Refactoring the SWMM 5 Help File – LID’s In SWMM5

LID’s in SWMM5

LID Control Editor

The LID Control Editor is used to define a low impact development control that can be deployed throughout a study area to store, infiltrate, and evaporate Subcatchment runoff. The design of the control is made on a per-unit-area basis so that it can be placed in any number of Subcatchments at different sizes or number of replicates.

The editor contains the following data entry fields:

Control Name

A name used to identify the particular LID control.

LID Type

The generic type of LID being defined (bio-retention cell, rain garden, green roof, infiltration trench, permeable pavement, rain barrel, or vegetative swale).

Process Layers

These are a tabbed set of pages containing data entry fields for the vertical layers and drain system that comprise an LID control. They include some combination of the following, depending on the type of LID selected:

· Surface Layer
· Pavement Layer
· Soil Layer
· Storage Layer
· Drain System
· Drainage Mat
LID Surface Layer

The Surface Layer page of the LID Control Editor is used to describe the surface properties of bio-retention cells, green roofs, rain gardens, porous pavement, infiltration trenches, and vegetative swales. These properties are:

Berm Height (or Storage Depth)

When confining walls or berms are present this is the maximum depth to which water can pond above the surface of the unit before overflow occurs (in inches or mm). For Rooftop Disconnection it is the roof’s depression storage depth and for Vegetative Swales it is the height of the trapezoidal cross section.

Vegetation Volume Fraction

The fraction of the volume within the surface storage depth filled with vegetation. This is the volume occupied by stems and leaves, not their surface area coverage. Normally this volume can be ignored, but may be as high as 0.1 to 0.2 for very dense vegetative growth.

Surface Roughness

Manning’s n for overland flow over surface soil cover, pavement, roof surface or a vegetative swale (see this table for suggested values). Use 0 for other types of LIDs.

Surface Slope

Slope of a roof surface, pavement surface or vegetative swale (percent). Use 0 for other types of LIDs.

Swale Side Slope

Slope (run over rise) of the side walls of a vegetative swale’s cross section. This value is ignored for other types of LIDs.

If either the Surface Roughness or Surface Slope values are 0 then any ponded water that exceeds the surface storage depth is assumed to completely overflow the LID control within a single time step.
LID Pavement Layer

The Pavement Layer page of the LID Control Editor supplies values for the following properties of a permeable pavement LID:


The thickness of the pavement layer (inches or mm). Typical values are 4 to 6 inches (100 to 150 mm).

Void Ratio

The volume of void space relative to the volume of solids in the pavement for continuous systems or for the fill material used in modular systems. Typical values for pavements are 0.12 to 0.21. Note that porosity = void ratio / (1 + void ratio).

Impervious Surface Fraction

Ratio of impervious paver material to total area for modular systems; 0 for continuous porous pavement systems.


Permeability of the concrete or asphalt used in continuous systems or hydraulic conductivity of the fill material (gravel or sand) used in modular systems (in/hr or mm/hr). The permeability of new porous concrete or asphalt is very high (e.g., hundreds of in/hr) but can drop off over time due to clogging by fine particulates in the runoff (see below).

Clogging Factor

Number of pavement layer void volumes of runoff treated it takes to completely clog the pavement. Use a value of 0 to ignore clogging. Clogging progressively reduces the pavement’s permeability in direct proportion to the cumulative volume of runoff treated.

If one has an estimate of the number of years Yclog it takes to fractionally clog the system to a degree Fclog, then the Clogging Factor (CF) can be computed as:

CF = Yclog * Pa * (1 + CR) * (1 + VR) / (VR * (1 – ISF) * T * Fclog)

where Pa is the annual rainfall amount over the site, CR is the pavement’s capture ratio (area that contributes runoff to the pavement divided by area of the pavement itself), VR is the system’s Void Ratio, ISF is the Impervious Surface Fraction, and T is the pavement layer Thickness.

As an example, suppose it takes 5 years to completely clog a continuous porous pavement system that serves an area where the annual rainfall is 36 inches/year. If the pavement is 6 inches thick, has a void ratio of 0.2 and captures runoff only from its own surface (so that CR = 0), then the Clogging Factor is 5 x 36 x 1 x (1 + 0.2) / 0.2 / 1 / 6 / 1 = 180.

Regeneration Interval

The number of days that the pavement layer is allowed to clog before its permeability is restored, typically by vacuuming its surface. A value of 0 (the default) indicates that no permeability regeneration occurs.

