Variable Time Steps and Time Slicing in HEC-RAS

It is often (very often) useful to subdivide time steps in time or space, using smaller time steps to resolve critical conditions in your unsteady 1D and/or 2D model. You will often need a small time step during large flows or in small cells, but don't want to use that time step for all time/space.
There are two common approaches for subdividing time steps: adjustable time steps and time slicing. The former subdivides time steps temporally, in response to they critical courant condition, the latter subdivides time steps spatially, applying different time steps to meshes with different resolutions. Both are really valuable, and this video helps make sense of when to use which...and how.

[@vojtechkouba9367]

I'am very grateful for people like you.

[@sorbeboss]

Stanford, the way you teach your classes are amazing. You are awesome! Keep it up!
From Brazil.

[@theicemanOQ]

Thanks, you help me a lot

[@phengxiong5361]

Many thanks for sharing this VDO

[@stanfordgibson]

Hello Pheng, I saw your comment on the dam breach videos. We will be releasing an entire, online, 2D class with version 6.2 that will include a dam breach workshop and several talks. Look for it in the 6.2 release notes.

[@GetnetMelaku-dn5rm]

Thanks sir. thats helpful for ma paper.
How to solve the error,
Minimum error exceds allowable tolerance at ???
I am from Afrika Ethiopian 🇪🇹

[@design-storm]

Love your videos! Any chance you can increase the volume on your mic? Thanks so much!

[@stanfordgibson]

Sure...I can try. I'm still learning those settings :)

[@edmondoseibarimah7642]

your Videos are very helpful. Stanford, please how do you model sediment accumulation in a concrete lined river channel

[@stanfordgibson]

Set the maximum erodible depth to 0 or the minimum erodible elevation to the thalweg elevation in the Initial conditions tab in the sediment editor. This will cause some oscillation, but it will only deposit, it will not erode below the original bed.

[@praveenchaudhary5199]

Cell size :2m
Input hydrograph :1 hr interval
I varied computational time step 10 min to 1 second, maximum courant no 2
Minimum courant no 0.95
But this couldn't work for run.
Please suggest me how to make stable model?? 🙏🙏

[@anggaeko299]

Hello Mr. Stanford Gibson. I am trying to model sediment transport with a slope >0.04 which makes the model unstable and produces an unrealistic shape (erosion >5 meters). Is there a way to make the hecras sediment transport analysis run well on the model with a slope >0.04 (Extrem slope)? thanks before

[@stanfordgibson]

Is it a 1D or 2D sediment model?

[@anggaeko299]

@@stanfordgibson 1D sediment transport model sir,..;then I tried to reduce the slope to 0.02 by modifying the cross section and the model became stable. but made the model different from the actual river conditions.

[@anggaeko299]

@@stanfordgibson When slope >0.04 (actual model) . There is always a critical depth that causes the warning "One or more cross section(s) eroded to the bottom" in some cross sections.
When I add a deeper "max depth" to the sediment data to overcome the warning, the model also becomes unstable following the input "max depth" data.

[@stanfordgibson]

@@anggaeko299 is it unsteady or quasi unsteady? Have you tried different transport functions? It sounds like there’s a mismatch between the slope, the transport function, and the bed material. It’s pretty common in steep models to over erode if there’s a mismatch between the bed material and the transport function. Steep slopes are often outside of the range of the transport function, so you may have to coarsen the bed artificially to compensate. Try different transport functions (among those intended for the bed material you have) to see which one works best, and you might adjust the transport function with the calibration tools

[@anggaeko299]

@@stanfordgibson I use quasi unsteady sir. I've also used all transport functions.
How to adjust the transport function with the calibration tools ?