Simulate pollutant washoff

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Simulate pollutant washoff

Step-by-step guide

In ICM, water quality simulations can model sediment buildup and the movement of sediment and determinants in a drainage system during rainfall.

Pollutant washoff refers to the pollutants that build up on a catchment surface and are washed into a sewer system by rainfall.

The volume of pollutants on the catchment surface and the speed with which they are washed into the sewer can be controlled in the water quality calculations during the simulation.

The graph below demonstrates the mass of pollutant on the catchment surface as a function of time:

A graph that represents the mass of pollutant on the catchment surface as a function of time.

  • The buildup time is a period of dry weather when surface sediment builds up.
  • The rate of increase decreases with time.
  • Generally, the buildup starts from zero; however, a specific mass of pollutant can be defined to start.

There are different methods to define the buildup within a simulation.

In a continuous simulation with periods of rainfall and dry weather, the process is applied automatically, with buildup occurring during dry periods, and washoff during wet periods.

In a simulation with a single event or several rainfall events, a consistent buildup time can be specified in the Run schedule QM Parameters.

  1. With a network model already created and opened, open the Run dialog.
  2. Click QM parameters.

The Run dialog with QM Parameters highlighted in red.

  1. In the QM Parameters dialog, specify the Buildup time in hours.

The QM Parameters dialog, with Buildup Time highlighted in red.

In a time series event, define individual buildup times as part of each rainfall event.

  1. From the Explorer window, within a model group, right-click an existing rainfall event and select Open.

In the Explorer window, the expanded context menu for a rainfall event, with Open highlighted and called out.

  1. From the Rainfall event dialog, open the Globals tab.
  2. Under the Catchment Sediment Data group, enter the Buildup Time in hours.

The Rainfall event dialog, with the Globals tab highlighted in red and opened, and the Buildup Time highlighted in red.

Review the impact of buildup time by comparing simulations with:

  • Zero buildup time.
  • Five days of dry weather calculated in the simulation parameters' buildup time.
  • Seven days of dry weather applied in the rainfall event itself.

To graph the first simulation:

  1. From the Explorer, click and drag base run into the graphics window.
  2. Select Results > Graph reports > Simulation report.

The three simulations highlighted in the Explorer; the Results menu expanded with Graph reports selected and Simulation report selected in the flyout; and the base run simulation open on the GeoPlan.

The Simulations dialog opens.

  1. Drag 5 days build up in sim parameters into the Simulations dialog.
  2. Drag 7 days build up in rainfall into the dialog.
  3. Click Produce Graphs.

The three simulations added to the Simulation dialog and Product Graphs highlighted in red.

  1. In the Parameter Selection dialog, under Parameters, select Washoff SF1.
  2. Click OK.

The Parameter Selection dialog, with Washoff SF1 and OK highlighted in red.

  1. In the graph, review how buildup time affects washoff.
  • With 0 days of buildup, there is almost no washoff.
  • After 5 days of buildup, there is a significant increase in washoff.
  • After 7 days of buildup, the washoff increases slightly more.

The graph of Washoff SF1 showing that as buildup time increases, washoff also increases.

Following buildup, or the dry period, rainfall causes runoff and reduces the mass on the catchment surface due to washoff.

In a continuous simulation, runoff is followed by accumulation again.

A graph of a continuous simulation showing a dry period of buildup, followed by a period of rainfall and runoff, and then a subsequent buildup period.

Runoff and washoff parameters can be controlled in the Surface Pollutant Editor. They are used to calculate pollutant buildup and washoff on catchment surfaces and in gully pots.

  1. To access the editor, from the Windows toolbar, expand the Grid Windows drop-down and select Surface pollutant editor.

From the Windows toolbar, the Grid Windows drop-down highlighted in red and expanded, with the Surface pollutant editor option being selected.

The editor displays a series of records that can be used to define pollution indices, global buildup and erosion characteristics for sediment, or parameters for sediment fractions.

In the Surface Pollutant Editor, a grid of records that includes runoff and washoff definitions for an example network.

Refer to the Autodesk InfoWorks ICM Help topic, Surface Pollutant Editor, for more information on each record type and the fields that they contain.

In the image below, the equations used by the model are overlaid on the editor, with color-coded arrows connecting parameters and their corresponding values.

In the Surface Pollutant Editor, the equations used by the model are overlaid on the editor, and color-coded arrows show the relationship between values defined in the editor and the parameters in the equations.

