Unrealistic Breach Condition Elevations

[JTardiff]

I am working to correct a few technical issues for a DAMBRK project tentatively completed back in 2004. I was able to resolve several of the issues using the DAMBRK manual, troubleshooting guides, and online forum postings. I still require assistance to explain or correct one remaining issue involving unrealistic downstream breach condition elevations exceeding the defined dam breach crest elevation. In other words, the elevation of the water is mathematically showing a “jump” in elevation immediately downstream of the dam following breach initiation.

My scenario comprises of a simple 20-foot-high concrete dam and fixed spillway with a reservoir storage area of approximately 60 acres. The user defined 8 downstream cross sections vary in width and elevation over a several mile long alignment, however no other major features have been included in the model (i.e. dams, bridges). The downstream cross sections were modeled using the method incorporating elevations, channel widths, storage widths and associated Manning’s Coefficients for each cross section. Boss DAMBRK Option #1 was used for defining the model alignment.

[Chris Maeder]

Without looking at the model, there are few items one could examine:

1. Are you running the model as a mixed flow regime? This may impact the computed WSEL profile.
2. Is it possible that the model is going through a hydraulic jump just downstream of the dam? This could be caused by having a higher Manning's roughness value or a constriction in the channel.
3. Do you have an instability in the model at the cross section in question? Perhaps changing from Option 1 to Option 13 would assist in solving that.
4. Have you reviewed the Reservoir Depletion Table that DAMBRK generates to make certain that everything looks OK? Perhaps this can shed some light into where your model is failing.

Sincerely,

Chris Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com