1D unsteady oscillation Frd from thousands to 0

[Antoine_Bard]

Hello everyone,

First of all thank you for reading and considering my request.

I have trouble running a 3 km long model in unsteady flow. It has about 80 XS from a Lidar built DEM and then I interpolate XS every 3m or 1.5 m when the channel has important width variation. For the moment I have drastically simplified the geometry and the manning values which are only asigned for the banks and the channel (0.083 for the banks and 0.035 for the channel). The slopes are relatively important about 0.015 m/m in average. It runs approximately ok in steady flow with warnings and notes which I am not able to solve. And it is barely able to solve the unsteady flow even if I apply a constant flow.
It used to fail while extrapolating above the XS parameters so I increased the possibilities in the Htab param, so it is finally able to finished an unsteady flow calculation but with giant oscillations in Water Surface and Energy Grade. The Froude number comes from more than 3000 to 0. How can I interpret this and which could be the issues to look at?

Any hints will be appreciated.

[rsbe]

have you put any warm up time steps? have you tried lowering the time step with which you calculate/run the model?

[Anonymous]

Appears that you have a situation in which the flow is going to be super-critical. Therefore, I suggest that you try using the “Hydrologic Unsteady Routing” option. This is Modified Puls routing and is applicable to situations, as with hydraulically steep slopes, where the gravitational and frictional forces are much larger than the temporal acceleration. This option is described on p. 6-179 of the RAS 5.0.3 User’s Manual. Another option to explore is using the “Mixed Flow Regime”. This was developed for situations where both sub- and super-critical flow occur. It is described on p. 16-2. There is an example data set for the mixed flow regime option included with the RAS examples.

[Antoine_Bard]

Thankx for your answer,

It turned out that the “mixed flow regime” solved my problem, as the slope is important; even small variation of the channel slope generate sub / super critical transition. I’ll have a look at the “Hydrologic Unsteady Routing” option as well.

[bora]

Hello,

I am having a problem with the profile view of a model run. I used hydraulic routing option and there is an elevation difference between the river bed and the water depth bottom at the upstream. What could be the reason? Thank you.

[cameron]

Is the geometry data exactly the same for the steady hydrologic routing model and the hydraulic model?

[bora]

Yes it is exactly same.

[Author]

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