Exploring the relationship between subcatchments, land uses, and runoff surfaces

Step-by-step guide

A sewer network has been created, consisting of nodes, conduits, subcatchments, and other structures. In InfoWorks ICM, there is a close relationship between subcatchments, land uses, and runoff surfaces, such that they can all be configured from the same window.

A subcatchment describes an area draining to one or more inflow nodes. A land use contains a list of runoff surfaces particular to the type of land use being defined, such as suburban or industrial. A runoff surface describes the runoff characteristics of a specific surface type, for example, a road or roof.

1. Open the transportable database .icmt file that matches this tutorial.
2. From the Windows toolbar, expand the Grid windows drop-down and click New subcatchments window.

The resulting grid window stores all the subcatchment information in a table format. There are 10 different tabs.

3. At the bottom of the Subcatchments window, right-click one of the yellow tabs and select Hide empty tabs.
4. Select the Runoff surface tab. For each surface type, individual characteristics can be defined that determine how the runoff of rainfall occurs on that surface, the volume of runoff, and the rate at which it enters the drainage system.

In this model, there are 4 runoff surfaces currently defined. ID 1 is the default value given in the model. Then, there are three runoff surfaces: 10 - Road, 20 - Roof, and 21 - Permeable.

5. Click the scroll bar and drag to see the available columns of data.

Depending on which Runoff volume type and Routing model is used, the data columns become available. All the available options should be populated for each runoff surface. If the cells are grayed out, data does not need to be populated.

6. Change some of these values to see the different combinations and fields that are required.

Note: In the Model menu, there is a Runoff surface wizard that helps define standard values for surfaces.

7. In the Subcatchments window, navigate to the Land use tab. This tab contains default values for several factors, including the percentage of total area for each Runoff Surface, the Population Density, and the Connectivity.

The default value starts with every model. The next ID is Urban land use, which is applied to the model. The Runoff surfaces have been populated with the IDs from the Runoff surface tab. InfoWorks ICM allows up to twelve runoff surfaces per land use to support the modelling of more complex, mixed surfaces.

8. Switch to the Subcatchment tab. This is where the individual characteristics of each subcatchment or contributing area are defined.
9. On the Subcatchment tab, scroll to find the Land Use ID column. These are all set to Urban, since that was chosen on the Land use tab.
10. Scroll to the Population column. This is where the wastewater, trade, and infiltration flows into the sewer network are defined. These are the default values unless other values are specified.

Note: In the Model menu, under Subcatchment, there are tools to assist in applying population from point data or polygon data. These tools use internal or external GIS layers to distribute the population for subcatchments, based on geo-spatial information.

11. Look at the Rainfall profile and Evaporation profile columns. These relate to the Profile ID within a Rainfall Event and can be used to apply different geo-spatial rainfall events across the catchment.
12. The SPR calculation, WRAP soil type, and HOST soil class columns should be set appropriately based on the runoff volume method that is applied.
13. Look at the Area measurement type column and corresponding Runoff area % and Runoff area absolute 1-12 columns. These define the amount of area used to generate runoff from each of the twelve surfaces, as defined in the Land Use.

If the Area measurement type is changed from Absolute to Percentage, then the percentage columns become available.

Click the Runoff surface tab. Since there are only three surfaces listed here—Road, Roof, and Permeable—back on the Subcatchment tab, only the first three Runoff area columns are populated with data.

14. When done exploring, Close the Subcatchments window.