Refactoring the SWMM 5 Help File – RDII in SWMM5


Unit Hydrographs

Unit Hydrographs (UHs) estimate rainfall-dependent inflow/infiltration (RDII) into a sewer system. A UH set contains up to three such hydrographs, one for a short-term response, one for an intermediate-term response, and one for a long-term response. A UH group can have up to 12 UH sets, one for each month of the year. Each UH group is considered as a separate object by SWMM, and is assigned its own unique name along with the name of the rain gage that supplies rainfall data to it.

Each unit hydrograph is defined by three parameters:

· R: the fraction of rainfall volume that enters the sewer system
· T: the time from the onset of rainfall to the peak of the UH in hours
· K: the ratio of time to recession of the UH to the time to peak

To generate RDII into a drainage system node, the node must identify (through its Inflows property) the UH group and the area of the surrounding sewershed that contributes RDII flow.

An alternative to using unit hydrographs to define RDII flow is to create an external RDII interface file, which contains RDII time series data.

Unit hydrographs could also be used to replace SWMM’s main rainfall-runoff process that uses Subcatchment objects, provided that properly calibrated UHs are utilized. In this case what SWMM calls RDII inflow to a node would actually represent overland runoff.

See Also


RDII Inflow Editor

Unit Hydrograph Editor

Unit Hydrograph Editor

The Unit Hydrograph Editor is invoked whenever a new Unit Hydrograph object is created or an existing one is selected for editing. It is used to specify the shape parameters and rain gage for a group of triangular unit hydrographs. These hydrographs are used to compute rainfall-derived inflow/infiltration (RDII) flow at selected nodes of the drainage system. A UH group can contain up to 12 sets of unit hydrographs (one for each month of the year), and each set can consist of up to 3 individual hydrographs (for short-term, intermediate-term, and long-term responses, respectively) as well as parameters that describe any initial abstraction losses. The editor, shown below, contains the following data entry fields:

Name of UH Group

Enter the name assigned to the UH Group.

Rain Gage Used

Type in (or select from the dropdown list) the name of the rain gage that supplies rainfall data to the unit hydrographs in the group.

Hydrographs For:

Select a month from the dropdown list box for which hydrograph parameters will be defined. Select All Months to specify a default set of hydrographs that apply to all months of the year. Then select specific months that need to have special hydrographs defined. Months listed with a (*) next to them have had hydrographs assigned to them.

Unit Hydrographs

Select this tab to provide the R-T-K shape parameters for each set of unit hydrographs in selected months of the year. The first row is used to specify parameters for a short-term response hydrograph (i.e., small value of T), the second for a medium-term response hydrograph, and the third for a long-term response hydrograph (largest value of T). It is not required that all three hydrographs be defined and the sum of the three R-values do not have to equal 1. The shape parameters for each UH consist of:

· R: the fraction of rainfall volume that enters the sewer system
· T: the time from the onset of rainfall to the peak of the UH in hours
· K: the ratio of time to recession of the UH to the time to peak

Initial Abstraction Depth

Select this tab to provide parameters that describe how rainfall will be reduced by any initial abstraction depth available (i.e., interception and depression storage) before it is processed through the unit hydrographs defined for a specific month of the year. Different initial abstraction parameters can be assigned to each of the three unit hydrograph responses. These parameters are:

· Dmax: the maximum depth of initial abstraction available (in rain depth units)
· Drec: the rate at which any utilized initial abstraction is made available again (in rain depth units per day)
· Do: the amount of initial abstraction that has already been utilized at the start of the simulation (in rain depth units).

Note that any unit hydrograph or initial abstraction parameter left empty is assumed to be 0.

Right-clicking over the unit hydrograph and initial abstraction data entry grids will make a popup Edit menu appear. It contains commands to cut, copy, and paste text to or from cells that have been selected in the grid.

RDII / Routing File Format

RDII Interface files and Routing Interface files have the same text format:

· the first line contains the keyword “SWMM5” (without the quotes)
· a line of text that describes the file (can be blank)
· the time step used for all inflow records (integer seconds)
· the number of variables stored in the file, where the first variable must always be flow rate
· the name and units of each variable (one per line), where flow rate is the first variable listed and is always named FLOW
· the number of nodes with recorded inflow data
· the name of each node (one per line)
· a line of text that provides column headings for the data to follow (can be blank)
· for each node at each time step, a line with:
a) the name of the node,
b) the date (year, month, and day separated by spaces),
c) the time of day (hours, minutes, and seconds separated by spaces),
d) the flow rate followed by the concentration of each quality constituent.

Time periods with no values at any node can be skipped. An excerpt from an RDII / Routing interface file is shown below.

RDII Files

The RDII Interface File contains a time series of rainfall-dependent infiltration/inflow flows for a specified set of drainage system nodes. This file can be generated from a previous SWMM run when Unit Hydrographs and nodal RDII inflow data have been defined for the project, or it can be created outside of SWMM using some other source of RDII data (e.g., through measurements or output from a different computer program). RDII files generated by SWMM are saved in a binary format. RDII files created outside of SWMM are text files with the same format used for a Routing Interface File, where Flow is the only variable contained in the file.


Example File







Node Year Mon Day Hr Min Sec Flow

N1 2002 04 01 00 20 00 0.000000

N2 2002 04 01 00 20 00 0.002549

N1 2002 04 01 00 25 00 0.000000

N2 2002 04 01 00 25 00 0.002549

RDII Inflow Page

The RDII Inflow page of the Inflows Editor dialog form is used to specify RDII (Rainfall Dependent Infiltration/Inflow) for the node in question. The page contains the following two input fields:

Unit Hydrograph Group

Enter (or select from the dropdown list) the name of the Unit Hydrograph group which applies to the node in question. The unit hydrographs in the group are used in combination with the group’s assigned rain gage to develop a time series of RDII inflows per unit area over the period of the simulation. Leave this field blank to indicate that the node receives no RDII inflow. Clicking the button will launch the Unit Hydrograph Editor for the UH group specified.

Sewershed Area

Enter the area (in acres or hectares) of the sewershed which contributes RDII to the node in question. Note this area will typically be only a small, localized portion of the subcatchment area that contributes surface runoff to the node.

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