HEC-RAS 2D_Mapping

[Anonymous]

Good morning everyone.

I’ve built and successfully run a 2D-only model using HEC-RAS 5.0.3.
While running the animation for the flood depth, I’ve realized that many cells show a small flood extent (like little ponds) since the initial time steps of the simulation, even if they are far from the inflow point and not hydraulically connected to the main flood area yet.

Does anybody have an explanation for that? Is it related to the rendering option used (default – Hybrid Sloping/Horizontal)? And how can I remove the small ponds from the results without post-processing the results in GIS?

Thank you.

[Anonymous]

Runoff is getting there one way or the other. I have noticed in most cases, the cause of this is the lack of breaklines and the computational mesh representation of adjacent cells. As an example i have a pond where i would send flow into directly and small areas of inundation would end up outside with no explanation. The solution to this is to make sure there are break lines connecting cells that represent largely varying elevations

[Anonymous]

Thank you Luis.
I am simulating the spreading of a huge flow (dam break) through a large floodplain, very flat and with a lot of meandering, so I have no large variations in elevation. I will anyway try to enforce a breakline at least along the river channel and see if anything changes.

However, you suggestion looks more applicable to cases where the isolate ponding areas are spot in a cell already partially flooded (because of the way the sub-grid works). In my case, I get very small pondings at 20-30km from the flooded area, and already in the initial time-steps, so there’s no way these pondings are “real”. I’ve been able to get rid of large part of them in the output by removing the flood depth lower than 1mm through the Render Mode Options…but I’m still worried that those areas are symptoms of numerical instabilities or other problems in my model.

Did anyone else experience the same issue?
Thank you.

[cameron]

What is your initial condition for the 2D area? What is your timestep and are you using the full momentum equation? You need to follow the Courant Criteria for time step. My guess is you have a large timestep and using the diffusion wave equation which never crashes and with large timesteps causes bad results.

[Anonymous]

Hi Cameron.
I’m using a time-step of 1minute over a simulated period of 30 days.And yes, I’m using the diffusion wave.
The 2D area is initially dry, but I’ve included some days of fluvial only inflow before the dam break, so that the river channel is wet.
Anyway, fragmented flooding happens very far from the flow and already at the first time steps, in cells where flooding is not supposed to happen if not much later in the simulation.

I’ve noticed that if I change the Render Options removing flow depth less than any arbitrary threshold (even 0.0001m…basically nothing) the fragmented flooding disappear. I wonder if this is just a graphical bug other than an actual numerical instability.

[cameron]

Switch over to full momentum it you get a lot of errors your timestep is too high. One minute timestep is high. In general, your timestep should be small enough that water from one cell does not skip the next cell during the timestep.

[Anonymous]

I’ve switched to full momentum and reduced the timestep. The fragmented flooding in areas far from the flow still happens (since the first timestep). It can still be removed by changing the plot tolerance in the Render mode Options (0.001m is enough).
I’m more and more convinced it’s a problem of visualization (bug?) other than a calculation error, but I’d be interested to know if anybody else have got the same issue.

Thank you

[Author]

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