Regeneration Fraction

The fractional degree to which the pavement’s permeability is restored when a regeneration interval is reached. The default is 0 (no restoration) while a value of 1 indicates complete restoration to the original permeability value. Once a regeneration occurs the pavement begins to clog once again at a rate determined by the Clogging Factor.

LID Soil Layer

The Soil Layer page of the LID Control Editor describes the properties of the engineered soil mixture used in bio-retention types of LIDs and the optional sand layer beneath permeable pavement. These properties are:


The thickness of the soil layer (inches or mm). Typical values range from 18 to 36 inches (450 to 900 mm) for rain gardens, street planters and other types of land-based bio-retention units, but only 3 to 6 inches (75 to 150 mm) for green roofs.


The volume of pore space relative to total volume of soil (as a fraction).

Field Capacity

Volume of pore water relative to total volume after the soil has been allowed to drain fully (as a fraction). Below this level, vertical drainage of water through the soil layer does not occur.

Wilting Point

Volume of pore water relative to total volume for a well dried soil where only bound water remains (as a fraction). The moisture content of the soil cannot fall below this limit.


Hydraulic conductivity for the fully saturated soil (in/hr or mm/hr).

Conductivity Slope

Slope of the curve of log(conductivity) versus soil moisture content (dimensionless). Typical values range from 30 to 60. It can be estimated from a standard soil grain size analysis as 0.48(%Sand) + 0.85(%Clay).

Suction Head

The average value of soil capillary suction along the wetting front (inches or mm). This is the same parameter as used in the Green-Ampt infiltration model.

Porosity, field capacity, conductivity and conductivity slope are the same soil properties used for Aquifer objects when modeling groundwater, while suction head is the same parameter used for Green-Ampt infiltration. Except here they apply to the special soil mix used in a LID unit rather than the site’s naturally occurring soil. See the Soil Characteristics Table for typical values of these properties.
LID Storage Layer

The Storage Layer page of the LID Control Editor describes the properties of the crushed stone or gravel layer used in bio-retention cells, permeable pavement systems, and infiltration trenches as a bottom storage/drainage layer. It is also used to specify the height of a rain barrel (or cistern). The following data fields are displayed:

Thickness (or Barrel Height)

This is the thickness of a gravel layer or the height of a rain barrel (inches or mm). Crushed stone and gravel layers are typically 6 to 18 inches (150 to 450 mm) thick while single family home rain barrels range in height from 24 to 36 inches (600 to 900 mm).

The following data fields do not apply to Rain Barrels.

Void Ratio

The volume of void space relative to the volume of solids in the layer. Typical values range from 0.5 to 0.75 for gravel beds. Note that porosity = void ratio / (1 + void ratio).

Seepage Rate

The rate at which water seeps into the native soil below the layer (in inches/hour or mm/hour).This would typically be the Saturated Hydraulic Conductivity of the surrounding subcatchment if Green-Ampt infiltration is used or the Minimum Infiltration Rate for Horton infiltration. If there is an impermeable floor or liner below the layer then use a value of 0.

Clogging Factor

Total volume of treated runoff it takes to completely clog the bottom of the layer divided by the void volume of the layer. Use a value of 0 to ignore clogging. Clogging progressively reduces the Infiltration Rate in direct proportion to the cumulative volume of runoff treated and may only be of concern for infiltration trenches with permeable bottoms and no underdrains. Refer to the Pavement Layer page for more discussion of the Clogging Factor.

LID Drain System

LID storage layers can contain an optional drainage system that collects water entering the layer and conveys it to a conventional storm drain or other location (which can be different than the outlet of the LID’s subcatchment). Drain flow can also be returned it to the pervious area of the LID’s subcatchment. The drain can be offset some distance above the bottom of the storage layer, to allow some volume of runoff to be stored (and eventually infiltrated) before any excess is captured by the drain. For Rooftop Disconnection, the drain system consists of the roof’s gutters and downspouts that have some maximum conveyance capacity.

The Drain page of the LID Control Editor describes the properties of and LID unit’s drain system. It contains the following data entry fields:

Drain Coefficient and Drain Exponent

The drain coefficient C and exponent n determines the rate of flow through a drain as a function of the height of stored water above the drain’s offset. The following equation is used to compute this flow rate (per unit area of the LID unit):

q = C hn

where q is outflow (in/hr or mm/hr) and h is the height of saturated media above the drain (inches or mm). If the layer has no drain then set C to 0.

A typical value for n would be 0.5 (making the drain act like an orifice). Note that the units of C depends on the unit system being used as well as the value assigned to n. Click here for more advice on setting drain parameters.

Drain Offset Height

This is the height of the drain line above the bottom of a storage layer or rain barrel (inches or mm).

