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