unsteady modelling in steep reach

[woida]

dear all,
what would you recommend to obtain stability for an unsteady calculation in a very steep torrent (slopes up to 25 percent). The model is running well for slopes up to 15 percent, afterwards stability problems are showing up. The hydrograph is also quite steep, raising form a base flow of 1 cms up to 120 cms in 15 minutes.
Thanks in advance for a reply.

[hitch24]

Hi woida

I don't have an answer to your problem, but I am really interested how do you get a 15 percent steep torrent stable in unsteady calculations? Do you have a very regular geometry (a channel) or is it a natural torrent? I am struggling with 1 percent steep river, I get it stable but without 5 percent steep block ramps. I would appreciate any hint.
Did you try ramping down the model to create a restart file?

Greetz

Chris

[Hydraulics Tool]

10% is generally considered the upper end for RAS modeling. However, I have run models with brief sections of steeper reaches, but too much higher thatn 10% and you're losing accuracy. To get steeper reaches to stabilize you can increase n values (typically they are underestimated in steep reaches-see Jarrett's equation), you can add in more base flow, add in pilot channels, use mixed flow or a combination of all of these.