Dam Break in HydroCAD

[mherr78]

I have modeled a watershed that drains to a lake. This lake has an earthen dam and a control structure. I am interested in breaching the dam, for inundation study purposes. The model is not very complicated, as the water shed is about 6 square miles.

The HydroCAD support site describes the method to perform a dam break, by setting the starting elevation of the lake to the peak elevation without the breach, and starting the calculations at the time when the breach occurs.

The problem that I have is that I don't see a way to input the time to failure. The method as described seems to just start the computations assuming that the breach is fully developed at the peak, when in fact, the breach develops over time. In my case, the time to failure is 30 minutes.

My breach model has really high discharges, which seem completely unreasonable to me. I think that the peak discharges should be lower if the breach takes time to develop, as compared to an instantaneous breach.

Any suggestions, ideas, comments...?

 

[psmart]

Although HydroCAD was not originally intended for modeling a dam breach, some scenarios can be modeled as described at

http://www.hydrocad.net/dambreak.htm

I assume this is the document you're referring to.

Using this approach, you need to set the calculation time span to begin at the time of the fully-developed breach. There is no separate setting for the breach time.

The next HydroCAD release is expected to have a new "dam breach outlet" which will allow for a progressive breach, as well as setting the breach time independently of the calculation time span.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[mherr78]

Thanks for the reply. I look forward to the next release. I'm not comfortable with my results when I perform the breach in the manner you describe. I was really trying to avoid using HEC-HMS or an unsteady model in HEC-RAS, but I think I don't have much of a choice.

 

[psmart]

I assume you're referring to the high flow rate at the time of breach? Yes, it will be high, since it's a complete, instantaneous breach.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

mherr78: I would be interested to know more about the break you're trying to model. Specifically, what shape is the break, how does it progress over time, and how is the break initiated (specific time, WSE, or something else). Any details will help us meet our customer's needs with the next release. Anyone else doing a dam break analysis please chime in!

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

Dams usually take time to breach. The failures are not instantaneous. Modeling this as such will give unreasonably high flows in my opinion.

A scenario could be modeled varying the shape of the weir (breach) with time, thus modeling the discharge over a likely time period. Seems to me like a reduced scale model of the dam could be constructed to see the likely progression of failure weir shapes and times.

Mike McCann
MMC Engineering
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