Connecting a River to an Off-channel Storage Area using a Lateral Structure

Written by Chris Goodell | November 7, 2012

An off-channel storage area in HEC-RAS can be a very useful way to simulate flooding in interior areas, adjacent ponds and lakes, urban areas next to rivers, green storage, or just about any area that you expect to flood but will be better represented as ponded water versus actively conveying water.  Connecting rivers to off-channel storage areas is done via lateral structures.  Although it is possible to use lateral structures and storage areas in steady flow modeling, typically lateral structures and storage areas are used in unsteady flow modeling, where quantification of storage and hydrograph attenuation are very important.

Here’s a simple example of using a lateral structure to connect a river to an off-channel storage area (this happens to be the LeveeBreach.prj project that comes with the HEC-RAS installation).
To begin, first make sure your cross sections are all included and correctly entered.  Then you draw in (or import from GIS) your storage area.  To do that, simply click on the Storage Area button on the top of the geometry schematic and start clicking points to define the perimeter of the storage area.  Double-click to complete the storage area.  Now you are ready to connect the river to the storage area.
Select the Lateral Structure button on the left side of the geometry schematic.    When you do this the first time, the following graphic will be blank, but in this case, the lateral structure (which is being used to simulate a levee) is already entered in.  The figure below shows the lateral structure in profile view.  The stationing plotted on the x-axis is the lateral structure stationing (which you will define in the Weir/Embankment editor).  It is NOT the same as the river stationing.

The River, Reach, and HW RS (Headwater River Station) define the location of the lateral structure in your system.  The upstream end of your lateral structure will be located at the HW RS (but can be shifted downstream of this station in the weir/embankment editor).  Notice that lateral structures are stationed from upstream to downstream (i.e. 0 is the most upstream point on the lateral structure).  The vertical lines in the graphic represent cross sections that are spanned by the lateral structure.  The vertical line that sits at station 0 is the HW RS.  The boxes on the bottom of the vertical lines represent the invert elevation of the respective cross sections, and the boxes on the top represent the end points of the cross section (on the side of the cross section that the lateral structure is located: left or right).  The red dots represent the bank stations of the respective cross sections.

Next, give a description for the lateral structure in the Description box and then define where its headwater position is.  You can place the lateral structure in either of the overbanks (left or right side) or adjacent to either bank station (left or right).

The plan data Optimization button is just a shortcut to the plan file to quickly define whether or not you want to optimize the flow split over the lateral structure during the initial conditions run.  Typically you will want to optimize this if you have flow over the lateral structure at the beginning of the simulation.  If your initial conditions are below the lateral structure, leave this off.  The Breach button is a shortcut to the breach editor, if you want to breach this lateral structure during the simulation.

The “Tailwater Connection” is really the subject of this post-this is how you connect the river to the storage area.  Make sure you select “Storage Area” as your Type and then go choose the storage area you want to connect to by clicking the “Set SA” button.  Alternatively you could connect a lateral structure to another river/reach or you could connect it to nothing (send the flow over/through the lateral structure out of the system).

There’s still work to be done to define the Weir/Embankment (if not already done), but the Storage area and the river are now connected via the lateral structure.  If you want to make sure you are connected, look at the points of the lateral structure on the geometry schematic.  If you see thin black lines connecting the end of the lateral structure to the storage area, then you know RAS recognizes them as being connected (sometimes you have so zoom in close to see the “connection lines”).


If you are having difficulty connecting lateral structures to rivers and/or storage areas, I highly encourage you to open up this example in HEC-RAS and have a look around.  Normally you will find the example projects in C:Program FilesHECHEC-RAS4.1.0Example Projects.

The “4.1.0” might be different if you’re using a different version of HEC-RAS.  If you don’t see the example projects here, go to the Help menu item on the main HEC-RAS window and select “Install Example Projects…”



  1. gsharif

    on November 8, 2012

    Thanks chris, although I did something similar like this but using steady flow modelling, but i will go over with the example you mentioned and the notes in depth and will get back to you if get stuck! Thanks again – Sharif

  2. Chris G.

    on November 8, 2012

    Sharif, you're welcome. Yes, you're not going to see much with lateral structures and storage areas in steady flow. Those are much more useful in unsteady flow modeling.

    Good luck!

  3. Yashodhan Joshi

    on September 22, 2013

    Can you tell me if this is possible by constructing a channel at right angle with the direction of flow? what would be the changes in surface profiles?

  4. Chris G.

    on September 23, 2013

    Yes. You could do this with a junction (diversion junction) or with a lateral structure connected to a separate reach. Either way works. RAS will not explicitely account for the momentum loss in turning flow 90 degrees to exit through the diversion channel, so you need to account for this with the discharge coefficient or with n values (or both).


    on December 10, 2015


    I am trying to run a simulation with a levee and I used the lateral structure and the Storage areas. I have done everything you mentioned! My only problem is that I cant see the flooding water in the storage areas when the average height of the weir is exceeded. The floodplain is generated up to the end of the cut lines.

    Can you please help me?

    Thanks in advance.

  6. Chris Goodell

    on December 10, 2015

    Check your TW connection. Also, make sure your model is not going unstable. Make sure your timestep is not too large or your n values too low.

  7. Unknown

    on February 10, 2016


    I am trying to model the effect of a canal in a system. I have added a storage area which represents the canal. I have used lateral structures and connected these into the storage area. When I model unsteady flow through the model, no water seems to be entering the storage connections and it is producing ridiculous results with water levels of about 30 m. Not sure what I am doing wrong or whether I am missing out a crucial step?

