SWMM Model

SWMM is the Storm Water Management Model developed for the U. S. Environmental Protection Agency. InfoWorks ICM incorporates some of the features of the SWMM runoff block for InfoWorks networks.

The following features can be modelled in this version of the software:

Note that Groundwater outflow can also be modelled, but is modelled using the InfoWorks method rather than the SWMM method.

SWMM runoff, like the other runoff models in InfoWorks, consists of three components. These are:

The SWMM runoff model (when not accounting for snow-melt) divides each subcatchment into the following three surface.

Surface Type Characteristics
Surface 1 Impervious with depression storage
Surface 2 Impervious without depression storage
Surface 3 Pervious with depression storage and Horton or Green-Ampt infiltration

We recommend that you set up three surface records, one for the impervious surfaces with depression storage, one for impervious surfaces without depression storage and one for the pervious surface. The impervious surfaces should be defined as fixed runoff surfaces with a runoff coefficient of 1.0.

Initial losses

Impervious areas Initial losses for impervious areas can either be defined as slope related or as an absolute depth. For a slope related initial loss we recommend a value of 0.000071m as in the Wallingford runoff model and the UK percentage runoff model.
Pervious areas An absolute initial loss is normally defined for pervious areas. For grassed areas a typical value is 0.0025m. Alternatively, a slope related initial loss can be defined.

Runoff volume

Impervious areas A fixed runoff of 100% after any initial losses should be defined.
Pervious areas

Pervious areas either have Horton or Green-Ampt infiltration.

Define the initial moisture store for the Horton model or initial moisture deficit for the Green-Ampt model.

Note

If infiltration models are mixed, ensure that the correct initial conditions are set in the Catchment Runoff data.

Routing

The SWMM runoff model uses a non-linear reservoir and the kinematic wave equation to route subcatchment runoff to the manhole. To use SWMM routing, specify the subcatchment width and Manning's roughness on impervious and pervious surfaces. Engman analysed measured rainfall-runoff data to produce the following table of Manning's Roughness Coefficient (n) for overland flow.

Note

For the SWMM model, the value in the Runoff routing value field is always the Manning roughness (n) whatever has been selected as the runoff rate type.

Ground cover

Manning's roughness coefficient (n) for overland flow(n)

 Range
concrete/asphalt 0.011 0.01 - 0.013
bare sand 0.01 0.01 - 0.016
gravelled surface 0.02 0.012 - 0.033
bare clay-loam (eroded) 0.02 0.012 - 0.033
range (natural) 0.13 0.01 - 0.32
bluegrass sod 0.45 0.39 - 0.63
short grass prairie 0.15 0.10 - 0.20
Bermuda grass 0.41 0.30 - 0.48

Runoff Routing Models