Breach weir in a 2D mesh

[SaraA]

Hi,

I have a very strange problem in a 2D mesh in HEC-RAS 5.0.5.

I have several breaklines in the 2D mesh which I am trying to breach at some point. I made the breach plan data for one of these breaklines. When I run the unsteady simulation I got the following message for the breakline (SA 2D connection at 6) trying to breach:

The SA 2D Connection at 6

has a weir breach lower than the cells they are connected to
for 2D Area 2D-area

The model runs until the end without any more messages and issues.

However, when I go to see the results in Ras mapper, the model has good results until I reached the hour of the breach, at this hour the water dissapears completly from the model, and the Depth file from this hour of the breach until the end of simulation is empty. No water at all.

Someone knows why can be cause of this?

Thank you,

Sara

[Scott Miller]

It sounds like the connection that is being breached has an embankment profile that follows the terrain. When the breach forms, the breach goes lower than the terrain. Is this correct?

It may be that the developers did not write a method to handle the exception, so the model runs. The breach below the terrain may be draining the model. Try running the time of the breach at a short output step, and see whether the water disappears instantaneously, or drains out.

In order to fix the problem, I expect it will work to generate the terrain with lower elevations at the vicinity of the breach. Keep the embankment profile, but make sure the terrain at the breach is no higher than the bottom of breach.

[cameron]

I agree with Scott about the issue. You need to either adjust the breach elevation or modify the terrain.

[SaraA]

Hi Scott and Cameron,

Thank you so much for you advices.

I checked an certainly it is an issue with the terrain and the embackments over the terrain.

Do you know what it is the best option to erase the height of the embackments into the terrain in Arcmap? and in this way, to uniformize the DEM with the cell height of the neighbours cells.

I am looking on internet how to do it, but I do not find the way to eliminate the embackments in the terrain.

Thank you,

Sara

[cameron]

Depending on your cell size, you may not have to erase anything. If the cells that are along the 2D connection extend far enough away from the top of embankment (beyond the toe) and have elevations which you want to breach to, then you don’t need to do anything. If this is not the case, you would have to make the grid cells along the dam larger.

If you do want to remove the dam/embankments what I do is buffer the centerline of the embankment a width so that it covers the embankment, then generate a ton of vertices along the buffered line, convert to points, extract elevations, create TIN from points and buffered line, convert to raster, and mosaic to original terrain.

I like the TIN option as it keeps it smooth, if you go from points to raster it can cause a rounding effect.

[Scott Miller]

Since you have ArcGIS available, it is good to know how to modify elevation grids. You can use the spatial analyst extension for this.

http://desktop.arcgis.com/en/arcmap/latest/extensions/spatial-analyst/enabling-the-spatial-analyst-extension.htm

In the spatial analyst toolbox is a tool called raster math. Without going much into how to use it, I’ll just point out a very useful formula.

[output] = con(isnull([input2]),[input1],[input2]

input1 would be the existing terrain
input2 is a modification you want to make to the terrain

The formula checks where there is no data in input2. It converts the null cells to input1 values, and puts input2 values where there is data in input2. You can create elevation values for input2 either as polygon or polyline vector data with an elevation value field. Convert the vector data to raster data, and just be sure to set the environment to have the same extent and cell size as the terrain raster.

I use this method to cut channels in elevation data to drain closed depressions. When I have recovered enough live storage this way, then I use the hydrology>fill tool in spatial analyst. Fill makes a prettier graphic for management.

This full technique is not needed for patching the terrain in HEC-RAS. You can create a polygon patch with an elevation field, convert that patch to a raster, and export it as a TIFF. RAS Mapper allows you to layer this patch on the top when you generate a new terrain. Use your original terrain source, and lay a small rectangle with lower elevation values across the embankment.

Sorry for the long post.

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