Problem with Flow Hydrograph BC in 2D model

[linus_ch]

Hey everyone

I am trying to implement a simple 2D model of a straight double trapezoidal reach including a widening. The goal is to investigate the numerical solution of a hydraulic jump performed by HEC-RAS. The model uses a constant roughness, which was defined by the default manning’s n value and has some breaklines, defining the channel geometry.

Unfortunately I am in trouble with the upper boundary condition which I implemented as a flow hydrograph with a constant discharge. I know that this boundary condition should calculate the normal flow depth based on the underlying terrain model. But HEC-RAS seems to have problems in my case to evaluate the correct normal depth. There is always an offset of approximately 1.5 m as it can be seen in the figure bellow (I checked that with a hand calculation).

I also tried different slopes for the energy line to distribute the flow along the boundary line, but this seems to have no influence on the calculated normal depth. Changing the grid resolution and adapting the computational interval did not solve the problem too.

Does anyone know what i have to change to get a nice boundary condition?
Thanks

[cameron]

Did you try extending the inflow across the entire inflow boundary?

The model treats the inflow/outflow lines as cross-sections in which it then calculates the wse based on slope and Manning’s.

[Anonymous]

Yes i tried this option too, and also the case where the boundary is present only on the bottom part of the stream…

[Lonnie A]

You might try ramping up to your steady flow instead of throwing all at the mesh at once.

[linus_ch]

Unfortunately this is also not the solution. I already set an appropriate initial conditions time in the calculation options and tolerances…

[Author]

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