines use a trapezoidal approximation for low flow calculations (Yarnell equation and class B flow check with the momentum equation). The HEC-RAS program uses the actual bridge opening geometry for all of the low flow methodologies.
Also for low flow, the HEC-2 program uses a single pier (of equivalent width to the sum total width of all piers) placed in the middle of the trapezoid. In the HEC-RAS software, all of the piers are defined separately, and the hydraulic computations are performed by evaluating the water surface and impact on each pier individually. While this is more data for the user to enter, the results are much more physically based.
For pressure flow calculations, HEC-2 requires the net flow area of the bridge opening. The HEC-RAS software calculates the area of the bridge opening from the bridge and cross section geometry. Because of the potential error involved in calculating the bridge opening area by hand, differences between the programs may occur for pressure flow calculations.
The HEC-RAS software has two equations that can be used for pressure flow. The first equation is for a fully submerged condition (i.e. when both the upstream side and downstream side of the bridge is submerged). The fully submerged equation is also used in HEC-2. A second equation is available in HEC-RAS, which is automatically applied when only the upstream side of the bridge is submerged. This equation computes pressure flow as if the bridge opening were acting as a sluice gate. The HEC-2 program only has the fully submerged pressure flow equation. Therefore, when only the upstream side of the bridge is submerged, the two programs will compute different answers for pressure flow because they will be using different equations.
When using the HEC-2 special bridge routines, it is not necessary for the user to specify low chord information in the bridge table (BT data). The bridge table information is only used for weir flow in HEC-2. When HEC-2 special bridge data is imported into HEC-RAS, the user must enter the low chord information in order to define the bridge opening. This is due to the fact that the trapezoidal approximation used in HEC-2 is not used in HEC-RAS, and therefore the opening must be completely defined.
When entering bridge table (BT records) information in the HEC-2 special bridge method, the user had to enter stations that followed along the ground in the left overbank, then across the bridge deck/road embankment; and then along the ground of the right overbank. This was necessary in order for the left and right overbank area to be used in the weir flow calculations. In HEC-RAS this is not necessary. The bridge deck/roadway information only needs to reflect the additional blocked out area that is not part of the ground. HEC-RAS will automatically merge the ground information and the high chord data of the bridge deck/roadway.