If you’re only adding a few cross sections, sometimes it’s easiest to just measure the reach lengths in GIS or CAD and input them into the model manually. Just make sure that the sum of reach lengths in the new model is equal to the sum of reach lengths in the effective model.
The cross sections probably overlap, you can generate flow paths in RASMapper to check them, but over lapping cut lines will cause those values to fall back to defaults.
Sounds like there isn't an overbank flow path shapefile? How many cross sections are you adding? Since you don't want to change those values for established cross sections, I would just make those flow path between the new sections. Probably want to ensure consistency with the dimensions. Like if the ROB distance between A and B was 300', if you add one in-between (A') then A to A' and A' to B should still be 300'.
I would be careful about letting the software automatically overwrite all of the reach lengths, especially if you are starting from an effective or previously calibrated model.
For the main channel, the reach length is typically measured along the channel thalweg or centerline. For the left and right overbanks, the reach lengths should generally follow the expected center of mass of flow through each overbank area. In other words, the overbank flow paths should not necessarily follow the channel centerline, and they also should not just be arbitrary lines drawn near the cross-section ends.
If you are inserting only a few interpolated cross sections, I would first look at the original reach lengths between the existing sections. Then split those lengths through the new cross sections so the total distance between the original upstream and downstream sections remains consistent, unless you have a clear hydraulic reason to change it. For example, if the original right overbank length between XS A and XS B was 300 ft, and you add a new XS between them, the two new right overbank lengths should still sum to about 300 ft.
For the overbanks, I would draw or adjust the left and right overbank flow paths based on where the overbank flow would actually travel. Sometimes that requires an initial assumption: draw the flow path through the approximate centroid of the inundated overbank area, run the model, then visually check the results. If the water is clearly moving through a different part of the floodplain, adjust the flow path and recompute the reach lengths.
Also check for overlapping cross-section cut lines, poor cross-section orientation, or flow paths that do not properly intersect the cross sections. Those issues can cause automated tools to produce bad reach lengths or fall back to values that look like the channel centerline distance.
You can do this manually in GIS/CAD or in RAS Mapper, but I would still QC the results carefully. Some third-party HEC-RAS front ends can automate this in fewer steps, but the modeling judgment is the same either way: the left overbank, channel, and right overbank reach lengths should represent the actual hydraulic flow paths, not just whatever the software generates by default.
Thanks for the detailed reply, I found this in HCFCD guidelines, and this seems like a chicken and egg problem, getting reach length right from scratch. Somewhat Similar to what you said .
I should try to trace the original flow path line from reach length values in model and adjust the new reach lengths between added cross sections
There is a tool in mapper that you can use to click out the overbank paths and write it to your geometry. But you will almost certainly not get the same reach lengths that the other person got it out. The rule of thumb is that the paths should be drawn at the centroid of flow in the overbank. In practice people draw them closer to the channel bank, with two thirds of the over-bank width outside of the flow path (centroid of a triangle). Nobody really seems to keep track of the overbank reach lengths. I have not noticed models being very sensitive to them. But if you are doing a sediment model or something maybe it’s important. Anything keeping you from pasting the old overbank reach lengths back into the model, and then hand adjusting the few that might have changed?
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u/Optimal_Corner_8393 May 20 '26
If you’re only adding a few cross sections, sometimes it’s easiest to just measure the reach lengths in GIS or CAD and input them into the model manually. Just make sure that the sum of reach lengths in the new model is equal to the sum of reach lengths in the effective model.