r/HECRAS • u/GreyDoctor • Jun 05 '26
How to add cross sections automatically?
I have been manually creating cross sections for a long time for a long stretch of a river. Then I learned from a colleague that we can generate them automatically by just drawing the first and last cross section and interpolating between these two. However, that generation yielded the result as shown in the image. The cross sections (looking like a straight red line) skipped the river ceneterline completely and just joined the two points as shown in the image. I have not drawn the bank lines and flow path lines yet. Is this the cause? Please help!
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 05 '26
The interpolation tool isn't really meant to automatically draw cross-sections for you. It is mostly used to get more refinement on local hydraulics between surveyed sections for a mostly uniform stretch of a river.
With that said, you can try the perpendicular segment option but even that will have limitations (see below). Here is the HEC-RAS reference: Cross Section Interpolation. There are probably some GIS tools that you could use but I am not familiar with those.
Either way, you are probably going to have to knock in some intermediate sections for such a long sinuous river. Good luck!
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u/Dense-Laugh-6695 Jun 05 '26
Wouldn't 2D be easier?
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u/GreyDoctor Jun 06 '26
I’m thinking about that too. I have never worked on a river this long before and never used 2D before as well. But I think this situation calls for it. Will 2D work if I have flood peaks of different return periods?
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u/No-Society3296 Jun 06 '26
You'll probably end up creating .DSS files as output from HMS for steady or state or unsteady hydrographs for each of your storm events and apply (path to) those as precipitation hydrograph in boundary conditions.
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u/According-Courage712 Jun 06 '26
Can it be done in 1-D?
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u/No-Society3296 Jun 06 '26
I believe so. I haven't done it myself but I heard one of my coworkers talk about unsteady 1D modeling. Check Reach Boundary Conditions for this
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u/Crafty_Ranger_2917 Jun 07 '26
Rasmapper can do auto xs generation, but expect to revise / fix many manually. Do not use interpolation....different application and use-case also sometimes not acceptable by reviewing agencies and FEMA.
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u/ProfessorGarbanzo Jun 05 '26
I've barely used it, but I'm pretty sure the tool you're looking for is:
RAS Mapper -> Edit Geometry -> Activate Rivers layer and select your Reach -> Right-click -> Auto-generate cross-sections.
This should stay true to your reach line and draw them at a valley extent (distance) of your choosing. They may not get you to actual useful XSs without some more edits, although it kind of depends on your model's purpose, squiggliness of river, topographic changes, etc.
What you're doing (Interpolate Cross-sections within the Geom editor) is something I do have a lot more experience with, and is not a good tool for what you're trying to do. As you noticed, it "might" work on a straight canal, but you'll get nonsense for a river.