Water Quality Simulations (InfoWorks)

Water quality simulations in an InfoWorks 1D network are used to simulate the build-up of sediment in the network and the movement of sediment and determinants through the drainage system during a rainfall event.

For information on water quality modelling in the 2D network, see the 2D Water Quality Simulations topic.

The pollutant modelling process relies on a good set of initial conditions and an initialisation simulation should be carried out for a reasonable period prior to the intended water quality event simulation. The final state of the initialisation simulation becomes the initial state for the model. See Initial Conditions for more information.

The simulation that provides the initial state for the model will usually be, but does not have to be, a dry weather flow simulation. In some circumstances, the user may use a simulation that includes a rainfall event.

Stages to carry out a Water Quality simulation

There are four main stages to carrying out water quality simulations.

Setting Up Data

Set up the water quality specific parameters for the network.

• Add pollutant information to Trade and Wastewater Events.
• Set up Pollutographs to represent pollutant inputs associated with inflow or level hydrographs.
• Choose which build-up/washoff model is to be used for the water quality simulations. This is set in the Water Quality and Sediment Parameters.
  • If the Innovyze build-up/washoff model is being used, you may want to use the Surface Pollutant Editor to modify its properties. However, a set of default values is provided which will be used if the properties are not changed.
  • If the SWMM model is being used, ensure that:
    • The pollutant properties have been set in the Washoff section of the Water Quality and Sediment Parameters properties.
    • The properties have been set for each SWMM land use on the Build-up/washoff land use tab of the Subcatchments Grid.

• Ensure that the sediment characteristics (particularly the diameter (d50) and specific gravity) for each sediment fraction is defined in the Water Quality and Sediment Parameters. (Note that although it was previously possible to define them using the Surface Pollutant Editor, this is now deprecated and provided for backward compatibility only).

• Optionally use Pipe Sediment Data to override the fixed Sediment Depth values for conduits in the network. Using pipe sediment data allows you to set differing fixed sediment depths for your pipes without having to make multiple copies of the network. Alternatively, you could define multiple scenarios with different fixed sediment depths.
• If 1D diffusion is to be included in the simulation, then ensure that the diffusion parameters are set for the appropriate link data fields. See 1D Diffusion for further information.

Setting Initial Conditions - Initialisation Run

Set up the initial state of the network. You can do this partly by carrying out a dry weather flow simulation. You can also use a buildup period for surface sediment at the start of your modelling run. See Carrying Out Simulations below.

• Carry out an initialisation run to generate initial conditions for the network. This run will probably be a dry weather flow run. The dry weather flow run does not include a rainfall event. All other parameters and inputs must be identical to those of the modelling run.
• Optionally, you can set the initial mass of sediment on the catchment surface before the start of the simulation. Initial sediment mass on the catchment surface is one of the initial conditions set in the Rainfall Event.

To set up an initialisation run

1. Set up the run as described in the Runs topic. For a Dry Weather Flow run you should NOT include a rainfall event. However, you should include all the other inputs, such as Trade or Wastewater events, or Pollutant Inflows, that you will use in later simulations.
2. Check Use QM.
3. Include a Pollutograph if you have point pollutant inflows.

   If you include a Pollutograph you must also include an Inflow and/or Level event. The Pollutograph provides time-varying concentration values. For each node that has time-varying concentrations for one or more pollutants, the Inflow or Level event must contain an inflow hydrograph covering the same time period. See Pollutographs for more information.

4. Include Pipe Sediment Data if you want to override the Sediment Depth values set for your conduits.
5. If 1D diffusion is to be included in the simulation, the relevant parameters must be set as listed in the topic 1D Diffusion.
6. Click QM parameters. This displays the QM Parameters Dialog. In the dialog:
   • Select the Pollutants and Sediment Fractions to model. The choices should include the selections made when setting up your pollutant inputs (Pollutograph, Trade event and Wastewater event, and Washoff).
   • Enter a QM multiplier. Water quality calculations do not have to be carried out at every hydraulic timestep. The multiplier sets how often these calculations occur. For example, a QM multiplier value of three would mean water quality calculations every third hydraulic timestep.
   • A QM multiplier of zero (the default value) means that water quality calculations are carried out at all intermediate calculation timesteps as well as at every hydraulic timestep.
   • If Dependent sediment fractions is checked, the two fractions are averaged, and then modelled as one. Leave the box unchecked and the two sediment fractions are modelled completely independently, with no interaction.
   • Choose whether you want the varying sediment depth from the water quality model to affect the hydraulic calculations. See Sediment.
   • If you want, override the surface sediment buildup time from the event by checking the relevant box, and specify a new Buildup time.
   • Click OK when you are finished specifying the QM parameters.
7. Set an appropriate length for the simulation. This may well be the length of time since the end of the last storm event.

   If there has been a long dry weather period, you may want to shorten the time so you just run the simulation long enough for the water quality parameters (sediment depth and pollutant concentrations) to reach a steady state. This may take a few hours or a few days depending on the network.

8. In the State saving dropdown list, select the Save State at End option.
9. Click Run simulations to run your dry weather flow simulation.

Carrying Out Simulations - Modelling Runs

Once the initial state of the network has been set up, modelling runs can be carried out.

• Set up the modelling run with one or more rainfall events. The easiest way is to edit the dry weather flow run and assign a different name. The rainfall events are the only thing that should vary between your modelling runs and the dry weather flow run used to initialise the model.
• Set a Buildup Time for sediment on the catchment surface. This is effectively a period of dry weather prior to the start of a simulation, during which the surface sediment builds up to a steady state.
• Set the number of days since the Last Sweep of sediment on the catchment surface. This only applies if the SWMM build-up/washoff model is being used and may reduce the amount of available built-up sediment during the simulation.

When you run the water quality model, the following calculations are made:

• The selected Build-up/Washoff model calculates the build-up of sediment before and during the rainfall event.

• Street sweeping (SWMM model only) which may reduce the amount of available built-up sediment during the simulation.

• After the start of the rainfall event, the catchment sediment is eroded and enters the system.
• The rainfall runoff model determines the flow generated by the rainfall event for each subcatchment in each timestep. This data is passed to the relevant Build-up/Washoff model, which calculates the flow of dissolved and attached pollutants into the network.
• The change in sediment levels and the flow of the pollutants through the system is calculated for each timestep of the simulation.

To set up a modelling water quality run including one or more simulations:

1. Set up the run as described in the Runs topic.
2. If you carried out an Initialisation run to create initial conditions, you should include the same inputs as your initialisation run. If the initialisation run was not a dry weather flow run then remove any unwanted rainfall events. An easy way to start would be to open the initialisation run and change the name.
3. Add one or more rainfall events to the run. If you want to make use of the selected Build-up/Washoff model, you should set a value for the Buildup Time and Last Sweep (SWMM model only) on the Globals Page of the Rainfall Event Editor for each rainfall event.

   The Buildup Time should be the length of time since the end of the preceding rainfall event. If there has been a long period of dry weather, the Buildup Time should be long enough to allow the surface sediment to build up to a steady state.

   The Last Sweep (SWMM model only) should be the number of days since the last sweep (surface removal of sediment) at the start of the event.

4. If you carried out a Dry Weather Flow run to create initial conditions, drag and drop the Dry Weather Flow simulation into the Sim box in the Simulation State box.
5. Click Run simulations to run the modelling simulation(s).

Viewing Results

You view water quality results in the same way as the results of a normal hydraulic simulation. Additional results fields are available for a water quality simulation.

See Results for further information.