Timestep Control Options Dialog

The Dry Weather Flow Multiplier determines the maximum timestep in DWF mode. Select a value from the drop-down box. The value is a power of 2, as the simulation engine can only change its minor timestep by doubling and halving it.

Generally, select a value that does not greatly exceed the timestep of any data that varies in the dry weather periods. For example, if you use wastewater data that has a timestep of 1hr and your timestep is 60s then the maximum appropriate value would be 64. If you have constant flows in dry weather periods, a much larger value would be possible (say 1024).

Remember that this defines a maximum timestep. The simulation engine may choose a smaller timestep occasionally during the run to maintain accuracy.

Select the Check pumps box if you want simulations to revert to storm mode when any pumps are on in the system. The engine will revert to DWF mode immediately when all pumps are off if all the criteria for DWF mode still apply.

The Check pumps flag is normally best set if you have fixed pumps. In this case, you may want to consider remodelling the pumping stations using equivalent screw pumps to get better run times in DWF mode.

Select this option if you want to have time-varying results when the simulation is in DWF mode. Otherwise, there will be gaps in your results between the end of one storm mode period and the start of the next. If you select this option, results are output at the DWF mode timestep when the simulation is in DWF mode.

Results will occupy less space if you do not select this option.

Check this box if you want the simulation to end when DWF mode applies and no more relevant time varying data exists.

Note the following:

• The Duration on the Schedule Hydraulic Run View defines the maximum possible duration. If there is still time varying data or the simulation engine never enters DWF mode it will run for this duration. It is best to treat the Duration as a failsafe value in this case.
• Remember to apply criteria for all the relevant time varying data. If you do not apply criteria to all your time varying inputs, the simulation may end before the time varying inputs have entered the system.
• Apply a sensible lag time to all your criteria. This creates a run-on time after all the time varying data has finished to allow the system to drain down completely.

Select this option to include levels as a criterion for DWF mode

The criterion will be true if the measurement is at or below the value you enter here. The measurement for the level threshold is the maximum absolute rate of change in depth at the end of any link connected to an outfall.

Enter a time lag in minutes. The criterion must be true for this length of time before the simulation will switch to DWF mode

Select this option to include inflows to nodes as a criterion for DWF mode

Select Total flow into system if you want the threshold to represent the sum of flows into all internal nodes in the system.

Select Flow at each node if you want the threshold to represent the highest flow into any internal node in the system.

Note that the flow includes inflow defined in Inflow Data, flows from subcatchments draining into the nodes and inflows into conduits. The simulation engine assigns the conduit inflow to the node at the end with the highest invert or the upstream node if invert levels are equal.

The criterion will be true if the measurement is at or below the value you enter here.

Enter a time lag in minutes. The criteria must be true for this length of time before the simulation will switch to DWF mode.

Select this option to include rainfall as a criterion for DWF mode

The criterion will be true if the measurement is at or below the value you enter here. The measurement for threshold is the maximum rainfall intensity of any rainfall profile.

Note the following:

• InfoWorks ICM scales the rainfall intensity in the rainfall data by the areal reduction factor.
• If the Apply rainfall smoothing option in the Schedule Hydraulic Run View is checked, InfoWorks ICM uses a smoothed rainfall value (see Wallingford Procedure volume 1, section 6.7) to account for the effect of averaging rainfall over a catchment.

Enter a time lag in minutes. The criterion must be true for this length of time before the simulation will switch to DWF mode.

Select this option to include outflows as a criterion for DWF mode

Select Total outflow from system if you want the threshold to represent the sum of flows through all outfalls in the system.

Select Flow at each outfall if you want the threshold to represent the highest flow through any outfall in the system.

The criterion will be true if the measurement is at or below the value you enter here

Enter a time lag in minutes. The criterion must be true for this length of time before the simulation will switch to DWF mode

Select the items to include in the calculation of inflow below. You must check at least one item for subcatchment inflow to be a criterion for DWF mode.

Select Total inflow into system if you want the threshold to represent the sum of flows into all subcatchments in the system.

Select Flow at each subcatchment if you want the threshold to represent the highest flow into any subcatchment in the system.

The flow into each subcatchment is the sum of the flows you selected in the Include box.

The criterion will be true if the measurement is at or below the value you enter here

Enter a time lag in minutes. The criterion must be true for this length of time before the simulation will switch to DWF mode

Enter a range or logic name. If all other timestep control parameters indicate that the simulation should be run in DWF mode, then the mode will apply only if this condition is true.

Note

Ensure that the Dry Weather Flow Mode Condition and Storm Mode Condition are not contradictory. If the two conditions are both true, the simulation engine will oscillate between Storm Mode and DWF Mode. When in Storm Mode, the engine only checks the DWF Condition to see if it should switch. Once in DWF Mode, it will check both conditions to decide whether to switch back to Storm Mode

Enter a time lag in minutes. The criterion must be true for this length of time before the simulation will switch to DWF mode

Enter a range or logic name. When this condition is true the simulation will run in storm mode. This field may be left blank, in which case storm mode will apply if the DWF mode condition is false

Note

Ensure that the Dry Weather Flow Mode Condition and Storm Mode Condition are not contradictory. If the two conditions are both true, the simulation engine will oscillate between Storm Mode and DWF Mode. When in Storm Mode, the engine only checks the DWF Condition to see if it should switch. Once in DWF Mode, it will check both conditions to decide whether to switch back to Storm Mode