Flow through Dam Breach

[Weimsguy]

I have a simple dam with a concrete weir 150 ft wide x 8 ft breath and 139 degree notch angle at the center of the dam. The rest of the earthen embankment starts at the top of the weir and increases asymmetrically to the top of the dam. Ideally I would like to know the flow associated with the concrete weir, the flow over the earthen dam, and the flow through the dam breach. My initial model was to define the entire dam and weir as single Asymmetrical Weir that includes both the earthen dam and the concrete weir. The dam breach notch is set at 20 ft wide (assumed) with a 20 degree notch angle and set to extend to the bottom elevation of the downstream section just below the spillway. This model gives me the combined weir and dam flow as the primary device and the dam breach as the secondary device. I am examining various Dam Breach conditions of top elevation and start elevation to establish the maximum flow. My second model was setup with a trapezoidal weir for the primary outlet, the earthen dam as asymmetrical weir starting at the maximum elevation of the concrete weir and extending to the top elevation of the dam as the secondary outlet. Again trying various combinations of the dam breach notch as the tertiary outlet. Other variables examined are breach width, breach angle, and breach failure time as suggested in HEC TD-39 "Using HEC-RAS for Dam Break Studies". In each model, I get Warning 95 for the Dam Breach Outlet, which I guess is to be expected.

The results from my two different HydroCAD models produce significantly different results, as I would expect. I would like to know others opinions as to which would be the better way to model this dam. Sorry for the long explanation.

 

[psmart]

Sounds like you may have overlap between the trap weir and asymmetrical weir, which will produce excessive flow, but it's hard to tell without your HydroCAD project file and a sketch of your outlet setup.

Warning 95 should not be ignored. It means the breach has a closed top and is behaving as an orifice. I would re-check your input data.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Weimsguy]

I only get Warning 95 when I route the flow through the individual outlets (which also happened in your webinar on Dam Breach). The "significant difference" I noted in the original message was due to not all the settings being identical. The inflows I am having to run are starting to cause some oscillations in the models, which I am trying to minimize with the Time Increment and Finer Routing settings. I have attached the project file and sketch of the dam (in separate post).

 

[Weimsguy]

Sketch of dam.

 

[psmart]

Do you still have a question about the model?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Based on your sketch, the breach should start at the about 630.32 not 624.80. Using the lower value creates an effective breach orifice, which is the reason for the warning 95.

Secondly, I don't understand why you are triggering the breach at 627. Normally the WSE would be above the top of the breaching section when the breach begins.

The combination of these factors is probably the cause of the oscillations.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Weimsguy]

The model, in the state that is was posted, was an attempt to match some earlier work where I put the breach on the spillway, but did not trigger it until the water was about two feet over the spillway invert of 624.8 ft. The intent of the next model was to vary the breach location/elevation and geometry to find a different solution. I am just starting to work with the Dam Breach outlet using HydroCAD and was using how it is setup in HEC-RAS where the physical location was a necessary component of defining the breach. In all the HEC-RAS sample dam breaches, the breach for overtopping/weir did not start until after the water surface was above the dam or an elapsed time after the water surface had reached the specified elevation. The HEC-RAS overtopping breach has a downward vertical and a outward horizontal component. The piping/orifice failure could start at any elevation, time, or both. The HEC-RAS piping breach has both and upward and downward component along with an outward horizontal component. When the piping breach vertical component reaches daylight, it changes from orifice flow to weir flow.

If I place the breach elevation to start at 630, it does not happen; the max elevation is 628.09 with the asymmetrical weir and dam breach outlets. And 628.16 using the combination trapezoidal and asymmetrical weir with the dam breach.