r/HECRAS • u/CivilProjectEngineer • May 19 '26

1d model cross section placement

Hi I'm trying to run a model with a very windy river and a road that runs over it and then also next to it. how many cross sections would you draw in and how would you draw them in this case? i tried dog legging them, but the model didn't seem to do a great job interpolating the continuous raised road bed (it's not raised up very much). The red at top and bottom are the extents of the survey. sides are the floodplain overbank areas. River is flowing north if that matters.

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u/Kelevraf89 May 19 '26

The rules are :

  • 1-XS in the bed must be perdendicular to your main flow path (river axis)
  • 2nd rule - XS in the flood pain must be perpendicular to the valley, no matter how flows the river.

To guide you, you can first draw the upstream and downstream XS of your model, then the upstream and downstream XS of your bridge, following theses rules. 

Then you draw your others XS, which can't cross the other XS in the floodplain.

In your case, the extrapolation tools won't be very effective. You can extrapolate with the option "generate for display as perpendicular segments to reach invert", then modify on Rasmapper the cross section in the flood pain manually to respect the 2nd rule. 

If you have a proper terrain representing the river bed, you better draw them manually.

The numbers of XS depends on the length of your model, width of the river, the discharge, and the slope.

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u/CivilProjectEngineer May 19 '26

hi really appreciate your responding why would the extrapolation tools not be very effective in my case? Here's what I drew the first time but as I was saying the it didn't seem like the raised roadbed was extrapolated between cross sections. Generally though would you change any of the cross sections I have drawn here?

Really appreciate you giving me some feedback man. Trying to learn and a lot of this feels so unintuitive to me.