Combining Two Reaches or Splitting One

Written by Chris Goodell | September 17, 2015


Suppose you have two separate HEC-RAS rivers and wish to combine them.  There is no “combine two rivers” option in HEC-RAS, but it can be done using the Move Points/Objects option.  Take the very simple reach presented below:

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I wish to combine the Upper Tualatin with the Clackamas reach of the Willamette River.  In the figure above, they are disconnected-two separate rivers.  Select Edit…Move Points/Objects and
you’ll notice your cursor change to a pointer with an arrowed cross, and the vertices of the cross section and stream centerlines will highlight.

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It’s helpful to zoom in close so you can see the end point nodes and so that you don’t accidentally grab a cross section node.

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Now simply grab the end point of one river and drag it on top of the end point of the other and release the mouse button.

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HEC-RAS will assume you want to put a junction here and will ask you what you want to name it.  It doesn’t matter what you put in for a name, because you will be deleting the junction next.

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Make sure to uncheck the option to Move Points/Objects and then select Edit…Delete…Junctions.

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Delete the temporary junction and you’re done.  You may need to close and reopen the HEC-RAS geometry schematic to make it look visually correct.

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HEC-RAS will automatically take the name of the upstream of the two rivers for the new combined reach.  You can change the name of the river and/or reach using the Edit…Change Name menu item.  It is very important that your river stationing for the two reaches are consistent prior to doing this operation, or it will not work.  River stations must always be in descending order from upstream to downstream.
Now let’s say you want to do the reverse and split a single reach into two reaches.  This two involves a temporary junction.  But to get HEC-RAS to insert a junction, you must draw a temporary tributary river.  You can see in the figure below a new river has been drawn from the upper right to the point on the original river where I want to split it.  Double click the end of the line right on top of the existing river, so that HEC-RAS will recognize you want to create a junction.

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First HEC-RAS will ask you for the name of the new river/reach.  Again, the name doesn’t matter, we’ll be deleting it later.

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Next, HEC-RAS will ask if you wish to split the reach.  Say yes.

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Now you have to give a new name for the reach below the split.  I’ll call it Willamette Clackamas, to take it back to the original two reaches I started with.

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Now enter a temporary junction name.

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If HEC-RAS asks if you want to increase the schematic extents, select Yes.  Now we have a river that was split into two by creating a temporary junction.

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Since the temporary river and the temporary junction were only used to split the original river into two, we’ll remove them now.  Right click on the junction and select “Disconnect Reaches”.  Then go ahead and delete the temporary reaches using Edit…Delete…Reaches.
Now the rivers are disconnected, but they still don’t look like it in the schematic.  You can verify that they are disconnected by checking to see if the new river and reach name show up or by clicking on one and seeing how HEC-RAS highlights it.  Notice here only the lower of the two rivers is highlighted when I click on it.

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If you want to visually make them look disconnected, use the Edit…Move Points Objects option again to pull the end points of the streams apart.  Again, it helps to zoom in to do this.

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Uncheck Edit…Move Points/Objects, and we’re done.  If you want it to look exactly like before, you can drag the stream centerline endpoints all the way back to the cross sections, but this is not necessary and has no effect on the computations.

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Comments

  1. GRG

    on February 26, 2018

    A reviewer of asking me to split up a continuous reach into multiple reaches even though it has no tributaries or flow splits. Is it possible or recommended? When I do it using the above procedure, if I leave the junction the model uses the junction as boundary conditions between these fake reaches, but gives an error for having a junction where there is no trib or split, if I delete the junction it runs but requires boundary conditions be re-defined for each reach.

  2. Chris Goodell

    on February 26, 2018

    No, that I not allowed in RAS.

