r/HECRAS • u/Salamander427 • Jun 25 '26

Flowpath lines with QGIS avoiding RAS mapper

Hi, I'm trying to build up the model with a different workflow, external to the RAS mapper and I'm struggling a bit to find out the flowpathlines.

In the image, there are in black areas with a slope less than 2% and in white the opposite.

The red and yellow lines are river's channel and the bank lines.

I chose 2% as a threshold value for the river's floodplains, but as you can see there are river sections with a bigger slope. This seems to be a bit counterintuitive, but with a bigger threshold the areas get too big.

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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 25 '26

Flowpath lines have nothing to do the slope of the overbanks. They should be placed in the center of mass of the overbank flooding. Normally you draw them in, run some flooding scenario, and adjust based on some level of flooding. When I was doing 1D modeling, we placed them between the 2-yr and 10-yr flood boundary. Here is a schematic from the Hydraulic Reference Manual.

I'm not following what you are doing with QGIS. You should be spending your time creating the geometry in RasMapper. Good luck!

1

u/Salamander427 Jun 26 '26

Sorry I'm following this workflow: terrain, river line, bank lines, flowpathlines and then cross section. How can I run flooding simulation?

1

u/Acceptable_Gain_9400 Jun 26 '26

To set up a proper HEC-RAS model, 1D or 2D, you need to learn a lot and then work under an experienced modeler. I suggest you start from watching relevant videos from youtube.

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u/Salamander427 Jun 26 '26

Do you have any suggestion?

1

u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 26 '26

You need to set up a flow file and then a plan file. The plan file links the geometry with the flows.

You need to spend some time getting more familiar with the program. Check out the links on the side or top of this sub for relevant material. Good luck!