SWMM Runoff Method

Note: This method may not be available for your region. For more information, refer to the Regionalization topic.

Percentage Impervious - The impervious portion of the Inflows area. When the Use Land Uses/Soil Types layers box is checked, the Percentage Impervious is calculated from weighted averages of Land Uses.

Average Slope -The slope across the catchment.

Urban Creep - This scales the impermeable area by the amount specified. This can be used to take the increase of urban areas, or other factors into account. This value is only enabled (and used) when the analysis creep of the Analysis Criteria is set to "Use Catchment Areas ".

Equivalent Width – The width of the catchment.

Internal Routing - Extends SWMM's traditional surface runoff model to allow overland flow to be routed in three different ways: 

Routing Percentage - The specified fraction for 'To Pervious' and 'To Impervious' Internal Routing methods

Impervious Area:

Area - Total Impervious area calculated from Area * Percentage Impervious

Manning's n - PERN is an adaption factor used to introduce the effect of catchment roughness as follows:

n

PERN

0.01

0.4

0.015

0.5

0.025

1.0

0.10

3.0

Depression Storage Depth - The depth of depression storage on the Impervious portion of the catchment

No Depression - The percentage of the impervious area that has no depression storage.

Pervious Area:

Area - Total Pervious area calculated from Area * (1 - Percentage Impervious)

Manning's n - PERN is an adaption factor used to introduce the effect of catchment roughness as follows:

n

PERN

0.01

0.4

0.015

0.5

0.025

1.0

0.10

3.0

Depression Storage Depth - The depth of depression storage on the Pervious portion of the catchment

Infiltration Method - Selecting one of these methods will then expose additional parameters related to the infiltration method:

Horton / Modified Horton

Green Ampt / Modified Green Ampt

SCS Curve Number

Constant

Allows specification of a constant conductivity/infiltration rate using the Green Ampt. Method.