  3. Anonymous

    on April 19, 2018

    I imported the geometry of model 2 into an existing model 1 and the river reach did not plot, however, the river stations and their cross sections did. ? I first had to change projection of geometry 2 from NAD 83 to NAD 27 in order for it to align with geometry 1. I guess I'm curious why the river does not plot, although it did import? I re-digitized the river that did not seem to import using google maps and the station cross sections, but now I need to rename the river to match the river identified in the station. When I attempt to do so, I get a message stating "Existing river-reach: cannot be renamed because another river-reach already has that name." If I remove the river, it removes all the associated stations and cross sections, which I need. How is the best way to rename the river to match that what is identified in the river stations? TK (USBR)

  4. Chris Goodell

    on April 21, 2018

    Hmmm. Not sure what’s happening there. I would suggest trying to import again but use a different name for your imported river reach. Then once in RAS, move the reach end point to combine the reaches.

  5. Unknown

    on August 1, 2018

    I have two streams that I would like to join. I use the move/edit points tool to drag one node on top of the other, and it prompts me to create a junction. So I do. But the junction isn't where I wanted it to be, it moves all the way to the top of the river, creating a long straight line from one river that crosses over the other to get to its first most upstream point. How do I stop it from doing that?

  6. Anonymous

    on August 17, 2018

    Can you model 2 structures on the same reach in Hecras?

  7. Juan Ossa-Moreno

    on September 24, 2018

    Thanks a lot for this post, am relatively new with HEC-RAS and want to join to segments, but am having problems with the cross sections names. I think am having the problem you warn about, but am not sure how to solve it. I updated the name of the cross sections hoping this would solve the issue, but it did not. I also tried using GIS Tools–>Reach Invert Lines Table to check the location of points, but the location seems to be ok in both segments, so do not know what to do. Any advise would be greatly appreciated. Regards

  8. Juan Ossa-Moreno

    on October 2, 2018

    No worries, i managed to solve this. I changed the names and closed the geometry, and once i opened it again it was fine. Thanks again for the post!

  9. Aviv Shapira

    on January 13, 2019

    Thank you very much.
    This post was very helpful!

  10. Phil Goode

    on February 5, 2019

    I can split the reach into two parts with different names like I want, but when I delete the temporary items (River, Reach, Junction), HEC-RAS v5.0.6 joins the split reaches back into one reach.

  11. Chris Goodell

    on February 6, 2019

    Make sure to click on the junction and select "disconnect reaches", before you delete the junction.

  12. Phil Goode

    on February 6, 2019

    Thanks Chris. That step was not in the original article, but I'll add a note and try again.

  13. Chris Goodell

    on February 6, 2019

    Yeah, I think somewhere between when I wrote this and the current version of RAS, they made that change in the software. I'll add a note in the original post so that others will know to do this. Let me know if it doesn't work.

  14. Phil Goode

    on February 10, 2019

    This worked perfect and I got the results I wanted. As many others have stated, thank you for all your responses. This blog and the forum are an invaluable resource for HED-RAS users.

  15. Chris Goodell

    on February 11, 2019

    Awesome! Glad it worked Phil. Thanks for reading the blog and thanks for the kind regards!

  16. Anonymous

    on April 30, 2019

    Hi,

    I am beginner in HEC-RAS. I have a river having multiple reaches, tributaries and Junctions. Hec-RAS is asking me to provide boundary condition at start of each reach which is connected through junction. I assumed that HEC-RAS needs only one upstream boundary for river (instead of boundaries at start of each reach). Where I am making mistake, kindly guide me. Thanks in advance

  17. Chris G.

    on May 2, 2019

    For steady flow, at a minimum, you need to provide the flow at the upstream end of each reach. Below a converging junction, the flow should be the sum of the two tributaries. For unsteady flow, you just need an upstream boundary condition (at a minimum). However, you have to define the flows at each reach for the initial conditions.

  18. Unknown

    on May 22, 2019

    Unluchily,my river stationing for the two reaches are NOT consistent prior. Could you please tell me what I should do to make it work? Thanks a lot!

  19. Unknown

    on May 22, 2019

    I've solved my problem by change staions' names. Thank you very much!

