Those Nuisance Cell Errors and Wedge Depressions

Written by Chris Goodell | Kleinschmidt Associates

Copyright © The RAS Solution.  All rights reserved

If you've been doing some 2D modeling, you most likely have run into what I call nuisance cell errors.  You see them while your model is running and they can really slow down your simulation as the RAS chugs through iterations to try to come to a solution.

The errors here are in the second to last column and are in units of feet.  So as you can see, they are quite small, and normally I wouldn't be too concerned with them.  However, if the errors persist, they can really slow down your simulation time.  Sometimes adding several hours to the overall time, in my experience.  So while the errors are small, there is a need to get rid of them, at least the ones that persist for many timesteps.  

A common cause of nuisance cell errors is what I call "wedge depressions".  In the following figure, notice the wedge of water that sits below a well defined ridge line.  The red arrow points to it.  

As you can see by the blue hydraulic connectivity lines that there is hydraulic connectivity between Cell 1 and Cell 2.  But water fills up cells from the lowest spot, and in Cell 2, this is below the ridge.  So you get a narrow wedge of water (wedge depression) that builds up causing some local fragmentation and a resulting error.  Notice the water just abruptly stops at the cell face between Cell 1 and Cell 2.  Wedge depression doesn't always cause an error (by error, I mean RAS cannot converge on a solution within the error tolerance in 20 iteration tries).  But it did in Cell 2.  Notice Cell 4 has the same wedge depression, but it was not producing errors in this simulation (ahhh, the mysteries of HEC-RAS!).  

Fortunately, the fix for this is easy.  You draw a breakline along the ridge line so that a cell face resides on the ridge line itself.  This removes the low spot (and thus the wedge depression) in Cell 2.  the next figure shows the placement of the breakline (but before enforcement).  In this case, to preserve the spacing of the cells on the north side of the line, I specified a cell center spacing of 55 feet on the breakline (actually, I started with 40 feet, then 50 feet, but these produced the dreaded "red dots".  55 feet worked.)

After a bit more tweaking of the breakline alignment, here is the result with the breakline enforced and the wedge depression eliminated.  Notice without the wedge depression, flow is able to move more naturally towards the upper left from what used to be Cell 1.  Oh yeah...and no more nuisance cell errors!  

Yes...there's still some more fragmentation that could be addressed, but this is early on in the simulation and my objective is to ultimately present the max ws maps.  So my concern at this point in the simulation is to have decent results with fast simulation times.  I'm not concerned with little bits of fragmentation that don't throw errors.  

Bridges in 2D Areas-Important Update!

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants 
Copyright © The RAS Solution 2016.  All rights reserved. 

***Update:  I received a few very important comments about this post that I want to share.  

First, and this is very important:  These methods are not meant to be used to do any kind of detailed bridge design or bridge scour analysis (contraction, abutment, or pier scour).  The methods I present in this post are strictly as a means to include a bridge in a 2D area to gage its effect on the overall resulting flood inundation.  Meeting FHWA standards for bridge modeling requires much more information than these options will provide you.  


Second, Mark Forest posted a couple of very good comments that I would like to share. Please read his comments at the end of this post.


There have been a lot of requests for examples of how to model bridges in 2D areas.  First, let me state up front that in the current version (5.0.1) there is no direct way to put a bridge in a 2D area, like you would in a 1D reach.  That means, that we currently don’t have a way to use low flow bridge modeling approaches like Energy, Momentum, Yarnell, and WSPRO.  Likewise, you can’t use the high flow approaches (Energy, Pressure/Weir).  Hopefully this will be added to a future version.  

That being said, I’ll present three options for simulating bridges in 2D areas.  

To demonstrate these options, I used a dam breach model adapted from HEC’s Bald Eagle Creek data set.  The animation is included at the end of this article. 

