1D? 2D? or 1D/2D? How Should I Build my Model?

Written by ironcore | March 29, 2016

Now that the official release of HEC-RAS 5.0 is out with 2D capabilities, I’m getting a lot of questions about whether 1D or 2D (or 1D/2D combined) is the best way to set up a specific model.  The answer is very simple.  Like everything else…It depends!  Fortunately, there are some guidelines.  
1.  The general rule of thumb is that if the length-to-width ratio is larger than 3:1, a 1D model can possibly be used; otherwise, a 2D model is needed (source:  Desktop Review of 2D Hydraulic Modelling Packages, UK Environment Agency, 2009).  For example, if a river reach is 10,000 m long and has a 100 m wide floodplain, the ratio is 100 to 1, so a 1D model is likely okay.  For a river reach that is 10,000 m long but has a 5,000 m wide floodplain, the ratio is 2 to 1, so a 2D model will probably be needed.
2.  Features such as a narrow bridge crossing causes significant expansion/contraction are best modeled using 2D capabilities.
3.  If knowing the flood patterns around buildings and other discrete features is important, a 2D model will be necessary.
4.  Detailed animations showing floodwave progression in multiple directions at a local scale is best represented using a 2D model.  If simple water surface elevation graphics are needed, both 1D and 2D models can be used to produce these results.
When will a 1D model be suitable?
1.  Locations where flow isn’t required to ‘spread’ significantly (flow maintains primarily uni-directional flow patterns).
2.  Well-defined channel/overbank systems (channel is bounded by steep slopes, constricting the lateral expansion of flows).
3.  Simply-connected floodplains where flow in main channel is well connected to flow in the overbank and that flow in both is primarily uni-directional in nature.
4.  When elevation data of only limited quality/quantity are available.
When is a 2D model usually preferable?
1.  Anywhere flow is expected to spread
2.  Urbanized Areas
3.  Wide Floodplains
4.  Downstream of Levee Breaks
5.  Wetland Studies
6.  Lake or Estuary Studies
7.  Alluvial Fans
Other Considerations:
Like anything else, there is rarely a definitive answer to the subject question, rather a lot of gray area.  Frequently, a model could be constructed in 1D or 2D and provide excellent answers either way.  In this case, the experience of the modeler with 1D modeling or 2D modeling becomes very important.  Someone who is very skilled at setting up a 1D model to represent 1- and 2-D conditions (a quasi-2D model) may end up with a much better model than if that same person tried to build a 2D model without much experience in 2D modeling.  And vice-versa. 
There are pluses and minuses to going purely 2D.  First of all, if you can justify using Diffusion wave, a purely 2D model will most definitely be more stable and robust than a 1D or 1D/2D unsteady flow model.  You’ll be surprised how easy it is to set up and run.  Even if you do have to use Full Momentum, typically if your Courant Condition is well satisfied, the model will be more stable.  With multiple streams arranged with complicated junctions and loops, the 2D version will do a much better job – especially around junctions and flow transfers from one stream to another.  And you get to remove subjective modeling techniques like ineffective flow areas, levee markers, cross section orientation, etc.  Some downsides to a fully 2D model are: 
1.  Run times.  If your 2D area is very large and you have relatively small cells (i.e. a lot of cells), then run times can be long.  By a lot of cells, I’m talking about 100,000 to 1 million or more.  Making your model 2D in areas where you need detail and 1D everywhere else can help solve this problem. 
2.  Output.  Getting output from 2D areas is a bit more cumbersome and limited. Still, you can get quite a bit of stuff out of your 2D areas, it just might take more time. 
3.  In version 5.0, there is no direct way to model pressure flow at bridges in a 2D area. Hopefully this will change for the next version.
4.  Learning curve.  Being new to 2D modeling, there will be some overhead just learning how to do it. 
5.  Your client may not be okay with it.  Make sure your client is aware of the benefits of 2D modeling.  There is generally a perception that 2D modeling is more expensive.  This is not (should not) always be the case. 
And remember…
Make everything as simple as possible, but not simpler.
            -Albert Einstein (paraphrased)
For every complex problem there is an answer that is clear, simple, and wrong.
            -H.L. Mencken
For more information, make sure to give Chapter 6 of the new HEC-RAS 2D ModelingManual.


