The Area Take-off tool in InfoWorks ICM provides a number of user definable operations. Some operations are complex and can lead to accidental misuse if they are not fully understood. One such case is the “Proportional system type split” option (shown in Figure 1), which is responsible for dividing up any areas that are left over after the primary areas have been extracted from the subcatchment by the Area Take-off operation. The complexity arises when there is no area of a specific type to be taken off from a subcatchment. According to the Area Take Off technical note within the InfoWorks ICM help section (3.4 & 4), in such cases, no residual area will be contributing to the runoff generation process. Therefore, it is recommended that you don’t select the “Proportional system type split” option when you are not simultaneously extracting the different types of contributing areas (Roads, Buildings and Permeable surfaces) for the various system types (Storm, Sanitary/Foul etc.) in your model network.
A common engineering practice is to take off a defined layer (say, buildings) and to divide the rest of the catchment areas across the other surface types (i.e. Roads and Permeable surfaces). In such circumstances, if a subcatchment does not present any building surfaces, but does include roads and permeable surfaces, then performing area take-off on the building layer, while leaving the “Proportional system type split” option active, will generate a null total contributing area. This in turn will lead to erroneous simulation results. Therefore, deactivating the “Proportional system type split” option is preferable in such situations, leaving the user to manually define the system type and the surface number to obtain the right overall contributing area count and distribution.