A Summary of New Features of the latest (Aug 21, 2015) beta version of HEC-RAS 5.0

The following article written by Mr. David Markwood of AECOM provides an excellent summary of the latest beta (candidate) version of HEC-RAS 5.0.  Well worth the read!

https://www.linkedin.com/pulse/hec-ras-50-beta-update-david-r-markwood-pe

Transect for Extracting Flow in a 2D Area

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

*Update:  Version 5.0.2 now does flow transects in RAS Mapper with profile lines.  No longer necessary to use a 2D Area Connection to do this.

A very common output request in 2D modeling is for flow over (through) a transect line.  HEC-RAS 5.0 does not have a specific tool for extracting flow data along a line in a 2D area, however it CAN be done.  Here’s how you do it.
Inside your 2D area, draw a 2D Area Connection where you would like to extract flow.  In the figure below, I’ve drawn my 2D Area Connection line from bottom to top, which represents left to right, looking in the downstream direction. 

Notice that with the completely orthogonal cell orientation, but with a skewed 2D Area Connection line, the black and red striped connection line is “jagged”, following cell faces between the connecting cells.   To fix this, we need to use a breakline.  You could draw a break line on top of the 2D Area Connection (click the 2D Area Break Lines button on the top of the Geometric Data window), however, HEC-RAS also has a short cut for putting breaklines on 2D Area Connections.  If you right click on the 2D Area Connection (I’ve named if “Flow Line”), you’ll see an option to “Create 2D Flow Area Breakline”.  Select this for quick cell re-orientation around the 2D Area Connection Line.


Now you’re ready to enter data to the 2D Area Connection that you’ll use for extracting flow.  Click the SA/2D Area Conn button on the left side of the Geometric Data window. 
Make sure you connect “From” and “To” to the same 2D flow area.  Then choose “Normal 2D Equation Domain”.  This will force RAS to use the same 2D St. Venant equations over the terrain under the 2D Connection line as if the connection line wasn’t there.  BUT…you still have to define the Structure Type as a Weir and provide station-elevation data to define the crest of the weir, even though RAS will not use it.  Maybe this gets fixed in a later version.  HEC makes this easy though as you can extract the terrain profile from the 2D Connection cutline by clicking the button “Terrain Profile” as shown below.  Just copy and paste the terrain profile into your station-elevation table and you will  have defined a “zero-height” weir.  I guess it’s possible to use any crest definition, since RAS shouldn’t use the weir equation-it will use the normal 2D equation domain, but I haven’t tried it. 

Now you are ready to compute.  After computing, go to the Stage and Flow Hydrographs output window.

Make sure you select Type…Storage Area Connections (yes, it is technically an SA/2D Area Connection, but HEC hasn’t gotten around to renaming this yet in this window).  Now you can see the flow hydrograph for the flow line you’ve drawn.  You can check, but you’ll see the results are exactly the same as before including the flow line.

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. 

Status on HEC-RAS 5.0

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

Many of us were hoping to see the official release of HEC-RAS version 5.0 around now (end of Feb, early March, 2015), but unfortunately there have been some delays due to a large amount of bug fixes and HEC won't be releasing it as planned.  I haven't been provided with a new date, and I don't want to guess, but I don't think it will be too much longer.  Keep checking back here at The RAS Solution, or at HEC's website for the latest information on the official release.

The good news is this delay will give HEC some more time to include some more really cool and useful features.  Potential new features are shown below.

I don't have all the details on these features yet, but the 2D Area Breaklines tool looks to be a quick and convenient way to align cell faces in your 2D mesh around terrain features-this will save a LOT of time setting up and editing your mesh.  The 2D Area Mann n Region will allow you to draw polygons directly in the HEC-RAS Geometry editor for region-specific n-value factors.  It won't initially replace the need for a land-use shapefile, but will allow you to more easily run calibration on your n values.

In the meantime, the October Beta version of HEC-RAS 5.0 is still available and works very well. You can get the install file here or on the side bar of The RAS Solution.