Multiple Reach Unsteady State Analysis- Model Instability

[maputnam]

Hello,

I am attempting to model a system consisting of a main river (with upper and lower reaches) and a tributary using an unsteady state analysis. I have built the geometry using GeoRAS and have been unsuccessful in getting the model to run. In a bit of frustration I stripped the tributary from the geometry file and combined the upper and lower reaches. After this the model runs without issue. I am assuming that the Junction is causing my issue. Does anyone have any experience with this type of situation? Please help me see what I am missing.

Thanks,

-Mike

[cameron]

how did you set up the junction for the model? did you try the different options in the junction editor?

[maputnam]

how did you set up the junction for the model? did you try the different options in the junction editor?

I have tried both the “Force Equal WS Elevations” and the “Energy Balance Method” using the junction. The error I am getting is not contained to one cross section and seems to “Jump” all over the place. However the same cross sections, when the junction and parallel reach was removed, work just fine with no issue.

[cameron]

Have you tried adjusting the time step to be lower or adjusting the minimum flow? Is there a steep drop in invert elevation from tributary to the main channel? Have you tried adding cross-sections?

[jadams]

For unsteady simulations ensure you have meet the samuel’s equation for section spacing {0.15(depth)/Slope}. This will help with some stability issues. Also this does apply to near your junctions. Often when issues appear near them you may need sections closer in each direction to it to assist with resolving inherent instabilities. Also adjust your timestep by Courant condition {Spacing/V+c} but not less than 5secs or you can take the time of the flood rise / 20.

[Lonnie A]

If the flowline of the first section on the tributary is higher than the initial conditions depth of the receiving channel that will usually cause stability issues. In my models where there is a significant elevation difference across a junction I will put in a inline weir at the downstream end of the trib.

I basically copy the last section I’ve cut from the surface say 5′ downstream. Then place a inline weir between the two sections. The inline weir geometry is the same as the bounding upstream section. The downstream section flowline is adjusted to match up with the receiving channel.

[maputnam]

For unsteady simulations ensure you have meet the samuel’s equation for section spacing {0.15(depth)/Slope}. This will help with some stability issues. Also this does apply to near your junctions. Often when issues appear near them you may need sections closer in each direction to it to assist with resolving inherent instabilities. Also adjust your timestep by Courant condition {Spacing/V+c} but not less than 5secs or you can take the time of the flood rise / 20.

I have checked the cross section spacing vs. Samuel’s equation and find that Samuel’s equation species a maximum cross section spacing of 1113 lf. I have examined the data and found that cross sections are spaced, on average, about 422′ apart with a maximum of 1450 (only two cross sections in the entire model exceed the length recommended by Samuels equation). I have reduced the computation time step to 1 second; if a greater time step is used the model will crash.

There is a pretty significant difference in the inverts of these two reaches. However, the tributary has a pilot channel to account for this.

I have also managed to get the model to run with the tributary however the lateral structures included in the original model will not allow the model to run. Additionally, any change I seem to make in the geometry or the Htab parameters seems to cause the model to go unstable.

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