The records listed in the editor correspond to specific equations.

  • Buildup Parameters record (BuildupPar): Used in the surface buildup equation to calculate the mass of sediment buildup on the catchment surface. To increase the buildup rate, increase the dry period or the surface factor, PS. Note that only SF1 is used for washoff; it is not possible to use SF2.

The surface buildup equation.

  • Erosion Parameters Record (ErosionPar): Used in the rainfall erosion coefficient equation to calculate sediment deposits left on the catchment surface that are washed off. This defines the speed of washoff as a function of rainfall. For example, an increase in parameter C1 increases washoff for that rainfall event.

The rainfall erosion coefficient equation.

  • Potency Factor Equation record (PFEqnRec): Used to determine the potency factors (Kpn) of pollutants, which are used to relate the surface mass of the sediment to the surface mass of the attached pollutant.

The potency factor equation.

  • Gully Pot Buildup Equation record (GPEqnRec): Used to determine the buildup of dissolved pollutants in gully pots during the buildup period of the simulation and for each simulation timestep.

The gully pot buildup equation.

The Pollution Index is the water quality equivalent of a Runoff Surface that is associated with a subcatchment through the Land Use definition, with up to twelve surfaces.

In the Surface Pollutant Editor, the twelve runoff surfaces available for the Pollution Index highlighted in red.

To use the Land Use definition file, first, identify the index that is linked to the subcatchment.

  1. With the base run open, from the Windows toolbar, in the Grid windows drop-down, select New subcatchments window.

In a zoomed-in view of the Windows toolbar, the Grid windows drop-down expanded, with New subcatchments window selected and called out.

  1. In the Subcatchment grid, right-click any column header and select Find column.

The Subcatchment grid open with the first column header in blue, and the column header context menu active, with Find column called out and being selected at the bottom.

  1. In the Find column in grid dialog, select Land use ID.
  2. Click OK.

The Find column in grid dialog, with Land use ID selected.

  1. In the Subcatchment Grid, in the highlighted Land use ID column, note the ID of 1.

In the Subcatchment grid, the Land use ID column highlighted to show an ID of 1.

  1. In the Grid, select the Land use tab, where anything in row 1 also applies to this subcatchment.

In this case, Land use ID 1 relates to Pollution Index 1.

In row 1 of the Land Use grid, the Land use ID of 1 and the Pollution Index of 1 both highlighted in red.

  1. Return to the Surface Pollutant Editor and locate the subcatchment linked to Index 1.

The Surface Pollutant Editor with the subcatchment linked to Index 1 highlighted in red.

Several parameters apply to this subcatchment:

  • The two numeric values represent terms in the buildup equation and the dissolved pollutants equation, respectively. If needed, refer back to the color-coded image to visualize these links.
  • There are also two indices, “standard” and “a”, meaning that two further rows in this file apply.
  1. Locate the “standard” and “a” rows.

In the Surface Pollutant Editor, the row for index 1 and the related “standard” and “a” rows highlighted in red.

The “standard” and “a” rows include additional indices, including “bod_a” and “cod_a,” that define the attached pollutants and the dissolved pollutants.

  1. Again, identify all of the applicable rows.

The Surface Pollutant Editor, with the previously selected rows and all rows linked to the “standard” and “a” rows highlighted in red.

While this file may look complicated, it is only because of the complex way in which it is linked together. To manipulate the equations, the important thing is to identify the pollution index for each subcatchment and the subsequent parameters that it is linked to.

Next, use the buildup equation and the washoff equation to compare three scenarios:

  • A base run.
  • PS doubled, with the PS value doubled in the buildup equation to result in more sediment on the catchment surface, and therefore, more washoff.
  • Ka doubled, with the C1 parameter doubled in the washoff equation to result in a faster washoff.
  1. Open the base run.
  2. With the subcatchment selected in the GeoPlan, select Results > Graph reports > Simulation report.
  3. Drag the second and third simulations into the Simulations dialog.
  4. Click Produce Graphs.
  5. Select Washoff SF1.
  6. Click OK to review the results.

A graph of Washoff SF1 shows the results of the three simulations in kg/s.

For PS doubled, since there was a significant increase in the amount of sediment accumulating on the catchment surface, there is an increased volume of washoff.

Ka doubled, which had an increase in washoff rate only, shows the same volume of sediment entering the sewerage system as the base run; but it is entering faster, as a function of rainfall.