  1. Bryce W. Cruey

    on March 29, 2016

    Nice set of guidelines. Thank you posting this it is very helpful.

  2. Chris Goodell

    on March 30, 2016

    You're welcome! Thanks for reading.

  3. Pedro

    on March 30, 2016

    I would like to suggest to point the importance of field verifications (when possible) and the use of observed times series of stage and flow to calibrate the model (again, when possible, to statistical verifications). It is just a guess, let me know if what I wrote is nonsense. Thanks.

  4. Bruno Neves

    on March 31, 2016

    Hello! Is hec5 ready to run 2d sediments transport simulation?

    best regards

  5. Chris Goodell

    on March 31, 2016

    No, but I understand HEC are already working on putting it in.

  6. shahab afshari

    on April 9, 2016

    Hello Chris,

    I have a question regarding water quality analysis (water temperature modeling) in HEC-RAS 4.1. Basically, I am going to execute water quality analysis based on simple physical water and air properties, e.g. constant boundary and initial conditions of temperatures, constant air pressure, etc., all along a straight uniform channel which is ending to an estuary. I set up the water quality properties in the specified domain and saved the water quality data to be utilized in water quality analysis. Also, I performed the unsteady flow analysis, so it is initiated for water quality analysis. I executed the WQ analysis successfully, however, on VIEW option I the "WQ Spatial Plots" or "WQ Time Series Plots" are still frozen, that is, I cannot check out the Water Quality outcomes.

    Do you know why this happens, or whether I should enable any options prior to run WQ analysis?


  7. Chris Goodell

    on April 15, 2016

    That's strange. It's possible it is a bug. Did you try it in 5.0?

  8. shahab afshari

    on April 15, 2016

    Hello Chris

    Thanks for your response. I tried it in 5.0 using the same setting and inputs as I generated and saved in 4.1 version, and now it gave me the following error once I tried to run the water quality analysis: Error with program: WQNet.exe Exit Code = -1073741515

    I should note that the same disabled options in View tab: "WQ Spatial Plot" and "WQ Time Series Plot" are still disabled. I am using HEC-RAS 4.1 and 5.0 on the VirtualBox model of Windows 7 X64-bit, however I learned that on Actual Windows machine where I had 4.1 version the "Unsteady Flow Spatial Plot","Unsteady Flow Time Series Plot","WQ Spatial Plot" and "WQ Time Series Plot" remained frozen even after running Unsteady Flow Analysis.

    That is really disturbing that I could not find anyone who had the same problem and shared it via HEC-RAS forums.


  9. Unknown

    on April 22, 2016

    Is the 'Break the HEC RAS code' book still applicable to HEC-RAS 5.0?

  10. Chris Goodell

    on April 22, 2016

    Yes, it was written for Version 5.0.

  11. Randolf Sanders

    on May 9, 2016

    Hello Chris,

    I felt that this question kinda pertains to this blog and was wondering if you could clarify something for me regarding the placement of bank stations within a cross section. A consultant of ours is trying to persuade us that their model of a creek widening, with their right over bank placed within the main channel just above a newly established low flow channel, gives a better representation of the cross section even though they are only creating a 12 foot bench within a 70-80 foot wide channel. The main reason they are pursuing this is that it gives them about a 1/3 of a foot lower WSEL. What is the reason that HEC-RAS would show such a difference between WSEL and how does HEC-RAS calculate the over banks compared to the main channel. Any help would be much appreciated.



