It sounds like you did not run the DAMBRK simulation long enough in order that it routes the entire hydrograph through the model.
Also, you need to make certain that the Theta Weighting Factor (contained in the Boundary Conditions dialog) is set to 0.5.
The Theta Weighting Factor is used in the finite difference solution. If a value of 0.0 is entered then a value of 0.60 is used by the program. If a value of 0.5 is used, theta is set to 0.60 internally by the program, the water surface elevation safety net feature is turned off (allowing the flow to drop below the initial conditions defined in the inflow hydrograph), and the model is then capable of allowing negative flows to occur (flow up the river valley). If a value of 0.51 is used, Theta again is set internally to 0.60 and the model routing is performed by the diffusion method (the first two inertial terms in momentum Eq.*(9) in Chapter 6 are omitted) instead of by dynamic routing.
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com