Problems modeling large model

Chris,

Since I am modeling over 120 miles of river downstream of a dam, I have desired a long routing period to obtain desired parameters at the cross sections furthest downstream. The outflow hydrograph from one dam break scenario indicates a constant flow of 167,758 cfs from 2.8 hours to 80 hours after flood wave has passed a cross section 1.4 miles downstream of dam (start of next reservoir).

If I use this as input into next dam break model that includes majority of downstream reach, I am in error as I know the 133,700 acre-ft. reservoir will be drained in less than 9 hours. After the reservoir is empty, we expected the model to report no more than 10,000 cfs outflow as this is my constant reservoir inflow. How can I obtain an accurate outflow hydrograph? The current outflow hydrograph increases some maximum stages beyond reality. Is this a safety-net feature that I can turn off or do you recommend a different modeling method?

Thanks,

Dan Rottler, P.E.
Tacoma Power
253-502-8605

Dear Dan :

Couple of items come to mind:

1. You can set the Theta Weighting Factor (in the Boundary Conditions screen) to 0.5. BOSS DAMBRK will then set the Theta Weighting Factor to 0.60 internally, the water surface elevation safety net feature is turned off (allowing the flow to drop below the initial conditions), and the model is then capable of allowing negative flows to occur (flow up the river valley).

2. You can combine the two models into a single model. We have a "super huge" version of BOSS DAMBRK especially compiled for these situations. You can really reduce the time step size and get really accurate models--even for really large models.

Anything else, I'd have to look at the model and troubleshoot it for you.

If you have any additional questions or need any further information, please contact me again.

Sincerely,

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