The Low Flow Options tab contains several options which may be used to assist model robustness, particularly at low flows. These should not normally need changing, but can assist in overcoming problematic areas during a simulation.
The options and their descriptions area as follows:
| Option | Description |
|---|---|
| Global Slot Roughness | Will override the section Manning's n in all automated Preissmann slots in the model. It is felt that a higher value may give a better chance of holding the water up in a channel and thus prevent problems associated with low depths |
| Relaxation at low depths | Analogous to the relaxation at high Froude numbers, this option phases out the |
| Enforce Increasing Conveyance | Supplementary to the , this option ensures that any instances of conveyance within a section decreasing with increasing water level are replaced by interpolating between a previous conveyance value and the next water level which produces a higher conveyance |
| Refactorisation | Selection of this option performs a refactorisation of the solution matrix at regular intervals throughout a simulation. This will add to the simulation time, but may improve structural stability of the matrix |
| Refresh | Selecting this option recreates as many of the conditions as possible that would be obtained by stopping the run, exporting the current status of the model nodes and restarting the simulation at that time using the exported state as new initial conditions. It has been shown frequently that this approach can work when a continuous simulation fails |
| Reservoir Method | Selecting reservoir method=1 reverts to a much simpler (and less accurate) discretisation of the mass balance equation |
| Reservoir Emptying Factor | If a reservoir estimates that, at the current flow rate, more volume will be lost during the next timestep than is available in the reservoir, then this factor can be applied to limit the volume leaving the reservoir during this timestep. A factor of 0 has no effect; a factor of 1 should leave the minimum depth in the reservoir |
| Use Theta to weight property averages | Use Theta to weight time-based property averages in St Venant Equations |
| Use alternate weighting for velocity term | Use formulation of velocity term in St Venant momentum equation from eqn. 3.97 in Cunge, Holly and Verwey |
| Use alternate weighting for conveyance | Use formulation of conveyance term in St Venant momentum equation from eqn. 3.85b in Cunge, Holly and Verwey |
| Use Theta to weight friction slope term | Use Theta to weight friction slope term with respect to time in momentum equation |
| Bottom slot depth | Specify global value for total depth of conduit bottom slot, measured from the base to its opening. Default value is zero. |
| Bottom slot dh | Specify global value for height of bottom slot opening (top of slot) above invert level. Default value is zero. |
| Upstream conveyance weighting | Weight conveyance term with respect to upstream value in formulation of conveyance term |
| Relax dQ/dt term | Relax the dQ/dt term in the momentum equation. Uses the same methodology as for the convective acceleration term. |
| Add slot baseflow | Separates baseflow in Preissmann slot from the main channel flow in solving the St Venant equations |
| Apply conduit bottom slot | Select this option to apply a bottom slot in geometric conduits globally. This setting may be overridden in individual conduit units |
| Apply conduit top slot | Select this option to apply top slot in geometric conduits globally. This setting may be overridden in individual conduit units |
| Top slot height | Specify global value for total depth of conduit top slot, measured from its opening to its top. Default value is zero. |
| Top slot dh | Specify global value for depth of top slot opening (bottom of slot) below soffit level. Default value is zero. |
part of the convective acceleration term in the momentum equation when the water depth (water level minus lowest bed elevation) falls between the two specified values. Below the lower depth, the term is omitted; above the higher depth, the term is fully present. In the case of a conflict with the phasing out due to Froude number, the smaller of the two terms is used