  12. Chris Goodell

    on May 9, 2016

    There are a number of things that could contribute to this. Most likely the difference in n values plays a part. Also, the way RAS computes conveyance in the sub-sections and adds them together could be important. For the results to be believable, the user must sub-divide the cross section into the three sub sections at the points of significant change in conveyance. If this rule is being broken to achieve the 1/3 foot lower WSEL, I would have issue with it.


    on March 7, 2017

    Hello I make a 1D / 2D model. After the simulation I get the following errors:
    Performing Unsteady Flow Simulation HEC-RAS 5.0.3 September 2016
    Error with initial backwater/starting conditions
    Unable to compute starting flows
    Specifying Initial Flows, under Initial Conditions
    on the Unsteady Flow Data editor may help

    Writing Results to DSS
    Reading Data for Post Process

  14. Jasper Ternate

    on April 9, 2017

    Hi sir, I'm currently using hecras 4.1. I have a problem with the ras mapper output. After i run an unsteady flow analysis of a with data of a certain rainfall event, the ras mapper wont show the maximum water surface at all. When I run another event with higher rainfall data (50yr return period), it display the water but it is discontinued at certain point near the downstream. What do you think is the reason for this? I'm hoping you can see this post and thank you for this blog it is very helpful for me just a newbie in ras modelling.

  15. Chris Goodell

    on April 10, 2017

    RAS Mapper in Version 4.1 is pretty clunky. I'd suggest modeling in 5.0.3. RAS Mapper in the latest version is much much better.

  16. Jasper Ternate

    on April 10, 2017

    Thank you Sir, I'll try to download the latest version.

  17. Vimal Chandra Sharma @ IIT Hyderabad

    on July 4, 2017

    Dear Chris Goodell,
    The application of 1D/2D concept has been explained well. But still i need a clarity that, apart from the 'spread' of a river, how can we justify a 2D output from 1D when the both outputs(1D and 2D) separately showed (however both the results in 2D plane only). Even a 1D result shows flood extent from a cross section width, where a 2D output shows flood extent with a polygon, so here the dimensional differences from line to polygon is the difference between 1D and 2D? Kindly provide me more explanation on how can we differentiate 1D and 2D ?

  18. Vimal Chandra Sharma @ IIT Hyderabad

    on July 5, 2017

    Dear Chris,
    In an unsteady flow simulation, if i don't have the inflow (flood/stage hydrograph) for a basin as a boundary condition where rainfall is available,how can i give precipitation as BD. The option was not highlighted while entering data. Did i miss any step in the model preparation? Kindly answer me on this as i am searching for it since long time.

    Vimal Chandra Sharma

  19. Chris Goodell

    on July 5, 2017

    You can only add rainfall as a boundary condition to a 2D or Storage Area.

  20. Chris Goodell

    on July 5, 2017

    Please have a look at Chapter 6 of the HEC-RAS 2D Modeling Manual. There you will find a lot more information about this topic. Hopefully that will help.

  21. Vimal Chandra Sharma @ IIT Hyderabad

    on July 5, 2017

    Thank you so much for the answers


  22. Anonymous

    on August 17, 2017

    Hi Chris,
    I am adding 2D areas to an already completed 1D model with proposed and existing geometries. Is there a way to copy 2D areas that were created in one geometry into a different geometry without starting over?

  23. Chris Goodell

    on August 17, 2017

    Yes, you can use the import geometry data (HEC-RAS format) command from the geometry window. Note that in the import dialog, RAS calls 2D areas "Storage Areas".

  24. Anonymous

    on August 17, 2017

    That makes it easy! Thank you for single-handedly answering the world's HEC-RAS questions.

  25. Chris Goodell

    on August 17, 2017

    🙂 You're welcome!

  26. Calvince Wara

    on August 17, 2017

    Hi Chris
    Am trying to use HEC-RAS 5.0.3 to with main aim of developing and calibrating for a Rating Curves of a bridge within the river channel. Am using the observed flow measurement to do the calibration (Manually changing n, expansion & constriction values) so to match the observed values. I would like to put the observed at the weir structure location but I don't know how? It only allow the observed at the upstream cross section. Any advice and help is highly appreciated.

  27. Chris Goodell

    on August 18, 2017

    You can set your observed location at the cross section just upstream of the dam and then set the Dn Dist (Downstream Distance) so that the observed point is at the weir structure. However, I would caution using observed data at a weir structure. There is a lot of hydraulic stuff happening around weir structures that aren't accounted for in RAS (i.e. severe contractions and vertical accelerations). Calibrating to a point there might not be a great idea.