Drain Delay (for Rain Barrels only)

The number of dry weather hours that must elapse before the drain line in a rain barrel is opened (the line is assumed to be closed once rainfall begins). A value of 0 signifies that the barrel’s drain line is always open and drains continuously. This parameter is ignored for other types of LID practices.

Flow Capacity (for Rooftop Disconnection only)

This is the maximum flow rate that the roof’s gutters and downspouts can handle (in inches/hour or mm/hour) before overflowing. This is the only drain parameter used for Rooftop Disconnection.

Open Level

The height (in inches or mm) in the drain’s Storage Layer that causes the drain to automatically open when the water level rises above it. The default is 0 which means that this feature is disabled.

Closed Level

The height (in inches or mm) in the drain’s Storage Layer that causes the drain to automatically close when the water level falls below it. The default is 0.

Control Curve

The name of an optional Control Curve that adjusts the computed drain flow as a function of the head of water above the drain. Leave blank if not applicable.

LID Drainage Mat

Green Roofs usually contain a drainage mat or plate that lies below the soil media and above the roof structure. Its purpose is to convey any water that drains through the soil layer off of the roof. The Drainage Mat page of the LID Control Editor for Green Roofs lists the properties of this layer which include:


The thickness of the mat or plate (inches or mm). It typically ranges between 1 to 2 inches.

Void Fraction

The ratio of void volume to total volume in the mat. It typically ranges from 0.5 to 0.6.


This is the Manning’s n constant used to compute the horizontal flow rate of drained water through the mat. It is not a standard product specification provided by manufacturers and therefore must be estimated. Previous modeling studies have suggested using a relatively high value such as from 0.1 to 0.4.

LID Pollutant Removal

The Pollutant Removal page of the LID Control Editor allows one to specify the degree to which pollutants are removed by an LID control as seen by the flow leaving the unit through its underdrain system. Thus it only applies to LID practices that contain an underdrain (bio-retention cells,permeable pavement, infiltration trenches, and rain barrels).

The page contains a data entry grid with the project’s pollutant names listed in one column and the percent removal that each receives by the LID unit in the second editable column. If a percent removal value is left blank it is assumed to be 0.

The removals specified on this page are applied to the unit’s underdrain when it sends flow onto either a subcatchment or into a conveyance system node. They do not apply to any surface flow that leaves the LID unit. As an example, if the runoff treated by the LID unit had a TSS concentration of 100 mg/L and a removal percentage of 90, then if 5 cfs flowed from its drain into a conveyance system node the mass loading contribution to the node would be 100 x (100 – 90) x 5 x 28.3 L/ft3 = 1,415 mg/sec. If in addition the unit had a surface outflow of 1 cfs into the same node, the mass loading from this flow stream would be 100 x 1 x 28.3 = 2,830 mg/sec.

LID Controls

LID Controls are low impact development practices designed to capture surface runoff and provide some combination of detention, infiltration, and evapotranspiration to it. They are considered as properties of a given subcatchment, similar to how Aquifers and Snow Packs are treated. SWMM can explicitly model the following generic types of LID controls:

Bio-retention Cells are depressions that contain vegetation grown in an engineered soil mixture placed above a gravel drainage bed. They provide storage, infiltration and evaporation of both direct rainfall and runoff captured from surrounding areas.
Rain Gardens are a type of bio-retention cell consisting of just the engineered soil layer with no gravel bed below it.
Green Roofs are another variation of a bio-retention cell that have a soil layer laying atop a special drainage mat material that conveys excess percolated rainfall off of the roof.
Infiltration Trenches are narrow ditches filled with gravel that intercept runoff from upslope impervious areas. They provide storage volume and additional time for captured runoff to infiltrate the native soil below.
Continuous Permeable Pavement systems are excavated areas filled with gravel and paved over with a porous concrete or asphalt mix. Block Paver systems consist of impervious paver blocks placed on a sand or pea gravel bed with a gravel storage layer below
Rain Barrels (or Cisterns) are containers that collect roof runoff during storm events and can either release or re-use the rainwater during dry periods.
Rooftop Disconnection has downspouts discharge to pervious landscaped areas and lawns instead of directly into storm drains. It can also model roofs with directly connected drains that overflow onto pervious areas.
Vegetative Swales are channels or depressed areas with sloping sides covered with grass and other vegetation. They slow down the conveyance of collected runoff and allow it more time to infiltrate the native soil beneath it.

