October 24, 2019
HEC-RAS in Three Dimensions
Now this is the kind of post that gets you on The RAS Solution! Cutting edge creative solutions…
My Unsteady HEC-RAS Model is Unstable…Why?
This question (or some variation of it) comes up quite a bit on The RAS Solution: “I have an unsteady flow model. When I run it, it goes completely unstable. What is causing this?” Many times a screen shot of the computation window with the dreaded “red bar” is attached.
While the person asking the question genuinely needs help, what he/she doesn’t understand is that it is impossible for anyone to troubleshoot his/her model with this limited amount of information or without the model data files. There are an almost infinite number of possible reasons the model crashed.
If I could sum up my approach to troubleshooting unsteady HEC-RAS models in one sentence, it would be
“HEC-RAS likes things to change gradually”
If your model is crashing, look for places or times where something is changing drastically. It could be a sudden and significant change in the size and shape of the main channel from one cross section to the next. It could be a sudden increase/decrease in flow. It could be a sudden increase/decrease in stage. Whatever steps you take to try to stabilize your model, make sure you are confident that the steps you are taking will improve stability without giving up more accuracy then you’re willing to sacrifice. Haphazardly making changes and adjustments to your model without any forethought or strategy, in an attempt to make it stable could very well make it worse and get you nowhere-and waste a lot of time. Making methodical, logical, and beneficial changes to your model is a much better approach. By methodical, I mean understand what you are doing, and why it can improve numerical stability. Also understand what accuracy (if any) you are sacrificing to achieve the increased stability. Keep in mind, stabilizing your model may require more than one “change”. This is why it is important to understand the theory behind the computations in HEC-RAS. That understanding will allow you to make informed and intelligent decisions on what techniques to use to stabilize your model.
Here are some very helpful references that deal directly with how to troubleshoot HEC-RAS unsteady flow models that are unstable and/or crashing. If you’re having trouble with your unsteady flow model, please carefully read through these references. And consider taking an HEC-RAS training course if you can. The “Unsteady Flow” and “Dam Breach” classes both cover techniques for troubleshooting an unsteady flow HEC-RAS model.
- HEC-RAS User’s Manual Chapter 8, Performing an Unsteady Flow Analysis. Particularly the section on Model Accuracy, Stability, and Sensitivity. This manual (along with the Hydraulic Reference Manual) comes with the installation of the HEC-RAS software. You can access it from the main HEC-RAS window under Help…User’s Manual.
- Investigating Instabilities with HEC-RAS Unsteady Flow Modeling for Regulated Rivers at Low Flow Stages, Sharkey 2014. Master’s Thesis by Jennifer Kay Sharkey. A great summary of potential causes for instability in HEC-RAS models, especially those with low stages.
![[clip_image002%255B5%255D.jpg]](http://lh3.googleusercontent.com/-KpE2NKuTvIQ/UlRNE5yAIvI/AAAAAAAAA6E/B_iqpIdzCWg/s1600/clip_image002%25255B5%25255D.jpg)
Stabilizing a Dynamic Unsteady HEC-RAS Model. Post on The RAS Solution. Steps taken to stabilize a HEC-RAS model, along with the dataset used. http://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.html
- 10 Tips for Troubleshooting Unsteady HEC-RAS. Post on Ebb and Flow blog. http://www.freese.com/resources/blog/10-tips-troubleshooting-unsteady-hec-ras
- Common Model Stability Problems with Performing an Unsteady Flow Analysis. Lecture by Gary Brunner.
Comments
carles
on June 5, 2015thank's your information
West Cotter
on May 17, 2017Hi Chris,
I understand your idea about a sudden change and its impact on model stability. However, if one is going to model a dam breach or reservoir operation, the outflow will be, no doublet, a dramatic change and decrease of flow and stage in a very short time. How do you deal with model instability caused by such cases?
Chris Goodell
on May 17, 2017Great question. You are correct, which is why dam breach models are among the most difficult to stabilize in HEC-RAS. The Mixed Flow option is a great tool for minimizing the effects of large accelerations at the front end of a dam breach wave. Providing a good "cushion" at the front end of a dam breach wave will help as well (i.e. use a higher minimum flow value).
