Skewed Bridge

[sabz04]

Hi,

I am trying to understand what actually happens in HEC-RAS when a skew is applied to a structure. In my case, I have 3 culverts (9' span x 5' rise, 8.3' x 5, and 6' x 5') modeled as one structure. I have skewed the culverts 45 degrees, and asked the program to adjust US and DS cross sections. I moved the culverts over since the program didn't adjust for that. I moved them over by comparing the end stations before and after the skew, and determining the reduction ration of the stations. The issue is the downstream flowline was not contained in the structure previously before the skew. Now, after the skew it would seem logical that the downstream XS Flowline would be contained in the structure, but this is not the case. Can somebody please explain to me how the stations are changed before and after? I'm trying to back calculate how the stations became what they are.

Thanks!

*Note: Flow is towards northeast direction in attached photo.

 

[gbam]

When you have a roadway crossing and take your cross sections parallel to the roadway skewing them in the program shortens the horizontal distance. I typically do not take my sections in that manner today; I have in the past, though.

 

[sabz04]

How do you fix your cross sections if you don't let the program do the work? What's the alternative method?

Is it legit to redraw them at the skew (if they're georeferenced) and bring them back in, and just skew the structure in RAS? Do you think this would be a more accurate representation? Is there another way I don't know of?

 

[gbam]

The way I do it consistantly now is cut my cross sections perpendicular to flow. Then if it is a culvert, I use the appropriate chart for a skewed headwall. You can cut the xsecs at a skew (parallel to roadway) and import them then have the model adjust for skew. That is acceptable. refer to the hydraulic users manual.

 

[francesca]

Depending on complexity, I do one of two things. I either cut all x-sections perpendicular to flow except immediate up and downstream bridge x-sections (at the base of the abutments) and then apply the skew function to those and the bridge. For a more complex site - e.g. meandering channel, variable skew, bridge on a curve, etc. - I cut all x-sections perpendicular and determine a "perpendicular open area" for the bridge (put in a perpendicular bridge).

It's critical that you document how you did it and talk to the bridge designers. They like to pretend that "perpendicular open area" means "open area parallel to the roadway" and they like to stick in piers/abutments that are perpendicular to their bridge, fantastically increasing the scour potential and reducing the open area.