Subcatchment Data Fields (SWMM)

Subcatchment ID

A unique name for the subcatchment.

subcatchment_id

Text

254

0

The type of object to which this subcatchment drains. The choices are:

Outlet

The name of the node or subcatchment to which this subcatchment drains. The name can be either be typed in or selected from a dropdown. The list is automatically populated when an option is selected in the Drains to field.

outlet_id

Text

254

0

Area

Area of the subcatchment.

The default area is calculated from the subcatchment's geometry.

area

Double

CA

3

0

x

The X co-ordinate at the centre of the subcatchment boundary region.

If the system default flag (#D) is set, the X co-ordinate at the centroid of the subcatchment will be determined by ICM.

x

Double

XY

1

y

The Y co-ordinate for the centre of the subcatchment boundary region.

If the system default flag (#D) is set, the X co-ordinate at the centroid of the subcatchment will be determined by ICM.

y

Double

XY

1

The width of the overland flow path for sheet flow runoff.

Slope

The average gradient at which the subcatchment drains to the node.

catchment_slope

Double

S

3

0.005

Applicable when using spatial rainfall (from Events or TSDB).

Check this option to calculate rainfall for the subcatchment as the area-average of the rainfall of all rainfall polygons or radar cells that overlap the subcatchment. Any areal reduction/calibration factors set for the rainfall polygons are incorporated in the calculation of the area-averaged rainfall.

A rainfall profile is created for each subcatchment which is then passed to the simulation engine.

If this option is not checked, the rainfall polygon or radar cell that contains the centroid of the subcatchment will be used to provide rainfall data for the entire subcatchment. The rainfall rate for every radar cell is sent to the simulation engine as a separate rainfall profile, and the engine checks the centroids to determine which profile to use.

See Spatially Varying Rainfall in SWMM Networks and Using Spatial Rainfall in SWMM Simulations for further details.

Note

The simulation engine can process up to 2,147,483,647 TSDB generated rainfall profiles in a single simulation. Therefore if radar images with more cells than this are imported, spatial rain zones must be used to crop them to less than 2,147,483,647 cells for any given simulation (and similarly for subcatchment-averaged rainfall, where per-subcatchment profiles also need to be taken into account).

Specifies which model is to be used to simulate runoff for this subcatchment. The choices are:

See Subcatchments for details.

Displayed when Runoff model type is set to SWMM.

The value of Manning's n for overland flow over the impervious portion of the subcatchment.

Displayed when Runoff model type is set to SWMM.

The value for Manning's n for overland flow over the pervious portion of the subcatchment.

Displayed when Runoff model type is set to SWMM.

The depth of depression storage on the impervious portion of the subcatchment.

Displayed when Runoff model type is set to SWMM.

The depth of depression storage on the pervious portion of the subcatchment.

Displayed when Runoff model type is set to SWMM.

The percentage of the impervious area with no depression storage.

Displayed when Runoff model type is set to SWMM.

The type of runoff routing. The choices are:

Displayed when Runoff model type is set to SWMM.

The percentage of runoff routed between sub-areas.

Displayed when Runoff model type is set to SCS_curvilinear or SCS_triangular.

The time of concentration.

If the system default flag (#D) is set, the time of concentration is determined by ICM.

Displayed when Runoff model type is set to SCS_curvilinear or SCS_triangular.

The hydraulic length of the subcatchment. That is, the distance between the most upstream location of the watershed to the point where the unit hydrograph is derived.

Displayed when Runoff model type is set to SCS_curvilinear or SCS_triangular.

The shape factor or peak rate factor as defined in the SCS National Engineering Handbook - Section 4 (NEH-4).

SCS determined the hydrograph shape factor to be 484 for most watersheds. Therefore, the default value is 484 for a hydrograph where the volume under the falling side of the triangular unit hydrograph is equal to 1.67 times the volume under the rising limb of the curvilinear unit hydrograph. Actual values may vary, for example, from 100 in very flat swampy country to 800 in steep rocky terrain.

This shape factor is used in the formulation of peak discharge and the peak of the unit hydrograph.

Effectively, the shape factor lengthens the time base of the unit hydrograph and lower the peak.

Note

When using the SCS_curvilinear model and you specify a Shape factor other than the default value of 484, the resulting unit hydrograph will not conserve water.

Displayed when Runoff model type is set to SCS_curvilinear or SCS_triangular.

Determines if the initial loss before runoff begins is to be specified by Depth or Factor.

Displayed when Initial abstraction type is set to Depth.

The total depth of precipitation before runoff begins.

Displayed when Initial abstraction type is set to Factor.

A decimal fraction that represents the amount of precipitation before runoff begins.

