Dam Breach – 20% channel

[bobd]

I am trying to model a dam that sits at the top of the watershed. The stream channel below is very steep, 20% or more. I cannot get anything to work. I know Hec-Ras recommends slopes less than 10%. Anybody got any suggestions?

[cameron]

I would suggest using a lot of interpolated cross-sections, increase the base flow, lower time step, or a pilot channel.

[bobd]

Thanks. I’ve tried all of those (except a pilot channel) and still not getting a stable run.
I’ll give that a try

[cameron]

Another option would to do it in 2D (version 5.0).

[Robert]

Hi

Could You send any pictures/printscreen from your model? Or you can share your project and I try to solve your problem.
Your project can you compress using: Hec-RAS- File- Debug Report!!!

Regards!
Robert

[Anonymous]

Suggest that you try using the “Hydrologic Unsteady Routing” option. This is Modified Puls routing and is applicable to situations, as with steep slopes, where the gravitational and frictional forces are much larger than the temporal acceleration. This option is described on p. 6-156 of the RAS 4.1 User’s Manual. There is also an example data set with this option implemented provided with RAS 4.1; it’s called “ModPuls Routing Test.” This option can be turned off for the downstream portion of your reach where the dynamic terms in the St. Venant equations may become more important.

[bobd]

Robert, attached is the project file. Any help would be appreciated. I am continuing to work on it as well
PondC.zip

[bobd]

Thanks Michael, I have never tried the ModPuls Routing options. I will give it a try!

[cameron]

The inline structure in the model does not have an outflow so the model is going dry downstream and causing it to crash. If the inline structure does not have an outflow, than I would add a dummy flow just downstream of the inline structure just to keep the model stable. Also the model you posted will never cause the inline structure to breach as the wse never gets high enough to trigger the failure since only 1 cfs is being added to the storage area.

You should also turn on the Mixed Flow Regime as flow will probably go below critical depth with how steep it is.

You also have an initial condition of 50 cfs which is small and you may want to bump this up. For dam breach models, having an initial condition of up to 10% of the peak discharge is ok since dam breach models have very large peak discharges, large inundation areas, and other uncertainties that may have a bigger impact.

Another thing you will probably want to do is to include ineffective areas to help smooth out transitions from small to large cross-section topwidths.

[bobd]

Cameron, Thank you for the input. This impoundment will have zero inflow and outflow. How can I set the initial conditions to show the impoundment is full.

[cameron]

In the unsteady flow options menu there is a place to set initial water surface elevations at specific cross-sections (Internal RS Initial Stages). Use this option and specify the correct water surface elevation at the first cross-section upstream of the inline structure.

[bobd]

Thanks for all of your help, but I still cannot get this dam to breach. The only way I can get flow is to increase it in the Initial Conditions. A breach on a reservoir this size should give a peak flow of approx. 3000 cfs. I give up!

[cameron]

You can change the trigger to how the dam fails to time instead of water surface elevation. Set the dam to fail 1 hour (or something else) into the simulation run and at 1 hour, the dam fails.

[Robert]

Hi

I was looking your model and I think, that your geoemetry isn´t correct. In the appendix I send you an example of correct gemetry (level-pool routing). You have to look a possition of objects: basin, 2 CS, dam and CS.

When I solve dam break analysis, so I cretae a two models. The first model is solved an estimation of flow hydrographs from breach. The second Is solved a downstream routing.

How did you estimate breach parameters: breach bottom width, failure time? Did you use: comparative analysis, empirical questions and mathematical models?
What type dam do you simulate?

regards!
Robert

[bobd]

Thanks to everyone for the suggestions. I have been using Boss Dambrk and Hec-2/Hec-Ras for years but new to modeling dams in Hec-Ras. I did get it to run with a reasonable solution!
Again, thanks!
Bob

[Author]

Posts