HEC-RAS cross section placement 1

[alex12345]

I want to compare the water surface during a 1.25 year and 100-year storm event on an existing bridge and the effects if adding a bypass culvert next to it. I get a slight increase in surface elevation when I add the new culvert which doesn't seem right. I'm also looking for advice on ensuring that the program knows that the stream curves. Does it matter how many bends a cross section can have? Where do you start each cross section, at some arbritray elevation or do you create a straight baseline adjacent to the creek?

 

[francesca]

There are plenty of other threads that cover HEC-RAS cross-section selection. No, it doesn't matter how many bends a cross-section has, as long as it remains perpendicular to the direction of water flow.

When you've run your model, you should look at the error messages and add or interpolate cross-sections where it requires them. Make sure you have a cross-section at every widening, narrowing, bending, etc. location. The HEC-RAS manuals (both software and technical) are very descriptive.

You need to start your cross-sections at an elevation that is higher than the 100-year flood elevation. Since you don't know this elevation ahead of time, you need to assume one and be conservative. If you get it too low, you'll have an infuriating time extending the cross-sections.

Adding bridges and culverts to a HEC-RAS model is a tricky business and you need to make sure that you've dotted your Is and crossed your Ts. Remember to check "pressure flow" if the culvert or bridge will be submerged during the 100-year flood. Again, the manuals are excellent and there are plenty more threads here that deal with this.

 

[alex12345]

Francesca,

My project also will have a multiple opening criteria where I want to add a bypass culvert adjacent to the existing arch bridge. So another problem I see is that since my culvert is longer than the bridge width by 40 feet, it conlicts with the bridge sections. I'm also doing a lot of grading on both side of the culvert. Can I add cross sections within the contraction and expansion areas or will that result in erroneous values?

 

[francesca]

Yes, you can have as many cross-sections inside the expansion and contraction reaches as you like (or as the program asks).

 

[lamelihla]

Hi!

Francesca, you said that
"...,as long as it remains perpendicular
to the direction of water flow" in your second thread above.

HEC-RAS requires the user to put cross sections at just upstream and just downstream of the bridges.

And what if a bridge itself was not set perpendicular to the flow?

Which one should be preferred, placing cross section parallel to the bridge or placing it perpendicular the flow?

Thank you!
Melih

 

[francesca]

If you assume that the water will flow perpendicular to the bridge (i.e. flow through the bridge and not into the piers) then parallel to the bridge would be the same thing as perpendicular to the direction of water flow. If, however, your piers are simply concrete pillars in the water, you may not be able to make this assumption. Check with the user manual, but I'd probably say that for the immediately up- and downstream bridge cross-sections parallel to the bridge would be appropriate.

 

[LHA]

Definitely parallel the openings with the cross sections. You correct for the bridge/culvert skew with that function in the culvert creation window...I don't have it opened, but I think it says "Angle of Skew", or something to that effect? I recall it defaults to 0 degrees, so you may have not even noticed it.

Engineering is the practice of the art of science - Steve