West Cotter
on May 19, 2017Thanks Chris
A further question about unsteady flow analysis. We are thinking to use HEC RAS for flow routing, either for flooding, forecast flow, or for planed water release (flow scenarios) from an upstream dam. If we want to know how flow affects downstream in terms of water surface elevation, unsteady flow is an appropriate method based on some studies, such as Hicks and Peacock (2005) and Alaska Energy Authority (2013).
I calibrated HEC RAS with unsteady flow analysis (with more interpolated cross sections and a good calculation time step). Then when I use this “calibrated” model for a planned water release (i.e., daily flow for 15 days, flow goes high then low….), but this HEC RAS model goes to instable again. Why? Because planned water release changes too rapidly? It seems that error was related with a bridge, and I have about 15 bridges in this model; should I delete bridges?
Is this a major reason that HEC RAS is not commonly used for operation flow forecast, because each flow situation could cause instability? Sorry for bit long questions.
Chris Goodell
on May 19, 2017It's impossible to say why your model goes unstable without seeing it. There are any number of reasons. That being said, nearly all instabilities occur due to sudden and abrupt changes in something (could be stage, flow, velocities, flow area, energy slope, etc). My suggestion is to get your model stable for extreme releases of flow, then calibrate it. Then your model should be robust enough to handle all operational scenarios. If the errors are originating at a bridge, try refining your htab parameters (i.e. add more resolution). Yes, new users can have difficulties with model instabilities, but like everything else, with practice, you get good at solving them. And I don't look at model instability as a negative thing, but rather a nice warning to you that there is something wrong with the setup of your model. Software that never goes unstable is either not using the full dynamic wave equation, or is making assumptions for you to stabilize the model. Personally, I'd rather the software not make assumptions for me, I'd rather have control over it myself.
MOAZZAM ALI RIND
on June 14, 2017hello chris
i am facing a unique error which i am unable to figure it out and had tired alot but still it prevails.the issue is when i run unsteady simulation at 1 day time step it doesn't show error and model runs successful but when i see the mapper for results the water vanish away all of sudden just after first 2 days of simulation and no flow shown for the remaining season . similar model when run on 1 hour time step gives a convergence error on exactly same time from which the flow disappears on wards. i am using 5.0.3 version and please help me out and give me your email so that i may share my model with you
Chris Goodell
on June 15, 2017Both a 1 day and 1 hour time step are very coarse. Your model is probably unstable. Suggest checking courant number for a more suitable time step.
Boniface Kinnear
on July 6, 2017Good day all. I'm having a slightly different issue which I'm hoping members can shed a light on. I'm running a 2D simulation for an urban catchment with one upstream boundary condition (24 hydrograph with a peak flow rate of 4.3m3/s) and a normal depth downstream boundary condition. The model runs well (using diffusion wave. note flow momentum crashes) however I'm getting an unusually high depth (in the order of 5m). I did a 1D model and the average depth is on 0.5m.
Is there a reason for these usually high and dpeths?
Chris Goodell
on July 7, 2017Did you check your Courant number?
Ujas Pandya
on August 22, 2017when ever i run unsteady flow analysis i always get stuck in it and my simulation gives this error!
SOLUTION SOLVER FAILED! Unstable for Initial Iteration at 17SEP2006 2400
Chris Goodell
on August 22, 2017Check your initial conditions and boundary conditions. Check all of your input. Make sure your n values aren't too low. Reading through the literature referenced above is the best place to start trying to troubleshoot this.
Michael Burke
on August 24, 2017Hi Chris, I am having trouble stabilizing a 1D/2D model for a large storm event. The model consists of a 1D reach with a 2D overflow area connected by lateral structures along the river bank. Once water overtops the bank and enters the 2D area, floodwater flows away from the main channel towards
an auxiliary outlet. This auxiliary outlet is a restrictive culvert underneath a tall railroad embankment. I modeled the storage immediately upstream of this railroad culvert as a 1D storage area in order to input a flow hydrograph from the contributing subbasin. The 2D area drains into the 1D storage area through two SA/2D connections.
The model runs fine (minimal error) for all storms up to the 100yr. However, the 500yr storm will not run, and I get a lot of 1D/2D flow errors before the model ultimately crashes with the following error: "error with program: RasUnsteady64.exe Exit Code = -1073741819". The1D/2D flow errors are occurring where the 2D area drains into the 1D storage area.