  28. Anonymous

    on February 8, 2018

    describe your initial conditions in the model

  29. Brendan Pauls

    on February 17, 2018

    Hi Chris,

    I am building a dam breach model, with the reservoir modeled as 1D, and the area downstream of the breach modeled as 2D. I am consistently getting the error "Minimum error exceeds allowable tolerance at 18MAY2017 24:00:00", and then it states the cross section where the 1D and 2D are supposed to be coupled.

    I feel like I have tried every method I can find for fixing instability issues in unsteady models, but no luck. My gut says there is an issue with how the 1D and 2D models are coupled together. Flow does not seem to be getting passed to the 2D area properly.

    When I open the results in RAS Mapper (albeit they are incomplete), it shows the minimum depth of the 2D area as an impossibly high number (103632 with 50 zeros behind it).

    Is there some simple step I could be missing to pass flows from a 1D to a 2D area? I have tried:
    -Setting initial flows
    -Initial elevations
    -Mixed Flow
    -Setting 2D Flow Options for Initial Conditions Time and Ramp up Fraction
    -Iterating between 1D and 2D
    -10 second time step

    Any thoughts would be appreciated.


  30. Chris Goodell

    on February 17, 2018

    Hi Brendan-you’ve encountered perhaps the most common source of instabilities for 1D2D-the 1D 2D connection itself. I assume you’ve verified the connection is actually established. You’ve listed the typical ways to fix these types of errors. But also you want to make sure that the conveyance of the connections cross section is the same or as close to the same as possible by:
    1. Make sure the cross section is placed on the 2D boundary.
    2. Make sure the cross section and 2D area Are made from the same terrain.
    3. The cross section and the connecting cells have the same n value definition.
    4. Try to locate the connection in a place where flow is generally 1D in nature and there are no ineffective flow areas.

    Hope this helps.

  31. Chris Goodell

    on February 17, 2018

    I meant to say the conveyance of the cross section is the same or close to the same as the total conveyance in the bounding cells.

  32. Brendan Pauls

    on February 18, 2018

    Thanks for replying Chris, I appreciate it! Unfortunately nothing seems to be helping to get this model running. I'll keep plugging away at it.

  33. Unknown

    on March 26, 2018

    Hi Chris, I´m trying to model a wetland that has it is originated by the end of a lake. The problem is that there is no river in the area… is that possible to do it??

    Thank you so much

  34. Chris Goodell

    on April 5, 2018

    Yes. If you can assign boundary conditions to it, you can model it.

  35. FKI

    on August 1, 2018

    Hello chris.. i wanna to build 2D model based on 1D. Is there are some methods, like must to connect with lateral structure, and the other to make that? Thanks..

  36. FKI

    on August 1, 2018

    Hi Chris.. i'm going to make 2D model Hecras based on 1D model. If i want to make it, do I connect without lateral structure?. So that if the river is blow up, the flow will be directly inundating flow area. Thanks.

  37. Anonymous

    on September 27, 2018

    Hi Chris,
    I’m Working in modeling flood in an urbanized area. I create a 2D area for the model then a land Cover for the 2D area and I create a land cover also for the main channel to change the manning’s value
    To calibrate the model, I start changing the manning’s value for the main channel.
    I would like to ask you if there is another way to calibrate a 2D model

  38. Chris Goodell

    on September 27, 2018

    You could also focus on other uncertain input parameters. But yes, generally that's how it's done.

  39. Prince Mathur

    on November 5, 2018

    Hi Chris,
    I am using HecRAS2D 5.0.4 version to build a model that includes one major stream inlet and outlet and other minor tributaries. To get an flood extent we need to give flow boundary condition at each of locations of upstream segments.
    For model calibration practice, we need to add flows at different junction to conserve flows as per increased in drainage area because there are no other flow adding in hec 2d unsteady flow other than at upstream segments. If I am not wrong then what is the best practice to calibrate the 2D model wrt observed flows.

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