Theoretical question…Unsteady time step

[Anonymous]

Hi, I’m wondering if there is some link between the time step used to define the Up stream Boundary condition (flow hydrograph) and the computational time step… Let say I use a time equal to 1 minute in the definition of boundary condition and the computational time step is set very lower (e.g. 1 second). How Hec ras manage this? Does hec-ras use a linear interpolation between two points in the bc hydrogrpah and use the the computational time step to know the current flow value?

Thank you

[TypicalEngineer]

Hey Miguel,

The very short answer to your question is yes and you will find a handy relationship in the RAS 2D Manual on page 4-5. You might want to read a little about the CFL condition if you are not already familiar.

I think RAS defaults will keep you in the clear but this is something you should intentionally specify and experiment with.

Since the diffusion wave equations uses an implicit solution (not inherently bound by previous time step) scheme you can use those equations on 4-5 to keep your C between the suggested 1-5 for a delataX and deltaT combination.

At least that is how i understand it. Hope that helps!

[Anonymous]

Thanks for the reply, but maybe I wasn’t clear enough… I was asking what hec-ras does “behind the scene”, and not what I have to do to choose the right computational time step. I mean, let say I define an upstream boundary condition as a flow hydrograph and that this data is define with a Data time interval equal to 10 minutes. Than, I run the unsteady simulation with a a computational time step equal to 1 second. How does Hec-Ras know the flow value to use at the boundary condition at each time step? Does the program use a linear interpolation between 2 points of my data (which are distant 10 minutes, in time) to know the flow data value after each time step (1 second)?

Thanks

[Author]

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