FYI, I am using a 20-ft grid, 12 sec timestep, full momentum eqn. I have been adjusting (loosening) different parameters and have increased the iterations to 20 (max).
Any help on this issue is much appreciated. Thank you.
Chris Goodell
on August 24, 2017Seems like you could use a smaller timestep to satisfy a Courant number = 1. Also, you might try adding some 1D/2D iterations, if you haven't already.
Anonymous
on April 19, 2018I've having problems running an unsteady model that completes its run, but with results that don't make any sense. For example, there are very high positive and negative velocities where the steady-state run of the model predicts positive velocities less than about 5 fps. I've tried numerous things to debug the model including the following: reducing time step to 0.5 s, removing all levees, structures and ineffective flow markers, interpolated sections, revised the HTABs, running the inflow hydrograph at a steady state value, setting the minimum and initial flows to be the same, run an initial discharge for 10,000 time steps, increased the Manning's values, turned on the mixed flow option, checked the section spacing against Samuel's equation, and checked the time step against Currant's equation. When reviewing the computation log there are so many values that appear to be in error I haven't been able to isolate the section causing the problem. Reviewing the documents you've posted here and reading this an other blogs hasn't provided much in the way of insight. I've never seen something like this before, nor have any of the modelers I've asked for assistance. Any recommendations for getting past this kind of hurdle? Many thanks,
Chris Goodell
on April 21, 2018Sounds like your model is unstable. There are so many reasons this could be. Use the output plots to try to find the source of the instability as you animate through your profiles. Use a relatively small detailed output interval to help identify the source. And remember, RAS likes things to change gradually. Any sudden changes can lead to errors and instabilities.
Anonymous
on April 23, 2018Right now the model is set to run with a steady state hydrograph of 25 cfs. The model is also set to run with 10,000 time steps of warmup. A review of the computation log indicates that the errors begin during the warmup. The first errors in the log file during warmup is at the downstream boundary. I tried moving the boundary to another section as well as changing the boundary condition. Neither of these steps resolved the issue. I then tried to remove sections from downstream to upstream until the errors resolved themselves. I had to delete 2/3 of the sections before I was able to have a successfull model run. I then tried to add sections back into the model and the errors resumed. This seems to suggest that the problem is a run-time parameter and not the sections (unless 75 sections can all cause problems). I am unable to identify the problematic parameter or section(s). Any other thoughts? Thanks,
Chris Goodell
on May 6, 2018First off, if you're running a steady state hydrograph, i wouldn't expect you would need 10,000 warm up timesteps. Or even 10 warmup timesteps. This should be a very straightforward exercise for RAS. My guess is there is something wrong with the terrain making up your cross sections. But I can't really say without seeing your model.
Antonio Francipane
on May 22, 2018Hi Chris,
I have noticed that when I run a 2D simulation in HEC-RAS results can be very different on the base of mesh resolution and computation time interval. Sometimes, a bad choice of these two components can led to get very high values of inundation maps. I was wondering if there was a way to make a choiche of the best mesh resolution and time interval to get good results.
Thanks a lot for your time and attention.
Antonio.
Chris Goodell
on May 22, 2018Antonio-
Mesh resolution needs to be small enough to capture good velocity distribution in conveyance areas you are concerned with. Time step should then be set to get Courant numbers as close to 1 as possible in those areas.
Francesca Carisi
on January 22, 2019Hi, I have a model that goes unstable at the first time step. with Version 5.0.6. But if I run exactly the same model with Version 4.1, it ends the simulation without problems. Any idea of the reason? Thanks!
Chris Goodell
on January 22, 2019Hard to say without seeing your model. But there are a few things different with 5.0.x and previous models that can affect the initial conditions. Htabs defaults are handled a little different. You might check your htab parameters. That's the first thing that comes to mind.
Chris Goodell
on January 22, 2019This comment has been removed by the author.
ck
on July 8, 2019Hi chris
I am having serious viewing problems of posts. the classic view no longer scrolls down, there are no scroll bars to scroll down or pan across. The list and threaded vies also do the same. I can't read anything on the forum other than small sections of posts.
Chris G.
on July 11, 2019I just tried everything on my computer and it seems to be working as usual. I wonder if there is some kind of view setting that got changed? I'm not sure how to help, I can't replicate what you're seeing.
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