SWMM - Unit Hydrographs

Unit hydrographs (UHs) estimate rainfall-dependent inflow/infiltration (RDII) in a sewer system:

A HU set contains three hydrographs:

1. short-term response
2. medium-term response
3. long-term response

Each unit hydrograph is defined by three parameters:

1. R: the fraction of the volume of rain that enters the sewer system
2. T: the time from the beginning of the rain to the peak of the HU in hours
3. K: the relationship between the HU recession time and the peak time

It is not necessary for all three hydrographs to be defined, and the sum of the three R values need not equal 1. Note that any unit hydrograph or initial abstraction parameter left blank is assumed to be 0

Different initial abstraction parameters can be assigned to each of the three hydrograph responses:

1. Dmax: maximum initial abstraction depth available (in rain depth units)
2. Drec: rate at which any initial abstraction used is made available again (in units of rainfall depth per day)
3. Do: amount of initial abstraction that was already used at the beginning of the simulation (in units of rain depth).

A HU group can have up to 12 HU sets, one for each month of the year. Each HU group is considered as a separate object by SWMM and is given its own unique name along with the name of the rain gage that provides rainfall data to it.

To generate ISDN at a drainage system node, the node must identify (via its Inflows property) the HU group and the surrounding sewer collector area that contribute to the ISDN flow.

Note: An alternative to using unit hydrographs to define the RDII flow is to create a file of external ISDN interface, which contains ISDN time series data.

Hydrographs can also be used to replace the main SWMM rain-runoff process that uses subcatchment objects, provided properly calibrated UHs are used. In this case, what SWMM calls the RDII inflow to a node would actually represent surface runoff.

When creating a new hydrogram, provide the following properties:

• Identification
  • Description - Use a friendly description when the item name is not enough
  • Name - name of the item
    Avoid names that are too long, or that contain the following characters: '*', '>', '<', '/', '\', '"', ':', ';', '|', '=', '`', '#', ',', '?'
  • Priority - Information about the current state of the item. It might be:
    • Neutral: no problem
    • Critical: Some issue that prevents the item from being calculated or used
• SWMM
  • Monthly Data - Monthly Data, for each month data is available, or
  • Pluviometer - Pluviometer that provides the precipitation data for the HU group