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[Dave Oldford]

Here is my situation. I have two dams next to each other, I called them GF-2 & GF-3. I managed to get GF-2 working perfectly. Because these dams are next to each other, i figured the results would be very similar. However, I would need to change the first 5 cross sections of GF-2. Because after the 5th cross section GF-3's inundation path would follow GF-2's inunadation path.

Here is my problem, I go to change some minor cross section heights and lenghts, say GF-2 had a cross section at 1465 ft MSL and GF-3 had a cross section at 1470 ft MSL. But for some reason this minor change is giving me all sorts of problems.( non-convergence errors, floating point etc.. )

And another problem I found, was when I changed the cross sections lenghts. In my problem GF-2 is longer than GF-3. So when i go to shorten the lenghts I get the same errors. Now i know the dam lenght is only so long so I can't go making it longer... inorder to get the program running. This would not be practical, nor would I be simulating what's accually out there. I'm starting to loose faith in this program. There is no way that something so small should cause such proplems. Is there anyone out there that can give me some advice(without charging me $450 US for 2 hrs work) as the someone at one of the BOSS DAMBRK offices did.

Thank you
Dave

[Chris Maeder]

DAMBRK can be temperamental at times. I have listed below some suggestions that you can try in an attempt to get your model to run. These items are only meant to help clear up any possible problem that you are having, and can be removed once you have found out what has caused the problem:

1. Try modeling steady-state conditions at first. A flat hydrograph and no dam-break failure. See if it will run steady-state.

2. Make certain that there is enough flow contained in the river. DAMBRK is a "wet" model, meaning that it must have some sort of base flow to "start-up". Therefore, to get the model up and running, I suggest that you artificially raise the amount of base flow that exists at start-up. For example, use a minimum flat hydrograph of 1000 cfs. Have a turbine flow of 1000 cfs as well, to make certain that the base flow is getting through to the river.

3. If the model runs steady-state, try changing the hydrograph to a more "realistic" hydrograph. Again, make certain to make small adjustments. If this causes the model to fail, the cross-section geometry may need to be smoothed.

4. A "general rule" is to allow the cross-section top-width to contract down to 0.5 and expand up to 2.0 from one cross-section to another (i.e., a cross-section top-width of 100 m can contract to 50 m or expand to 200 m). Any greater contraction or expansion should be "absorbed" using storage top width (i.e., a 100 m top-width could expand to 700 m by defining a channel top-width of 200 m and 500 m of storage).

5. Make certain that there is no large flat bottom inverts. For example, a channel bottom width of 100 m with a small base flow will prevent the model from having a sufficient depth of flow to start-up.

6. If subcritical and supercritcal profile conditions exist, make certain to use the "mixed flow option."

7. A specified dam-failure cannot occur too rapidly. The specified inflow hydrograph cannot change too quickly.

8. Try to simplify the model, and then add complexity to it. Try to reduce the number of items that could be causing your model to fail to run. For example, if a bridge is specified downstream of a dam, temporarily undefine it (changing the "Flow Structure at Cross-Section" entry to "NONE"). Then see if the model runs.

9. A small enough time-step value must be specified. For example, this generally ranges from 0.05 to 0.005 hours.

10. If a bridge is causing problems in routing your model, is there another way to model it? Could you model it as a dam with a spillway rating curve? Could you assume that the bridge gets washed out (conservative estimate)? Could you model it as simply a cross-section geometry constriction?

Sincerely,

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