Cutting HEC-RAS bridge cross-sections from a topo map

Chris

I have a quick question about bridges. When I import a HEC-RAS project, I noticed that only the bounding, exit and approach sections are brought in, although the bridge geometry is displayed on the bounding section grids. My question is whether I have to cut cross sections from the 3D map for all of these, or if by defining only one, the others are then placed. Also, if each needs to be cut, does the actual bridge face need to be cut?

Thanks
Kyle Griffith
USGS WRD Helena, MT

Dear Kyle :

Some good questions! You need to cut all 4 cross-sections at the bridge:

1. Downstream "Exit" Cross-Section. This is generally at a distance equal to four times the amount of expansion, downstream of the bridge. This cross-section is downstream far enough, where the effect of the expansion of stream flow due to the bridge opening constriction is no longer present.

2. Downstream "Face" Cross-Section. This is generally at a distance a few feet (5 to 10 ft) downstream from the actual downstream face of the bridge. This cross-section geometry represents the geometry at the bridge face, without the bridge abutments. However, the bridge abutments are separately defined at this cross-section, as well as roadway geometry, low chord geometry, and ineffective flow areas.

3. Upstream "Face" Cross-Section. This is generally at a distance a few feet (5 to 10 ft) upstream from the actual upstream face of the bridge. This cross-section geometry represents the geometry at the bridge face, without the bridge abutments. However, the bridge abutments are separately defined at this cross-section, as well as roadway geometry, low chord geometry, and ineffective flow areas.

4. Upstream "Approach" Cross-Section. This is generally at a distance equal to the amount of contraction, upstream of the bridge. This cross- section is upstream far enough, where the effect of the contraction of stream flow due to the bridge opening constriction is not yet present.

Note that a very good discussion of this is presented in the HEC-RAS Bridge Tutorial Lesson provided with the RiverCAD User Manual.

Sincerely,

Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com