Help with Dam Break

[Anonymous]

Hi, everyone. I have used HECRAS a fair amount in the past, but primarily using existing models to add/modify culverts for road crossings to see what (if any) impact it had on flood elevations. I have modeled dam breaches with HECRAS to some degree, but I would always create an outflow hydrograph via excel simulating an earthern dam breach. I would then enter this tabular data as my flow data and route it through the channel.

Using the dam break tools in HECRAS is new to me. I’ve spent a lot of time learning how to set up my profile and stations in Civil 3D, exporting them in HECRAS format, importing and adjusting the reach and section data directions in the correct format (reach stations counting down to zero as you move downstream, with cross-section data moving from left to right looking downstream… correct?). I have inserted an inline structure, but where do I go from here? I have read TD-39, and although it gives information on the types of simulation as well as values, it really doesn’t cover setting up the information. Do I simply have a steady-state flow coming into the pond to fill it and then breach the pond? Do I use unsteady flow at all? I assume I need to simulate the rider and culvert in the pond? Or does that matter if I simply plan to breach the dam?

I’m familiar enough with HECRAS that hopefully just a simple step-by step would be all that I need. Thanks!

[Anonymous]

Jonathan: There is lots of necessary info. in TD-39 for simulating dam breaks. However, as you point out, to get started doing this kind of modeling its useful to have a short introduction on how to put together a basic model. I have some lecture notes on how to do this. Be glad to share them with you.
Michael

[Anonymous]

Michael–

Thank you! I would very much appreciate seeing anything you have that would help me get everything set up. Are you able to send them to me via the RAS solution, or do you need my e-mail address?

[Anonymous]

I have put together some notes on how build a RAS model for the situation in which the pool is being modeled as a storage area at https://www.dropbox.com/s/675ecz6s67a10tu/RAS dam modeling steps.pdf?dl=0. There is an example data set with RAS 4.1 (Bald Eagle Creek Example Dam Breach Study) that represents the pool with cross sections.

[Anonymous]

I’m not sure if the link was truncated, but I get a 404 error when trying to go to the Dropbox site.

I think I have my channel and dam modeled (I am modeling it via full dynamic wave routng since I have the actual lake cross sections). Is there really a benefit to this versus level pool routing?

Right now I have the dam ponding the water, etc., using steady flow. When trying to do unsteady flow I just don’t know what to put for the boundary conditions. The downstream I have a friction coefficient, but what do I use for the start station? Can I not simply have a steady inflow, the pond is full, and then at a particular elevation the dam breaches and it calculates the outflow hydrograph based on my breach parameters??

[Anonymous]

Jonathan: In general the pool should be modeled with cross sections where there are significant water surface slope and velocities in the reservoir area. In practice, one seems to have either cross sections or storage-area info., not both. So data availability dictates the method. Usually, the boundary conditions are an upstream inflow hydrograph, such as the PMF, and a downstream rating curve. If the dam is overtopped by this flood, the breach can be set to begin formation after a prescribed amount of overtopping occurs. For “sunny day” failures the breach formation can be set to begin at some clock time or pool elevation. The breach hydrograph is calculated dynamically using the breach size and upstream and downstream stages at any point in time. Details of the computations are in TD-39.
Hope this helps,
Michael

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