    Thanks in advance

  8. Chris Goodell

    on February 10, 2016

    Sounds like your model is unstable. Make sure that at the beginning of your simulation, the water level is low enough so that it is not overtopping the lateral structure. That will help stabilize your model for initial conditions. Also, make sure your cross section spacing and time steps are appropriate and your n values aren't too low. Make sure you have a reasonable lateral structure coefficient (i.e not too high).

  9. Anonymous

    on August 25, 2016


    I'm trying to route water from a cross section to a storage area via a pump. I'm setting the "highest elevation in pump line" to the top of the cross section and the storage area has the same min. and max. elevation as the cross section. The model keeps wanting to raise the initial elevation of the storage area beyond the dimensions I've given. Do you know whether the model isn't made to do this or what might be causing this?

  10. Chris Goodell

    on August 25, 2016

    Storage Areas have no maximum stage. As long as water can get to them, they are able to continue filling up. The model is made to work as you wish it to, but make sure you are accounting for all water coming into and out of the storage area.

  11. Anonymous

    on August 25, 2016

    The weird thing is, the storage area is only connected to that one river station through the pump and I've set up the pumps (water surface elevation triggers) to not turn on at all to test the storage area elevation. Even without any water going to it, the model still wants to raise the initial elevation of the storage area.

  12. Chris Goodell

    on August 25, 2016

    That is weird. I would be happy to take a look at it to see what's wrong if you want to send me your model (.prj, .p##, .g## and .u## files).

  13. Anonymous

    on August 25, 2016

    I greatly appreciate that. Let me put together my file and anything else you might need to know into an email to send over.

  14. Unknown

    on June 12, 2017

    Hello, I am trying to connect a 2D flow area to a reach with a lateral structure. The lateral stucture intersects several cross sections over a distance of 2000 meters. My cross sections are well defined, but the HW connections don't compute all of the cross sections. The HW connections are blocked between the first two XS with a distance of 2000 meters between those (instead of 200 meters). Any idea to detect all of the XS in HW connections ?

  15. Victor Mars

    on March 28, 2018


    I would like to know more about the tailwater connection type 'Out of the system". Does this type of connection mean that as soon as the water flows over the lateral structure, it "disappears" from the system ?
    I ran a 1D test model to experiment this connection and I am a little bit confused when i look at the results.
    In my model, the water is still flowing over the structure and flooding the part of the XS behind the structure. However, when I look at the Stage and Flow Hydrograph, i can see that my flow leaving is equal to the difference of my upstream volume minus my downstream volume.

    I don't know if I have been clear but I would just like to know a bit more about how is this type of connection working and what's happening in HEC-RAS when you're using it.

    Thanks a lot in advance and thank you for this website who helped me so many times.

  16. Chris Goodell

    on April 5, 2018

    Yes, flow over the structure is computed by the capacity of the features on the structure itself (weir, culvert, gates, etc) without consideration of tail water effects. The flow will leave your reach and disappears.

  17. Segolene

    on June 6, 2018

    I also wonder about the "Out of the System" tailwater connection type.
    My model is a steady flow modeling, with one unique chanel.
    The flow injected uspstreeam into the model is 70 m3/s.
    I have leaving flow at the lateral structure (Q=5 m3/s) but I still have 70 m3/s in the cross sections downsteam of the lateral structure.
    I don't undersatnd why do I still have 70 m3/s downstream of the lateral structure with "Out of the System" connection. Do you have an explanation about this problem?
    Thanks a lot in advance.

  18. Aishwarya Kaushal

    on September 18, 2020

    I have been working on constructing a steady flow model for two rivers in Birmingham, Alabama. I have an issue with the Missing River/Missing reach popping in every time even after clearing the data for the rivers and removing all the cross-sections. Even after saving these changes, I see a missing river on my Ras mapper and have not been able to remove it even after redoing the whole model. Do you have any suggestions?

    The missing river/ reach is not a part of my project and is to be removed.

  19. Chris Goodell

    on September 22, 2020

    did you try to delete those “phantom” reaches? In the Options…Delete Reaches in the geometry window.

  20. Joy Gr.

    on November 2, 2020

    I’m trying to model a 265-meter rectangular pipe outflowing into the sea for the 10-year discharge. The pipe invert is at -2.00m everywhere and the top varies from +1.65 to 1.15.
    Usually modelers set the downstream boundary condition to sea level or normal high tide. When I do that, the water surface hits the lid just at the most upstream xs.
    However, I’m not comfortable modeling the pipe like that, as I feel the seawater filling the pipe below zero elevation is not moving like a stream, but somehow obstructing the flow.
    My gut says the pipe will convey the discharge under pressure, like a siphon.
    Any ideas?
    Many thanks in advance and congrats on the great work you are doing.

  21. Chris Goodell

    on November 2, 2020

    Great question Joy. Since RAS doesn’t account for the density differences between salt and fresh water, any differences you want to model will have to be “faked” in by you. slightly higher roughness values seems reasonable, although very subjective. As to your question as to whether the saltwater in the pipe will act as a wedge, or will it fully mix with the freshwater I think depends on the size of the culvert and the velocity in the pipe. You could make the case that a relatively small pipe with relatively high velocity would likely have a fully mixed characteristic, while a relatively large pipe and relatively slow velocity could set up a saltwater wedge. Given the uncertainty, I’d look at both cases and design off of the more conservative.

  22. Joy Gr.

    on November 3, 2020

    Hi, Chris,
    Thank you for your helpful and prompt response. I truly appreciate it a lot.
    Take care and keep them coming!

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