Tuesday, January 26, 2010

Lateral Structure flows into Multiple Cross Sections

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Not sure if this will be changed for the soon-to-be-released version 4.1, but for now, the option to drop flow from a lateral structure into multiple cross sections is a little awkward.

First thing you do is specify that you want to drop your lateral flows into cross sections. Then set your upstream TW river station (the downstream one will be greyed out).

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Then, go to the Weir/Embankment editor for your lateral structure. At the bottom left, you'll see weir stationing reference. Select TW flow goes: "over multiple XS's". Then on the right hand side you'll see a table for specifying weir stationing for your TW cross sections.

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21 comments:

  1. Storage area connected to a XS, an inline structure at least 2 cross sections away from the upstream end and then the boundary condition defined as a Lateral Inflow.

    Is this a way to simulate a dam with a reservoir upstream? Instead of simulating the reservoir with cross sections (dynamic routing).

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  2. YES! That's the way. The trick is making sure RAS recognizes the storage area as "connected" to the XS. You do that by turning on the "move object" mode in the geometry, and then clicking and dragging the end of the reach to a point inside of the storage area.

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  3. So there is nothing wrong (hydraulically speaking) to put the boundary condition as a lateral inflow at the XS located after this connection (SA to XS)?

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  4. Technically nothing wrong with that, although if you want to route the flow through the reservoir, to see the reservoir's affect on attenuation of the flow, then it would be better to add the lateral inflow to the Storage Area itself, rather than the cross section. That's how I do it. Just make sure your pilot flow at the dam is consistent with the base flow in your lateral inflow hydrograph. And also make sure you set an internal RS initial stage (unsteady flow data editor, under Options...) at the cross section just upstream of the dam that is consistent with the initial conditions storage area elevation.

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  5. Can this method of lateral structure flows into multiple cross sections be used to put a storage area behind the lateral weir in order to do a level pool routing breach lateral to the reach? Didn't find yet where or whether a storage area can be connected to a lateral weir to input a flows laterally into a reach, not the opposite way of modeling flow out of a reach over a lateral structure and into a storage area. Thanks

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    Replies
    1. Jon-Great question! And yes, you can do it. Set up the lateral structure in the reach and connect it to the storage area. When the storage area exceeds the threshold breach elevation, the lateral structure will breach. RAS monitors both sides of the lateral structure for a water surface that exceeds the breach threshold. Be aware, that discharges through the breach will be listed with negative values since flow is going from the SA to the reach. I put an example RAS project that demonstrates this on my Dropbox site. Feel free to download it.
      https://www.dropbox.com/s/o8pe4w2zwnj2kj6/BreachSAtoReach.p01.zip

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    2. Thank you for the response Chris. Got everything running great thanks to your example.

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    3. The referenced HEC-RAS data set is now located at: https://drive.google.com/folderview?id=0B0bpiyLiUeRXellfVG81TGRXVWM&usp=sharing

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  6. Hello Chris, do you mind putting the breachSAto reach zipfile back into dropbox? I am working on a similar project. Thank you!

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    1. Sorry about that. Everything got moved into a Google Drive Space (more free GBs!). Here is the new link: https://drive.google.com/folderview?id=0B0bpiyLiUeRXellfVG81TGRXVWM&usp=sharing

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  7. Hi,
    I am modeling a river where there is a power canal separated by a lateral structure in HEC-RAS.
    1. I have laid out the canal as a river tributary. Is this a correct approach? Is there any other approach to better represent the system?
    2. Modeling as a tributary creates a junction where the river and the canal meets. Now, I am not being able to draw a lateral weir across this junction. Whenever I put the Head water (HW) River station, no reach thalweg appears as supposed to. Can I go modeling without this structure?

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    Replies
    1. 1. Sounds like a good approach to me.
      2. You can leave the junction there if it makes sense physically. It sounds like you are hesitant about connecting with a junction. You could have 2 separate and parallel reaches, connected in multiple locations with lateral structures. Each reach would have to have its own upstream and downstream boundaries, but that's easy to do.

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  8. Thanks Chris. Just another related question, can I somehow merge two river reaches into one reach (by eliminating the junction) and somehow save all the cross section data? I can not imagine drawing the layout again!!

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    1. Yes, just left click on the junction and choose "Delete Junction".

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  9. Good Afternoon Chris, I have a question regarding the combined use of inline and lateral structures. I am working on a model where the reach "Zs" across a roadway. That is, the reach runs parallel to the roadway upstream of the inline structure, crosses the roadway at a skew through the inline structure, and then resumes parallel to the roadway downstream of the inline structure. Assume the roadway grade is flat. What is the best way to model this - a lateral structure upstream of the inline structure, ending just upstream of the inline structure with flow connected to downstream cross sections? How would cross sections be drawn for this - cross sections ending at the CL of roadway (assume to by high point)? If the roadway is flat do I just propagate the inline structure roadway profile to the bounding lateral structure cross sections (thus capturing all over-the-road flow by either lateral structure weir flow or inline roadway weir flow)? I realize this will likely result in the inline structure cross sections being bound by frictionless walls at the edges of the cross section. Please advise if a more detailed description/sketch would be helpful. Thank you!

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    1. I think you are on the right track. If it's strictly a low flow model, no overtopping of the roadway, then just draw your cross sections perpendicular to the flow lines, terminating at the CL of the roadway. You may have to dogleg them to make the turn through the bridge opening.

      If you expect overtopping, and that is what you are most interested, then yes, I think a lateral structure would work well. But there are any number of ways to set this up, depending on the terrain around the bridge, and where you expect the water to go during high flow. For example, will some of it continue to flow parallel along the roadway, while some overtops? Will it all go over upstream of the opening? If you want to link me to a sketch, or even an aerial photo, I'd be happy to give you my recommendation for cross section/structure layout.

      Chris

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    2. Thanks for the timely response Chris!

      That would be fantastic if you could look at an aerial, I would greatly appreciate any recommendations you might have. Here is a public link to a Google Drive stored pdf:

      https://drive.google.com/file/d/0ByfCIbICQwdUWWNYeHZDRU80dkE/view

      I appreciate your time.

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    3. Here's my suggestion. Keep in mind this is from me sitting at my desk, never having been to the site. Hope this helps.

      https://drive.google.com/file/d/0B0bpiyLiUeRXOERjNmlIOEMxTm8/view?usp=sharing


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  10. Chris,


    I am working on a project where there are two parallel stream channel that have the potentional to influence one another when larger storms occur. I have two questions:

    1. I setup geometry to use lateral structures across multiple cross-sections to convey the flow from one reach to the other. Does a lateral structures need to be input to allow flow conveyance in either direction or can one be used.

    2. When running steady flow analysis the following statement results..."In reach 2 the lateral weir extends past the tailwater reach." Why does this occur and how can this be resolved.

    Thanks

    Darrell

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    1. 1. You can have just one lateral structure between the streams. Flow can go both directions over a lateral structure. If it's a long reach, you may want to break it up into multiple lateral structures though.
      2. Sounds like your tailwater connection is not consistent with the reach on the tailwater side of the lateral structure. You have to go into the tailwater connection table and make sure that the tailwater reach's reach lengths are consistent with the stationing of the lateral structure weir. It's very confusing, so it's helpful to sketch out your reaches, cross sections, and weir stationing to find the problem.

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