Monday, May 23, 2011

Modeling Junctions for Unsteady Flow Analysis

Written by Aaron A. Lee   | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.

In the current version of HEC-RAS (v 4.1.0) there are two methods of modeling the hydraulics at a junction for unsteady flow. By default RAS selects the Force Equal WS Elevations (Forced) method, which forces the upstream bounding cross-sections’ water surface equal to the downstream water surface. This method may be adequate for some situations like high depths and shallow bed slopes, but can also cause major instabilities in your model if depths are too low and/or bed slopes are too steep. The alternative is the Energy Balance (Energy) method, which uses the energy equation across the junction to solve for WS elevations. The model presented in this post is part of a dam breach simulation and will demonstrate that there can be significant differences between the two methods. This simulation is a hotstart run which seeks to identify stability issues by starting the downstream stage artificially high, and slowly lowering it to the true solution over the run time. The river system in this model has a normal flow combining junction with a steep transition. Special attention will be paid to the steep transition, especially at low flow conditions. The Figure 1 below shows the 3D view of the model extents, which includes the Middle Reach, Tributary C and Lower Reach.


image Figure 1


For a normal flow-combining junction, the water surface elevations at the upstream bounding cross-sections are based on the computed downstream WS elevation. Longer lengths between the bounding cross-sections will generally make your results less accurate and less stable. By looking closely at the above figure you can see that the bounding cross-sections are spaced far apart, which corresponds to long junction lengths. The results for both methods are shown below in a series of profile plots. Figures 2a and 2b show the junction approximately halfway through the simulation. Figure 2a shows the Energy Balance method and Figure 2b shows the Forced Equal Water Surface method. The water surface is high enough that there are no differences between the two methods. For reference, Tributary C is the steeper of the upstream reaches.


image Figure 2a


image Figure 2b


Significant differences develop in Figures 3a and 3b as the downstream stage is lowered. At the same time-step, the two profiles are dramatically different. The Forced method produces a large drop at the junction (Figure 3b), while the Energy method produces only a minor instability (Figure 3a). The large drop (shown in Fig. 3b) occurs because RAS must balance the momentum equation from the upstream bounding cross-section at the junction (an unrealistically low water surface) to the cross-section immediately upstream. The only way to provide a balance is to overestimate the upstream WS elevation, which is why the profile for 3b is much higher than 3a. Notice the spike in the energy grade line. The same problem occurs for the Energy method, but at a much smaller degree.


image Figure 3a

image Figure 3b


Figures 4a and 4b show Tributary C only, just prior to the model crashing for the Forced Equal Water Surface method. There are obvious oscillations in the profile plot, which indicates a very unstable solution. As the stage is lowered downstream, the WS elevation at the junction also becomes lower. At a certain point the WS elevation at the junction approaches the invert for the upstream cross-section; and the model crashes. Figure 4b shows a zoomed in view of the WS elevation relative to the invert of the channel as the channel runs dry.

image Figure 4a

image Figure 4b


The best solution is to shorten the junction lengths as much as possible, which is done by adding cross-sections closer to the junction. By adding additional cross-sections you are decreasing the length over which RAS makes its calculations, which helps to remove the problems with low water surface elevations over a junction. If surveyed data is unavailable, then start by copying the most downstream cross-sections of the upstream reaches to a location closer to the junction. The positioning of these new cross-sections will be based on the judgment of the modeler, who should know the actual conditions of the river system. Make sure to adjust the downstream reach lengths and junction lengths accordingly.


