I looked at your DAMBRK model.
Very interesting geometry. However, your model flow rates are so small and you have such unusual river bottom geomtry you are running into problems. For example, in DAMBRK it warns you:
WARNING: River bottom elevation (HS) at cross-section 15 is greater than or equal to river bottom elevation at cross-section 14. This adverse slope may cause problems later in the routing computations, particularly if the base flow is small.
There are many cross-sections with these types of conditions. However, this is really part of a large reservoir (the dam is at the downstream end of the model). Hence, the cross- sections contained in the reservoir will not cause problems until the reservoir starts to dry up.
Note also that your model runs out of time steps before it is able to route the entire inflow hydrograph. You need break the model into smaller runs, or use larger time steps. We also have a 9,000 time step version of DAMBRK.
Your DAMBRK model is so unusual from what I have seen before, I am not surprised that you are running into weird effects. There is no rising limb inflow hydrograph--infact there is no hydrograph at all. You are basically releasing water at certain time intervals. There is no dam failure that occurs in your model.
Also, in your Boundary Conditions you have not set the Theta Weighting Factor. This can cause problems with the time-dependent turbine flow. If it is not specified, then it will not allow the base flow rate in the channel to drop below initial flow conditions (33 cms-- starting turbine base flow rate). Therefore, even though the warning message reports the wrong flow rate (see below) because it is looking at the inflow hydrograph flowrate (we never assumed that the base turbine flow rate would be greater than the inflow hydrograph value), that is why you are not seeing the turbine flow rate adjust downward.
NOTE: Inflow hydrograph safety-net feature will be activated during analysis since theta weighting factor has not been set to 0.5 in the boundary conditions, therefore inflow hydrograph values will not drop below 0.1 cms
THETA WEIGHTING FACTOR
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), 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.
If you specify a Theta Weighting Factor of .50 (to allow a lowering of base flow), then your model stops running at time 10.253 hours, exactly when the turbine flow shuts off from 33 cms to 0.1 cms. Hence, the turbine base flow rate is dropped and there is a shock in the system and insufficient flow in the downstream flow channel. This problem might be corrected by not cutting the flow rate off so rapidly and not reducing the flow rate so much (to such a small value).
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chrism@bossintl.com
Linear Mode
