Unit Hydrographs (SWMM)

Unit Hydrographs can be specified as part of the node data and are used to determine RDII (Rainfall Derived Infiltration and Inflow) entering the system.

This method could typically be used to model the extra inflow during and immediately after rainfall events, caused by seepage of rainwater into defective pipes, ill fitting manhole covers etc.

The Unit Hydrograph defines the proportion of the rainfall falling on the node that enters the system as RDII and the time at which this rainfall enters the system.

Each unit hydrograph object can contain a set of three hydrographs; one for a short-term response, one for an intermediate-term response, and one for a long-term response. Up to 12 of these sets, one for each month of the year, can be grouped into a Unit Hydrograph Group.

Unit Hydrographs in InfoWorks ICM

Unit Hydrographs are defined using the RTK method, and are specified on the Unit Hydrograph Grid Window of the Nodes Grid or the Unit Hydrograph Property Sheet.

The Unit Hydrograph is associated with a node by selecting the appropriate unit hydrograph from the dropdown list in the Unit hydrograph field in the Node grid window of the Nodes Grid or property sheet.

To generate RDII into a drainage system, the Unit hydrograph group and the area of the surrounding sewershed that contributes RDII flow must be specified for each applicable node.

RTK Method


Unit Hydrograph Data Fields (SWMM)

Unit Hydrograph Group Data Fields (SWMM)

Nodes Grid