Dry Weather Flow Mode

The InfoWorks ICM simulation engine can switch between two modes:

• Storm Mode applies by default. The simulation starts in Storm Mode and stays in that mode unless you have specified criteria for switching mode, and all the criteria are satisfied.

  In Storm Mode, the maximum simulation timestep is the Timestep defined in the Run Parameters section of the Schedule Hydraulic Run View.

• Dry Weather Flow Mode (DWF Mode) can apply during periods when there is no time-varying rainfall entering the system. Criteria must be defined, and the simulation must satisfy all the criteria, before Dry Weather Flow Mode is applied.

  In Dry Weather Flow Mode, the maximum timestep is the Timestep multiplied by the DWF Multiplier defined on the Control Page of the Timestep Control Options Dialog.

Terminating Simulations

In addition to using a much longer timestep during periods of dry weather, the Timestep Control feature gives additional control over how long a simulation will run.

The simulation can be set up to end when the calculation has passed the end time of all the time-varying data, and the simulation engine has switched into Dry Weather Flow Mode:

• Check the Stop Simulation At End Of Time-Varying Data option on the Control Page of the Timestep Control Options Dialog.
• Set up criteria for the simulation to switch to Dry Weather Flow Mode.

When using this feature, use the Duration value in the Run Parameters section of the Schedule Hydraulic Run View as a fail-safe value. The duration should be set to a value greater than that required by the simulation.

If the simulation never satisfies the criteria for stopping early, it will stop after the length of time specified in the Duration field.

Using Timestep Control

Set up Timestep Control criteria on the Timestep Control Options Dialog, accessed by clicking the Timestep Control button on the Schedule Hydraulic Run View.

The dialog has a number of pages:

• The first page, the Control Page, defines general control parameters
• The other pages define the criteria used to decide when to switch in and out of Dry Weather Flow Mode.

See the Timestep Control Options Dialog topic for details.

Hints on Using Timestep Control

Some common pitfalls users have encountered:

• If using timestep control to determine when to end a simulation (Stop Simulation At End Of Time-Varying Data) make sure that the duration is set to longer than you expect the simulation to run. Entering a duration value of 99999999 is OK as the engine will reset it to the longest possible duration.
• If you use timestep control to determine when to end a simulation (Stop Simulation At End Of Time-Varying Data) make sure you include the relevant criteria for your time varying data. If your simulation is based on rainfall check for rainfall. If your simulation is based on inflows check for inflows. The lag times determine the run-on.
• If you only use outflow or level criteria to determine when DWF mode starts, you may miss the start of storms. You will also need node, rainfall, or subcatchment criteria to determine when flow starts entering the system
• Similarly, if you only use node, rainfall, or subcatchment criteria to determine when DWF mode starts, ensure that the lag is sufficient for the system to drain down or you may miss details at the end of storms.