  20. Chris G.

    on May 29, 2019

    How about redoing your stationing so that they are consistent?

  21. Daily Logue

    on September 17, 2019

    I used this technique to split a river into 3 reaches, I then updated the geometry (center line and topo) for the central reach. When I then delete the junctions, the old centerline reappears over the new one. How do I fix this? Or is it just there graphically and will not affect calculations?

  22. Chris G.

    on September 17, 2019

    Try disconnecting the reaches from the junction, rather than deleting the junction. See if that works.

  23. Daily Logue

    on September 17, 2019

    It will allow me to disconnect the upstream reach, but if I try to disconnect the downstream reach it gives me an error message and closes HEC-RAS. Even with the upstream portion disconnected the extra center line shows up. It doesn't seem to affect the calculations though

  24. I have the same problem as Daily Logue....I still can't connect the 3 separate reaches together. Still the old reach is highlighted even after deleting the temporary junctions. Please help!

    on July 5, 2020

    I have the same problem as Daily Logue….I still can’t connect the 3 separate reaches together. Still the old reach is highlighted even after deleting the temporary junctions. Please help!

  25. Chris Goodell

    on July 6, 2020

    Make sure you have unique river and reach names for your disconnected reaches.

  26. Jola

    on October 6, 2020

    When I try to reorganise my reaches in a way described above I have encountered the following problem: HEC-RAS create 4 the same junctions (I have noticed 4 the same names of junctions in the junction editor). When I try to delete this junctions I receive information that there is en error delete junction and combining reaches. What can be wrong?

  27. Chris Goodell

    on October 14, 2020

    Not sure what you’re doing wrong. Maybe try disconnecting the reaches instead of deleting the junctions and see if that works.

  28. Anon

    on February 25, 2021

    I am new in HEC-RAS. I am trying to perform an unsteady analysis. I have a junction that splits the main channel into a diversion channel. On performing the simulation, the program shows an error that in the main channel ‘Downstream RS was not found in the geometry’. I have tried giving internal RS intial stages near the junction. But I can not provide boundary condition. I have used Energy Balance Method for flow computation. Also is there other way to model the diversion channel?

  29. Chris Goodell

    on March 4, 2021

    It sounds like one of your boundary conditions is trying to reference a cross section that doesn’t exist. Check that your boundary conditions are referenced to cross sections that exist.

  30. Steve

    on September 16, 2021

    I really needed help on how to add junctions. Thanks for this amazing piece of information!

  31. Chris Macek

    on February 10, 2022

    Can you have two completely separate stream networks? I am trying to model two separate streams that drain into one detention pond. It is a 1D steady model.

  32. Chris Goodell

    on February 18, 2022

    Yes you can do that. They each need to have their own boundary conditions though.

  33. brad

    on January 19, 2023

    Hello,
    I have 2 models provided from consultants I have imported in RAS mapper/Geometric editor. I want to model the area between them and connect the two. It simply requires drawings the river, cross-section etc. between the upstream and downstream. Hec-Ras does not like me creating a junction for these connection points because it claims junctions are only used to combine rivers or diverge rivers, not connect them.
    I was wondering if anyone could provide guidance for this as I am hopeful I do not have to remodel the work provided.

  34. Lachu M. Dhungyel

    on February 27, 2023

    Hi,
    I am a beginner and have to perform dam break using ground survey cross sections. When I imported the geometry and made necessary adjustment, the downstream most reach comes at the top while assigning the boundary conditions and flow scenarios. This gives me an error and not knowing how to resolve it. If experts kindly suggest possible solution.
    Thank you.

  35. Chris Goodell

    on February 28, 2023

    Interesting. Your stream centerline is required to be drawn from upstream to downstream. I wonder if your stream centerline goes the wrong direction? Also, your cross section river station ID’s must increase numerically in the upstream direction. See if either of those two requirements are being violated.

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