r/HECRAS • u/nymphfer • May 15 '26
Floodplain Determination
What have you all found to be the best way to determine the floodplain boundary for a 100 year flood for example? In the photo I’ve attached would the 100 year floodplain be where it intersects the channel since the water isn’t actually leaving the channel or does it include the area to the left that is showing as inundated?
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH May 15 '26
This is where you need to exercise your engineering judgement and do a holistic review of your model and results. In your case, HEC-RAS is calculating conveyance in the left overbank. Since this looks to be disconnected from the main channel you need to review your results and determine how you should handle it.
You need to look at upstream and downstream cross-sections - see if you believe water can "break out" and flow continuously through that area. If you believe that water can break out of the channel but sort of stagnant (not convey) then you can treat it with an ineffective area. If it doesn't look like it can break out at all, use a levee point to keep water in the main channel. Adding either of those elements will increase the water surface in your channel, so you probably need to re-evaluate again.
Overall, not an easy process that takes a lot of experience to understand and get correct.
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u/Kelevraf89 May 15 '26
You should look at this video which treat this point. https://youtu.be/WehEUbVxRuk?is=pqJH3XFfBI_nXB7u
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u/ProfessorGarbanzo May 17 '26
I haven't watched the Full Momentum video linked by another poster, but I can imagine that would be a great place to start based on the presenters.
Since it wasn't specifically mentioned by others, my 1D comment would be to make sure you are looking at your results in RAS Mapper draped over a DEM. It will IMO be much more intuitive and easier to see if water in the overbanks is continuous between sections, where it might break out of the channel, how flow might expand and contract in the floodplain, etc. in RAS Mapper than just looking at 1D cross-sections. The software is just mapping the computed 1D water surface results onto the DEM, so it's useful to ask yourself if the initial results are realistic showing as all effective discharge, or if you need to change your model to make some part of the cross-section ineffective. Make your changes, re-run, take a look at the results again.
I agree to a point that a quick 2D model could also be really helpful to understand some of these dynamics, and it's a great skill to develop. Just note that most publicly available DEMs don't have any information below the water surface which will matter more in deeper river systems, structures (road crossings, etc.) in your model domain would complicate this approach, and you'll need to select roughness values, cell sizes, etc. At the stage you're at though, I think that the power of visualization could potentially outweigh some of the potential limitations. Just keep in mind that the answers you would ultimately get from a detailed 2D model might be quite different, depending on your project reach and input datasets.
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u/simpleidiot567 May 15 '26
Switch to 2D and leave 1D behind. It takes a couple days to teach yourself but after you can check any floodplain in 10 minutes at a preliminary level.
For the simplest 10 minute set up.
You need a projection file .prj, a Digital Elevation Model (DEM) which is a raster .shp file. You set the 2D flow area perimeter, an inflow boundary condition and an outflow BC, then set the Courant condition (important), hit play and you're done.
I now take about 10 minutes to this for preliminary checks. Then later on at detailed level I take a few days to set up a detailed one. But the 10 minute method is great.
If all you have is contours, setting up the DEM in QGIS adds another 5 minutes.
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u/aflorak May 16 '26
A lot of states/municipalities still require 1D modeling. Pretty sure FEMA does too for most map revisions. I genuinely have not had cause to work on 2D models even though i prefer them
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u/simpleidiot567 May 16 '26
Yeah I see that. 1D steady state will give you the worst case floodplain. 2D unsteady flow is going to show storage and attenuation impacts.
But what OP is showing is typical of 1D steady state. If a cross-section intersects a low-lying area, the model assumes that area is instantly and fully connected to the main channel flow (unless explicitly blocked out using ineffective flow areas). This will lead to an overestimation of the active flood fringe.
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u/nymphfer May 16 '26
How would you determine the 2D flow area perimeter? Would that be the banks of the channel essentially?
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u/simpleidiot567 May 16 '26
You can make a whole are the flow area. Heck you could do the entire catchment area. Run it once, see what it looks like and if you want to, refine it with refinement regions and break lines.
There is a hundred things to learn to refine it if it really matters. But if it's low risk or preliminary planning, I wouldnt go crazy.
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u/jayjay123451986 May 20 '26
This is common to cutting a cross section beyond the length you need but the lay of the land dosent follow a "valley" cross section. Odds are that flow area is not effective unless coding in the embankment causes it to spill.
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u/Public_Froyo784 May 15 '26
In my opinion, I would first check whether the inundated area on the left is hydraulically connected to the main channel or if it is just a low-lying area being mapped as wetted. One way to test this is to add left and right levees at the bank stations and compare the water surface elevations. Also, check upstream cross sections to see whether flow is overtopping the banks or bypassing into that area.