Initial Conditions

There are two parts to setting up the initial state of your model prior to a water quality run:

• Set up the initial state of the network by carrying out an Initialisation Run. This will probably be a Dry Weather Flow run.
• In the modelling run, set the initial state of the catchment by optionally including initial sediment depths for subcatchment surfaces, and by specifying a Buildup Time to allow the surface sediment to reach a steady state. If you are using the SWMM build-up/washoff land use model, then you can also specify the Last Sweep, which is the number of days since the last sweep (surface removal of sediment) at the start of the event.

Initialisation run

Ideally you should always carry out an initialisation simulation to set up the initial state of your model prior to your modelling runs.

InfoWorks ICM will use the state of the model at the final timestep of the initialisation simulation as the initial state for the modelling run.

The final state of this initialisation simulation is included as one of the input parameters for your modelling run on the Schedule Hydraulic Run Dialog.

Dry weather flow

In most cases, the initialisation simulation will be a dry weather flow simulation. The dry weather flow simulation should include all the same inputs as your simulation runs, except for the rainfall.

During a run that does not include a rainfall event (a dry weather flow run) the SWMM Build-up/Washoff Land Use and the Surface Pollutant models do not operate. You will need to initialise surface sediment masses as described in Surface Sediment below.

During the dry weather flow run:

• A layer of active sediment will build up in conduits. This active sediment can be transported by flows in the network. Active sediment sits on top of a fixed layer of bedded sediment that does not change during the simulation. See Sediment.
• Pollutant levels in the network will build up and eventually reach a steady state

Initialisation including rainfall

In some cases you will want to initialise your runs with a previous simulation that includes rainfall, perhaps with a short period of dry weather after the rainfall stops.

During a simulation that includes rainfall, the Surface Pollutant and the SWMM Build-up/Washoff Land Use models will run normally. The final state of surface sediment, attached pollutants, and dissolved pollutants in gully pots, will be saved at the end of the simulation. You therefore do not need to initialise surface sediment mass as described in Surface Sediment below.

Initialisation without an initialisation run

If you do not include the initial state for a run from a previous simulation, then InfoWorks ICM goes through a default initialisation process.

The hydraulic system is initialised in a steady state. In a similar way, the concentration of suspended sediment and dissolved pollutant in the flow throughout the system are set to an initial state based on the hydraulic steady state and initial water quality inputs to the system. The deposition of sediment to the bed is consistent with the carrying capacity of the flow field generated by the hydraulic steady state.

Surface sediment

You will normally initialise surface sediment mass by setting a buildup time for your modelling run. You set this in the Buildup Time field on the Globals Page of the Rainfall Event Editor. InfoWorks ICM calculates the buildup of sediment on the surface during this period prior to the start of the simulation. The sediment mass builds up towards a steady state.

You can also set initial sediment masses for subcatchment surfaces using the Rainfall Event Editor. You would not normally do this, but you may want to override values generated automatically during a buildup time.

You can use these initial sediment masses instead of, or as well as, a buildup time.

If you are using a SWMM model, you can also set the Last Sweep on the Globals Page of the Rainfall Event Editor. This is the number of days since the last sweep (surface removal of sediment) at the start of the event. Street sweeping may reduce the amount of available built-up sediment during the simulation.