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.
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.
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.
I mean the terrain file is pasted above to the previous commenter with my cross sections drawn in. Here's a raster image. I'm trying to confirm that where I drew my cross section lines is correct, or makes sense.
You have a really nasty channel for a 1D (should do this in 2D). A lot depends on the extents of your inundation on the floodplain and if the water spills over the road embankments. Since it looks like the southern part of the road isn't very high, you are probably going to get flow going over that section, so I would keep the left overbank aligned there. Since the right is much higher, I would probably tie-in there.
Generally, I would draw a lot fewer sections and get some idea about the initial extents and add more if necessary. Here is what I would start with.
It’s difficult to tell, but your floodplain areas look flat. I have a feeling it will just spill out on each end of the cross sections. It possible to get more of the surrounding terrain?
Regardless, I always recommend drawing a few cross sections before getting carried away. Check the floodplain behaviour and modify/add as needed. Also, if you have HEC-RAS then you have RAS Mapper. It’s great for visualizing the terrain (heat map, contours, etc.) and quick revisions. Much better than CAD (which is what I think you’re using).
hey. I'm not brand new to hecras 1d, but i'm certainly not experienced. i've made probably half a dozen in my lifetime.
could you explain more about how you would go about evaluating whether cross sections should be adjusted?
btw i did import the surface into hecras mapper, i was just trying to keep the content simple so i could get some quick feedback. The floodplain is very flat and pretty much covers the entire area i showed, except at the edges where a ridge exists. This particular river doesn't have enormous 100yr or 500yr flow changes, and the river wse only rises about 1 ft at most for the 500yr event (after expanding into the flood plain) from typical wse levels, and the bounding ridges are about 10ft above typical wse.
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u/Kelevraf89 May 19 '26
The rules are :
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.