Contributing Surfaces

Each subcatchment can be categorised into up to 12 different contributing surface types (runoff areas). However, in most cases only the first three are used where:

The runoff characteristics of roads and roofs are often very similar and in this case it is common for the two to be amalgamated into a single impermeable runoff area (usually Area 1).

Part of the subcatchment may be defined as non-contributing.

Defining Runoff Areas in InfoWorks ICM

The 12 contributing surfaces can be defined as either percentages of the contributing area or as absolute areas in either hectares or acres. Percentages or absolute areas are selected using the drop down men in the Area Measurement Type column of the Subcatchment Grid Window of the Subcatchments Grid.

It is advisable, but not obligatory, that the sum of the twelve runoff areas should equal the contributing area. In cases where the twelve surfaces do not add up to the contributing area, all runoff calculations are made from the sum of the runoff surfaces rather than the contributing area.

It is not possible for one cell in InfoWorks ICM to cover percentages, and absolute values. There are therefore a total of 24 columns to cover all combinations of measurement types. 12 of these columns are called Runoff Area (1-12) % and define the area in percentages.

The other 12 are called Runoff Area (1-12) Absolute and define the area in hectares or acres. (depending on which catchment area unit is being used).