Monday, April 13, 2015

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. 

image

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!

image

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. 

image

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.

 

image 

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

 

image

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. 

image

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.

 

image

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. 

14 comments:

  1. Hi Chris,
    Do you know if this BreakLine tool will ever create cells with more than 8 faces? I know RAS won't run with anything over 8 and I've run into that being a problem while manually adding cells.
    Also is there a current beta release with the BreakLine tool?
    Thanks,

    ReplyDelete
  2. Will there be a similar ability to automatically realign cells around structures drawn inside a mesh?

    ReplyDelete
    Replies
    1. I'm not sure if they'll make a structure a "breakline", but you will be able to easily draw a breakline on top of your structure to accomplish what you need.

      Delete
  3. Chris, is USACE going to publish any warnings or guidance on using this model in steeper alluvial fans? I found that even with a relatively small cell size (8 ft), "leaking" appeared to be a significant problem and the results differed from other 2D models in steep unconfined flow areas.

    ReplyDelete
    Replies
    1. I don't know for sure, but I hope so.

      Delete
  4. Chris: I don`t speak english very well so i apologize if i can`t explain myself in a correct way. I recently run into your blog and i have a question non related to the topic you posted about. My doubt is: it is possible to model a junction with two incoming and two outcoming reaches? I need to model a meander rectification (channeling) and i need to know the flow distribution in the XS where the natural channel crosses through the rectification channel, thanks

    ReplyDelete
    Replies
    1. Your English is very good! No, unfortunately, you can have up to 8 tribs entering a junction and one leaving, or up to 8 (dis)tributaries leaving a junction with one entering. You cannot have multiple tributaries entering and multiple tributaries leaving a junction. A better idea would be to model that junction as a 2D area anyway.

      Delete
    2. those are very bad news, so i need to start making subjective cosiderations because i have no time to learn how to use another program (or new tool in a newer version of ras). i`m considering to model that junction like 2 very close but different ones, one for incoming and one for outcoming reaches, thank you anyway for your time o knowledge!

      Delete
  5. Chris,
    Does this breakline tool ever produce cells with more than 8 faces? I have run into that being a problem with manually adding cells. Is this a glitch that will be resolved in future releases?

    Also is a BETA version with the break line tool out?

    Thanks.

    ReplyDelete
    Replies
    1. Tim, if you assign much smaller cell center spacing for your breakline than what is in the mesh, you'll find that you can get > 8 sided cells. The cool thing in the latest version is that RAS will automatically recognize cell violations and highlight them (actually, puts a red "dot" in the offending cell), so they're easy to find and fix. If you need to have cell center spacings along the breakline much smaller than that of the surrounding mesh, do the following:
      Start with a similar cell center spacing and then enforce the breakline. Next, assign a smaller cell center spacing and again re-enforce the breakline. Continue with this process, gradually reducing the cell center spacing for the breakline (enforcing the breakline each time) until you achieve the desired cell center spacing along the breakline. This is a very easy and convenient way to not only avoid cell violations, but to provide a gradual transition in cell sizes from your mesh proper to the cells adjacent to your breakline.

      Delete
  6. Is this version available or is just for Beta testers?

    ReplyDelete
    Replies
    1. Jose-Send me an email for more information...
      Chris G.

      Delete
    2. Hi Chris,
      I sent you an email with the same question. Looking forward for some information.

      thanks,

      Hao

      Delete