Reflex wave from an internal boundary, real or false?

[Marulke]

I am modeling an outlet channel from a hydro power plant. The channel is 500 m long, almost horizontal. The downstream end of the channel is connected to a larger river with a junction, forming a T.

I am running an un-steady model to investigate the water level changes in the channel when the hydro plant starts and stops. In 100 sec the flow goes from 1 m3/s to 80 m3/s, leading to a wave travelling downstream towards the junction.

The problem, (or maybe it is not a problem?), is that the junctions reflects the wave. When the wave reaches the junction, a new wave is formed in the upstream direction.

This is not necessarily wrong, waves may be reflected at expansions, contractions, junctions etc. However, I am surprised by the large amplitude. I know that several hydraulic models dont handle waves well at internal /external boundaries.

So finally, the question. Do any of you have experience with dynamic waves (not gradual flood waves) passing junctions in RAS?

[Anonymous]

The RAS Unsteady model uses the St. Venant Equations for the description of 1-D flow in channels. That means that the bulk momentum waves are modeled, not the surface waves. The governing equations that are solved assume a normal vertical velocity profile and that use of steady flow roughness (Manning) is OK for energy losses. Given that, the solution from RAS should be proper for wave reflections. There are options in RAS regarding how the hydrodynamic solution is carried out at junctions. I have used RAS unsteady flow in several tidal situations and it works well. A bit more description regarding this is given in the UNET Manual available from HEC.

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