The specified factor is converted to an initial abstraction using the following equation:

Ia = kS

where : I_a is the initial abstraction depth, k is the specified factor and S is the storage depth.

The storage depth (S), in mm, is calculated as:

S = 25400 / CN - 254

where CN is the curve number

Specifies how the infiltration of rainfall into the upper soil zone in this subcatchment is to be modelled.

The choices are:

The infiltration models are described in the SWMM5 Reference Manual, Volume 1 - Hydrology.

If the Runoff model type is set to Curvilinear or Triangular, the Infiltration model must be set to Curve_number.

A series of records that assign different soils to percentage areas of this subcatchment. These are entered on a grid that is displayed by clicking on the button.

Note that only the infiltration parameters defined for each soil, which are applicable for the selected Infiltration model, will be applied during a simulation.

If no soils are assigned to any areas in this grid, then the applicable infiltration parameters Initial infiltration, Limiting infiltration, Decay factor, Maximum infiltration volume, Average capillary suction, Saturated hydraulic conductivity, Initial moisture deficit, Curve number and Drying time, specified in the Non-soils infiltration section of the subcatchment's properties, will be used.

Similarly, if the soils, which are specified in the grid, apply to less than 100% of the subcatchment area, and if any applicable infiltration parameters are specified in the Non-soils infiltration section of the subcatchment's properties, then these will be applied to the remaining area.

Used by the Horton Infiltration model.

Initial infiltration () from the equation to calculate rate:

Used in the Horton Infiltration model.

Limiting infiltration () from the equation to calculate rate:

Used in the Horton Infiltration model.

The infiltration rate decay constant for the Horton curve (1/hours). Typical values range between 2 and 7.

Used in the Horton Infiltration model.

Maximum infiltration volume possible.

The fractional difference between soil porosity and actual moisture content.

Used by the Green Ampt model.

The SCS number of the curve associated with this subcatchment.

Used by the Curve Number model. A list of curve number values that can be used can found here.

Used in the Horton Infiltration and Curve Number models.

Time in days for a fully saturated soil to dry completely. Typical values range from 2 to 14 days

The identifier for the aquifer (if any) associated with this subcatchment.

Any aquifer already defined can be selected from the dropdown list. Alternatively, a new one can be added or an existing one modified using the button located to the left of the field. This displays the non-visual object menu, from which you can select the appropriate option.

The elevation of the subcatchment's ground surface above the aquifer.

The depth of surface water above the Aquifer node ID’s invert level.

Set to zero if surface water depth will vary as computed by flow routing.

The minimum level that the groundwater must reach before any flow occurs.

If no value is specified, the invert level of the Aquifer node ID will be used.

The level of the groundwater at the start of the simulation for the aquifer associated with this subcatchment.

If no value is specified, the value specified for the aquifer's Initial groundwater parameter will be used.

The moisture content of the unsaturated zone of the aquifer for this subcatchment at the start of the simulation (volumetric fraction).

If no value is specified, the value specified for the aquifer's Initial moisture content parameter will be used.

The button displays the Groundwater Flow Equation Editor which can be used to create or edit a custom lateral groundwater flow equation.

If a custom equation is specified, it will be used in addition to the standard lateral groundwater flow equation to compute groundwater outflow from the subcatchment. However, if the Groundwater coefficient, the Surface coefficient and the Surface groundwater coefficient fields are set to 0, then standard equation will be replaced by the custom equation.

The button displays the Groundwater Flow Equation Editor which can be used to create or edit a custom deep groundwater flow equation. If a custom equation is specified, this will be used instead of the standard deep groundwater flow equation to calculate deep groundwater flow.

Snow pack ID

The name of the snow pack associated with the subcatchment.

Any snow pack already defined can be selected from the dropdown list. Alternatively, a new one can be added or an existing one modified using the button located to the left of the field. This displays the non-visual object menu, from which you can select the appropriate option.

snow_pack_id

Text

254

0

0

LIDS controls

A series of records defining the LID controls structure parameters for the subcatchment. The records are entered on the Subcatchment LID Controls grid which is displayed by clicking on the button. See the Subcatchment LID Controls Grid topic for further information.

Each LID structure has one or more LID control object associated with it. The LID control objects are defined on the LID control tab of the Subcatchment Grid. A new LID control object can also be added using the button located to the left of the field. This displays the Insert New LIDS control Object option that allows you to add the new object.

Structure

A series of records defining the land uses which apply to this subcatchment. These are entered on a grid that is displayed by clicking on the button.

Database Table Name: sw_sub_coverage

A series of records defining the initial build up of pollutants which apply to this subcatchment. These are entered on a grid that is displayed by clicking on the button.

Database Table Name: sw_sub_loading

The total length of curbs in the subcatchment.

Enabled when the pollutant build-up Unit is set to Curb length.