Unsuccessful Unsteady Simulation

[Eli_Bro]

Hi All,

I was hoping someone might take pity on me and give me a tip on what I’m doing wrong here. I’ve got the hang of working up the model but now I can’t get any results. The models are unstable etc. If anyone feels charitable, here are links to the HEC-RAS files.

https://www.dropbox.com/s/xu5gtn668jty89t/Sistema-Bacias.p01.zip

https://www.dropbox.com/s/fq72rhzfvfzhyll/Sistema.p03.zip

I have four culverts that bring runoff from the south side of a road to a river bed on the north side. There will be land development around the river bed on the north side of the road. I want to be able to check that the existing culverts are sufficient and design the canals between the culverts and the river for the regularized flow coming out of the culverts – a flow less that the peak flows from the culvert basins.

In the first link I’ve tried to set up the system so that there is a storage area in front of each culvert. The idea being that these basins reduce the peak flow through the culverts. Can’t get that working though. So I tried to go with sections all along the drainage canals leading to the culverts. Still no luck. I was able to run a model with steady flows but in that case I’m just imposing a peak flow condition on every culvert which I don’t think is realistic.

Eli

[jadams]

In a quick look at your model I noted a couple things. You have a few sections that are near vertical drops. Is this intentional? If so that could definitely cause some instability. Also if you haven’t already I would run a draw-down model for your reaches to get your initial conditions. The flows you have in the reaches seem low for how steep the reaches are.

[Eli_Bro]

Thank you. Yes those drops are intentional. The reinforced concrete canals between the culvert outlets and the river will be designed to have maximum flow velocity of around 3.5 m/s. So preliminary, I took the peak flow from each culvert and did a quick calculation in a spreadsheet of the canal slope so that the velocity would be below the limit. The step downs I’ve put in so far are drastic and are located at points where the ground slope changes. Perhaps in the final design we’ll have more step downs of less height.

I will look into running a draw-down model but I admit that I don’t know yet what that means.

In regards to the last comment. I’m embarrassed to say that I don’t really know what the base flows are for these culverts/drainage ways. I’m working in Brazil – the model is georeferenced to SAD69 UTM Zone 24S. I’m told that these streams/creeks/drainage ways what have you, actually have sewage from the urban areas nearby. I would assume this means there is some base flow far above the flows I’ve specified. After saying all that, I’ll be putting in some base flow. My guess at the problem is that the solving routines of HEC-RAS can’t deal with such low flows at the beginning of the simulation with such high slopes and the solutions give these wild energy grades.

[jadams]

When a model jumps from steady state flow to critical flow or vice versa it creates a dramatic change in Energy grade. Often these changes will exceed the tolerances of the model and thus cause it to be “unstable.” One common step for “unsteady” flow models to counter this is to first run a draw down model and store the information for the initial conditions of the actual model. To do this you create a boundary condition at the lower point of the model that will have a stage that will completely flood all the reaches. You then lower the stage over a model period. Should the model crash the section where the water stage is at is usually the culprit. This allows you to troubleshoot several instabilities prior to an actual run. Once you have achieved a draw-down model that runs for the entire period you can tell the software to store the condition as a start-up file. For flow you can usually use 2-10% of the reach flow. This will be an iterative process because you are looking for the flow that will yield a stable model. Chris used to have a nice “How-to” on draw-down models. If he reads this and it’s still accessible I am sure he will provide a link.

[Eli_Bro]

Thanks again. I went looking for the draw-down exercise and did find the how-to that I think you were describing. I followed the steps and so far haven’t had a calm moment to see how to analyze the results and fix the problem. But as expected the draw-down run did crash.

http://hecrasmodel.blogspot.com.br/2010/12/how-to-create-hotstart-file-in-hec-ras.html

I checked out the curves for my model as described in this post and found many of my sections have these types of discontinuities in the conveyance curve. Obviously I need to study up!

http://hecrasmodel.blogspot.com.br/2012/12/htab-problems-with-using-drawdown.html

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