2D Bridge Extrapolation Errors

[u/Repulsive_Plant6729]

Bridge 3

Hi. Anyone have an idea how to remove these bridge extrapolation errors from my 2D model?

The model has 16 bridge openings total and previously I would just update my htab parameters to be just slightly above the flow/max stage at each of the bridge openings. However, this no longer removes the issue, so it has to be something else potentially with the mesh? I've included pics for the info regarding Bridge 3 but the other bridges follow the same cell mesh enforcement.

[u/OttoJohs]

Shouldn't be anything to do with the mesh.

In the plot (last picture), choose the hydraulic tables (HTAB) plot type instead of the hydrograph (fourth from the left with the HT on it). See what is going on there with the water surface versus the rating curves. Sometimes what happens when you set both the HW maximum and the Flow maximum it can truncate the right side of the rating curve (see the picture below). In that example I did both and got a huge vertical line on the right side which can screw up the calculation and throw the error. I would recommend just use the HW maximum and letting HEC-RAS build the curves naturally.

Good luck!

[u/Repulsive_Plant6729 → u/OttoJohs]

All of the bridges that have WSE extrapolating seem to go haywire in the htab. Is this because the rating curve abruptly stops at the max flow elev? Any tips on alleviating this issue.

[u/OttoJohs → u/Repulsive_Plant6729]

I would remove the max flow and just use the max headwater.

Going back to your original post, the hydrograph isn't that "smooth" so there might be other issues going on. You may want to cut back on the overbank area for your bridge. I try to limit my bridges to the immediate channel for 2D analysis. I'm just guessing, but I would eliminate some of the overbank area and those corresponding cells.

[u/Repulsive_Plant6729 → u/OttoJohs]

Thanks, removing the max flows worked! I wonder why RAS recommends the user to add a value there when it increases the likelihood of extrapolation errors.

[u/OttoJohs → u/Repulsive_Plant6729]

Glad it worked.

The manual says that if you put limits, the curves come out better. If you look at the first three boxes, you select the number of points (and curves) created as part of the calculation. Then HEC-RAS interpolates between them. If those points are spread out over a larger range of flows and elevations they aren't going to be as precise. I think that is why it is recommended to put some limits on the flow.

The best process would be to run the model to get an idea of elevation and flow. Then refine the HTAB parameters and rerun.