New HEC-RAS 2D Modeling User’s Manual

In advance of the final release of Version 5.0 (hoping for this summer), HEC has released its 2D Modeling User’s Manual, written by Gary Brunner, the HEC-RAS Team Leader.  This manual contains updates to all of the information that was put out in the previous guidance document, “Combined 1D and 2D Modeling using HEC-RAS” as well as more discussion on developing your terrain, creating the 2D mesh, combining with 1D elements, and avoiding errors and instabilities.

If you plan to run a 2D or combined 1D/2D model in HEC-RAS, please read this manual.  Mr. Brunner has done a fantastic job putting together this very well-written document with easy-to-understand instructions and guidance for setting up and running a 2D or combined 1D/2D model in HEC-RAS. 

You can get a free electronic copy of the HEC-RAS 2D Modeling User’s Manual here, or by clicking on the link on the side bar to the right. 
 
An excerpt from the Introduction:
“HEC has added the ability to perform two-dimensional (2D) hydrodynamic flow routing within the unsteady flow analysis portion of HEC-RAS. Users can now perform one-dimensional (1D) unsteady-flow modeling, two-dimensional (2D) unsteady-flow modeling (full Saint Venant equations or Diffusion Wave equations), as well as combined 1D and 2D unsteady-flow routing. The 2D flow areas in HEC-RAS can be used in number of ways. The following are examples of how the 2D flow areas can be used to support modeling with HEC-RAS:

Detailed 2D channel modeling

• Detailed 2D channel modeling
• Detailed 2D channel and floodplain modeling
• Combined 1D channels with 2D floodplain areas
• Combined 1D channels with 2D flow areas behind levees
• Directly connect 1D reaches into and out of 2D flow areas
• Directly connect a 2D flow area to 1D Storage Area with a hydraulic structure
• Multiple 2D flow areas in the same geometry
• Directly connect multiple 2D flow areas with hydraulic structures
• Simplified to very detailed Dam Breach analyses
• Simplified to very detailed Levee Breach analyses
• Mixed flow regime. The 2D capability (as well as the 1D) can handle supercritical and subcritical flow, as well as the flow transitions from subcritical
  to super critical and super critical to subcritical (hydraulic jumps).

2D flow modeling is accomplished by adding 2D flow area elements into the model in the same manner as adding a storage area. A 2D flow area is added by drawing a 2D flow area polygon; developing the 2D computational mesh; then linking the 2D flow areas to 1D model elements and/or directly connecting boundary conditions to the 2D areas.”

(Hydrologic Engineering Center, “HEC-RAS River Analysis System 2D Modeling User’s Manual Version 5.0”, Davis, CA.  April 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. 

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. 

Mac In Black Productions

You may notice some new background imagery on The RAS Solution.  Mac In Black Productions has been very kind to allow me to showcase some of his photos on the blog site.  He has some fantastic photos and does an amazing job blending nature and infrastructure.  I especially love his photographs of rivers, streams, and bridges.  Go follow his site and you'll see why: @macinblackproductions.

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. 

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.

Sediment Transport Features in HEC-RAS 5.0

Written by Stanford Gibson, Ph.D. |  Hydrologic Engineering Center
Copyright © Stanford Gibson 2015.  All rights reserved.

The new 2D features and mapping tools are the most anticipated new features in HEC-RAS 5.0. However, HEC also added a couple major new sediment features, as well as many minor features and a few substantial bug fixes (e.g. SI Units). The two major features are the ARS-USDA Bank Stability and Toe Erosion Model (BSTEM) and Unsteady Sediment transport.

USDA-ARS Bank Stability and Toe Erosion Model (BSTEM)
HEC collaborated with Andrew Simon (Cardno) and Eddy Langendoen (USDA-ARS) to couple the HEC-RAS mobile bed model with the USDA-ARS Bank Stability and Toe Erosion model (BSTEM).  This model coupling adds BSTEM’s lateral processes (geotechnical bank failure, groundwater lag and toe scour) to HEC-RAS’ vertical, deposition and erosion processes.  This tool has a separate User/Technical Reference manual available from HEC.


Goodwin Creek, MS, repeated right bank surveys compared with computed HEC-RAS/BSTEM cross section migration from Gibson et al. (2015).
 
Unsteady Sediment Transport:
Previous versions of sediment transport in HEC-RAS used the quasi-unsteady hydraulic model exclusively, simulating hydrodynamics with a series of steady flows. HEC-RAS 5.0 couples the sediment computations with unsteady flow. Hydrologic mass conservation is the biggest advantage of unsteady sediment transport, making reservoir models and even multi-reservoir cascade models much more practical in HEC-RAS. However, coupling sediment transport to the unsteady flow capabilities also brings several powerful features, native to the unsteady hydraulic analysis environment, into sediment transport analyses including: lateral structures, flow networks, mixed flow (figure), and especially operational rules (Gibson and Boyd, 2014). Version 5.0 even includes sediment based operational parameters, operating structures based on bed change and concentration (e.g. TMDL).


Initial and bed profile and water surface elevation for a mixed flow sediment transport simulation with equilibrium sediment load and hard bottom. Sediment deposited in the sub-critical reach.
 
Other Features:
HEC-RAS 5.0 also includes a range of other new sediment features including:
· HDF5 Sediment Output and a New Sediment Output Viewer
· Copeland (1993) Sorting and Armoring Method (Exner 7 in HEC 6T)
· Gradational Hotstart
· Sediment Flow Splits
· New Dredging Tools
· Bed Roughness Predictors
· New User Manual
· DSS Sediment Time Series Boundary Condition
· Specific Gage Capabilities




HEC-RAS 5.0 will be released shortly. To test a final beta version contact Stanford Gibson at HEC.


References:
Copeland, R (1993) Numerical Modeling of Hydraulic Sorting and Armoring in Alluvial Rivers, PhD Thesis, The University of Iowa, 284 p.

Gibson, S. and Boyd, P. (2014) “Modeling Long Term Alternatives for Sustainable Sediment Management Using Operational Sediment Transport Rules,” Reservoir Sedimentation –Scheiss et al. (eds), 229-236.

Gibson, S., Simon, A., Langendoen, E., Bankhead, N., Shelley, J. (2015) A Physically-Based Channel-Modeling Framework Integrating HEC-RAS Sediment Transport Capabilities and the USDA-ARS Bank-Stability and Toe-Erosion Model (BSTEM), SEDHYD Interagency Sediment Conference, April 2015, In Press.

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.