In general, when importing HEC-2 normal bridge data into HEC-RAS there should not be any problems. The program automatically selects the energy-based methods for low flow and high flow conditions, which is equivalent to the normal bridge method. The following is a list of possible differences that can occur.
In HEC-2 pier information is either entered as part of the bridge table (BT data) or the ground information (GR data). If the user stays with the energy based methods in HEC-RAS the results should be about the same. If the user wishes to use either the Momentum or Yarnell methods for low flow, they must first delete the pier information from the BT or GR data, and then re-enter it as separate pier information in HEC-RAS. If this is not done, HEC-RAS will not know about the pier information, and will therefore incorrectly calculate the losses with either the Momentum or Yarnell methods.
The HEC-2 Normal bridge method utilizes six cross sections. HEC-RAS uses only four cross sections in the vicinity of the bridge. The two cross sections inside the bridge are automatically formulated from the cross sections outside the bridge and the bridge geometry. In general, it is common for HEC-2 users to repeat cross sections through the bridge opening (i.e. the cross sections used inside the bridge were a repeat of the downstream section). If however, the HEC-2 user entered completely different cross sections inside the bridge than outside, the HEC-RAS software will add two additional cross sections just outside of the bridge, in order to get the correct geometry inside of the bridge. This however gives the HEC-RAS data set two more cross-sections than the original HEC-2 data set. The two cross sections are placed at zero distance from the bridge, but could still cause some additional losses due to contraction and expansion of flow. The user may want to make some adjustments to the data when this happens.
In HEC-2 the stationing of the bridge table (BT Records) had to match stations on the ground (GR data). This is not required in HEC-RAS. The stationing of the data that makes up a bridge (ground, deck/roadway, piers, and abutments) does not have to match in any way, HEC-RAS will interpolate any points that it needs.