In this example, cross-sections were placed within 20 ft of the junction. Junction lengths were changed from 573’ and 534’ to 35’ and 28’ for Tributary C and Middle Reach, respectively. In addition, cross-sections were added every 40’ on the steep section of Tributary C by interpolation. Figures 5a and 5b each show the profile plots for both the Energy and Forced method at the junction of Tributary C and Lower Reach.

image Figure 5a

image Figure 5b


By redefining the geometry around the junction the error is significantly reduced for both methods and the results appear stable. Both profiles are very similar in this case, showing only a slight difference in WS elevations. The dotted line-type represents the profile for the Forced method. It might not always be possible, or realistic, to place new cross-sections close to the junction. The Energy method allows this model to run to completion without the addition of new cross-sections, though the results appear to not be as good. The table below lists the WS elevations at the bounding cross-sections for each of the different plans: the initial Energy method, and the Forced and Energy method after adding additional cross-sections.













The initial plan has the geometry with the long junction lengths, which consistently calculates lower WS elevations than the plans with shorter junction lengths. Although the elevations were underestimated in the initial runs, they are still within 1 ft of the new profiles. For this model, the Energy method provides a stable solution at the junction without having to modify the geometry. However, given the steepness of Tributary C, the addition of cross-sections near the junction improved the accuracy and stability of the model output. Therefore, even though the Energy method can produce stable results for long junctions in steep reaches, adding more cross sections will improve the results.

9 comments:

  1. Hi, what should i do if the Hec-Ras cannot run the simulation, the red color comes out when the simulation is running? i am working with junction for unsteady flow analysis, Energy Balance Method and Force Equal WS surface. There are 2 combining junctions and 1 split junction. One of the upstream reach is steeper than others. The simulation succeeded in HEC-RAS version 4.0. Could you please give me some advises? Thank you.

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  2. It could be any number of things. Start with the errors that show up on the computation messages. Then see what is happening in the profile plot. Look for instabilities in your profile plot as you animate. Then look further at any cross sections that may reveal clues to sources of instabilities. Quite frankly, you are trying to stabilize an unsteady HEC-RAS model. It takes practice. Read up in the manual about what to do for an unstable model. Also, search this blog site some more. Sorry I can't be of more help, but your description is very general and it's impossible to tell you what is causing it to crash via a message board.

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  3. I have a very small stream coming into a larger creek and I'm having trouble getting my unsteady model to compute correctly at the junction. For the event I'm running (the 1%, 24-hr storm) the larger creek backs up into the small stream about a thousand feet. The model is for a dam breach on the smaller stream, and the no breach and breach profiles will converge well before the junction because of the backwater effects of the larger creek. Would it be incorrect to model the small stream and larger creek separately, then use the stage profile from the larger creek (at the location of the junction) as the downstream boundary for the small stream? It seems like this would account for the backwater effect, but maybe I'm missing something?

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    Replies
    1. That sounds like a good way to model this, as long as the dam breach effects have diminished by the time the floodwave reaches the junction.

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  4. Maybe 've missed it, any suggestion for setting initial flow in the first section after the junction?
    I've run two separate models without a problem, but when i join the models with a juction the evil red line become the master of my pc. it crashes in the very first steps, so i assume can be the initial condition... but i'm not sure... Help please, this thing is driving me crazy (sorry for my bad english :))

    ReplyDelete
    Replies
    1. The initial flow just downstream of the junction should be the sum of the initial flows of the 2 converging reaches. Junctions are notoriously hard in unsteady flow, especially in steep reaches with low stages. Have you read these posts also?
      http://hecrasmodel.blogspot.com/2008/12/unsteady-flow-and-junctions.html
      http://hecrasmodel.blogspot.com/2009/02/how-to-best-model-junction.html
      http://hecrasmodel.blogspot.com/2009/02/severe-energy-jump-at-junctions-for.html

      If this doesn't help, please send me your model and I'll see what I can do.

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  5. thank you
    i asked you one moment ago
    but you excellent statement " The initial flow just downstream of the junction should be the sum of the initial flows of the 2 converging reaches." solved my problem
    Dr Hossam

    ReplyDelete
  6. hello, in a river I have an inconsistency in the flow, the river is flooded but there are sections
    intermediate where water fits perfectly, the only way to resolve this error is varying mannings in the sections that are not flooded, is there any other way?

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    Replies
    1. ineffective flow areas and possibly levees.

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