2D Mesh “Leaking” Part 2 – 2D Area Breaklines

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

Continuing on with the discussion on leaking in 2D meshes, I want to highlight a new feature that will be included in the full release of Version 5.0 (hopefully due out early this summer).  The new feature, 2D Area Breaklines, will make aligning cell faces on to high ground features a snap!  Essentially, you draw breaklines along the crest of high ground features in your topography that will completely or temporarily act as a barrier to water flow.  A good example of this is a levee or berm.  As discussed in the previous post, we need to have our cell faces aligned to the high ground of these features so that the terrain is properly picked up in our mesh.  Here’s the same example found in the Muncie data set.  I’ve set the cell size to be rather larger through here, but certainly of an adequate size.  As shown, there is a high ground feature with a slightly lower overflow section that (when the stage is high enough) will allow flow to spill northward and continue up the floodplain. 

Notice that there is a cell that sits on top of the overflow section and straddles its high ground.  Running the model this way will allow flow to leak through the high ground feature, even before the high ground is overtopped, as shown below.  The example simulation below actually leaks water through the high ground a full 4 hours and 50 minutes before it is actually overtopped.  Not good!

In previous RAS 5.0 beta versions, to fix this you had to manually add cell points to align your cell faces.  In this example, that is not too hard, but you can imagine that with a long high ground feature and small cell sizes, this can take quite a bit of time. 

With the pending full release of Version 5.0, you can now let 2D Area Breaklines do the work for you. 

Select the 2D Area BreakLines button and click a series of points on your schematic to draw your breakline.  Double-click for the last point and at the prompt, give your breakline a name.

 

 

Left-clicking on the breakline will provide you some options for control over how your breakline re-adjusts cells.

 

Select Edit Break Line Cell Spacing to specify the spacing of cell points around the breakline.  In this example, I used a slightly higher center spacing so that the cells on the downstream side of the high ground feature extends all the way to the low spot (It is important to not have cells completely contained on a slope of a high ground feature like a levee or berm-I’ll save this topic for another discussion)

Now simply left-click again and select “Enforce Breakline in 2D Flow Area…” and RAS automatically redraws the mesh, respecting the breakline and the specified breakline cell center spacing. 

Now we can see that the high ground feature no longer leaks and only allows water to pass to the next cell once it is overtopped.

 

It’s important to note that there is some fragmentation downstream of the crest.  This is normal for water flowing over steep terrain.  You could go to the trouble of reducing the size of cells as I did in the previous post to eliminate this issue, but as with all modeling decisions in HEC-RAS, you must determine if that effort is worth it based on your overall study objectives. 

2D Mesh “Leaking”

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

To follow up my post on fragmented inundation, I want to highlight another 2D mesh issue we should all be aware of.  Unlike fragmented inundation, which is an artifact of how HEC-RAS discretizes the 2D domain and the way it maps the results, Leaking is a result of terrain features not aligning with cell faces and/or cells that are too large, and can produce very wrong results. 
Take the following example of leaking.



Here we see a high ground feature that is straddled by rather large cell.  HEC-RAS will preserve the underlying terrain on the cell faces, but the cell itself is resolved to a volume-elevation curve.  Since the high ground feature runs diagonally through the cell, it is not picked up by the cell faces.  As a result, HEC-RAS does not know that there is a barrier that should keep water on one side of the high ground feature before it is overtopped.  The consequence is that water leaks through the high ground and is available to move further down the channel even before the high ground is overtopped.  This is incorrect.  To better capture the high ground feature, cell faces in this vicinity should be aligned to the high ground feature so that the terrain is picked up on the cell faces, which will prevent leakage.  The following figure shows the resulting flood map at the same time in the simulation as the figure above. 



Here we see the entire mesh has been overall refined to a smaller cell center spacing, but in addition, much more resolution was added by manually straddling cell centers around the high ground feature.  Notice around the crest of the high ground feature, the cell centers were placed to align the cell faces with the contours.  This ensures that the high ground is picked up by the cell faces.  The result is a higher resolution flood map, but also, and more importantly prevents leakage through the high ground before it is overtopped.  Also important is to provide much smaller cells on the downstream slope of the high ground feature, to prevent fragmented inundation.  In hindsight, I probably went a little overkill on adding cell centers, but it didn’t really add any noticeable time to the simulation, so I’m good with it. 
Manually adding cell centers is not particularly precise, and can take a bit of time.  Fortunately HEC will be including a new feature in the full release of 5.0 that allows the user to define a breakline along high ground terrain features like this.  The mesh is then generated, automatically aligning the cell faces to that breakline. 

2D Troubleshooting – Fragmented Inundation

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

(Initial post 19 Feb., 2015.  Updated 20 Feb., 2015).  I continue to be more and more impressed with how user-friendly and robust the new 2D feature is in HEC-RAS.  However, there are some issues to be aware of, particularly in how results are mapped.  I want to take the next two blog posts to highlight two of the more important mapping problems to be aware of.  Today’s post covers what I call “Fragmented Inundation”, or fragmented mapping.  If you’ve run some 2D data sets already, you are likely to have seen this, especially if your 2D area starts off dry, you have steep terrain, and/or your cell size is too large.  Fragmentation usually shows up just as a part of a 2D area becomes wet (i.e. very shallow depths), and tends to go away as depths increase.  Very rarely do you see fragmentation in the maximum water surface plot.  Here’s an example of fragmented inundation:

Notice the isolated “ponds” of water in some cells, disconnected from the water in its neighboring cells.