Bio-retention cells, infiltration trenches, and permeable pavement systems can contain optional drain systems in their gravel storage beds to convey excess captured runoff off of the site and prevent the unit from flooding. They can also have an impermeable floor or liner that prevents any infiltration into the native soil from occurring. Infiltration trenches and permeable pavement systems can also be subjected to a decrease in hydraulic conductivity over time due to clogging.

Although some LID practices can also provide significant pollutant reduction benefits, at this time SWMM only models the reduction in runoff mass load resulting from the reduction in runoff flow volume. For more details on using LID controls within SWMM see the following topics:

LID Representation

LID Utilization

LID Placement

LID Results

LID Editors

LID Controls are defined and assigned to subcatchments through a series of three different editor forms:

· The LID Control Editor is used to define re-usable LID controls, designed on a per-unit-area basis, that can be placed throughout a study area’s subcatchments. It is invoked by adding a new LID Control object or editing an existing one from the main form’s Project Browser.
· The LID Group Editor is used to add any number of LID controls to a specific subcatchment. It is invoked by selecting the subcatchment’s LID Controls property from the subcatchment’s Property Editor.
· The LID Usage Editor is used to describe how each LID control added to an LID group is deployed within the group’s subcatchment. It is invoked from the LID Group Editor to specify the areal extent of the control and the portion of the subcatchment’s runoff that it treats.
LID Group Editor

The LID Group Editor is invoked when the LID Controls property of a Subcatchment is selected for editing. It is used to identify a group of previously defined LID controls that will be placed within the subcatchment, the sizing of each control, and what percent of runoff from the non-LID portion of the subcatchment each should treat.

The editor displays the current group of LIDs placed in the subcatchment along with buttons for adding an LID unit, editing a selected unit, and deleting a selected unit. These actions can also be chosen by hitting the Insert key, the Enter key, and the Delete key, respectively. Selecting Add or Edit will bring up an LID Usage Editor where one can enter values for the data fields shown in the Group Editor.

Note that the total % of Area for all of the the LID units within a subcatchment must not exceed 100%. The same applies to % From Imperv and % From Perv. Refer to the LID Usage Editor for the meaning of these parameters.

LID Placement

There are two different approaches for placing LID controls within a subcatchment:

1. place one or more controls in an existing subcatchment that will displace an equal amount of non-LID area from the subcatchment
2. create a new subcatchment devoted entirely to just a single LID practice.

The first approach allows a mix of LIDs to be placed into a subcatchment, each treating a different portion of the runoff generated from the non-LID fraction of the subcatchment. Note that under this option the subcatchment’s LIDs act in parallel — it is not possible to make them act in series (i.e., have the outflow from one LID control become the inflow to another LID). Also, after LID placement the subcatchment’s Percent Impervious and Width properties may require adjustment to compensate for the amount of original subcatchment area that has now been replaced by LIDs (see the figure below). For example, suppose that a subcatchment which is 40% impervious has 75% of that area converted to permeable pavement. After the LID is added the subcatchment’s percent imperviousness should be changed to the percent of impervious area remaining divided by the percent of non-LID area remaining. This works out to (1 – 0.75)40 / (100 – 0.7540) or 14.3 %.

Under this first approach the runoff available for capture by the subcatchment’s LIDs is the runoff generated from its non-LID area (after any internal re-routing of runoff (e.g., impervious to pervious) has been made). Also note that Green Roofs and Roof Disconnection only treat the precipitation that falls directly on them and do not capture runoff from other impervious areas in their subcatchment.

The second approach allows LID controls to be strung along in series and also allows runoff from several different upstream subcatchments to be routed onto the LID subcatchment. If these single-LID subcatchments are carved out of existing subcatchments, then once again some adjustment of the Percent Impervious, Width and also the Area properties of the latter may be necessary. In addition, whenever an LID occupies the entire subcatchment the values assigned to the subcatchment’s standard surface properties (such as imperviousness, slope, roughness, etc.) are overridden by those that pertain to the LID unit.

Normally both surface and drain outflows from LID units are routed to the same outlet location assigned to the parent subcatchment. However one can choose to return all LID outflow to the pervious area of the parent subcatchment and/or route the drain outflow to a separate designated outlet. (When both of these options are chosen, only the surface outflow is returned to the pervious sub-area.)

LID Representation

LID controls are represented by a combination of vertical layers whose properties are defined on a per-unit-area basis. This allows LIDs of the same design but differing areal coverage to easily be placed within different subcatchments in a study area.

During a simulation SWMM performs a moisture balance that keeps track of how much water moves between and is stored within each LID layer. As an example, the layers used to model a bio-retention cell and the flow pathways between them are shown below:

The following table indicates which combination of layers applies to each type of LID (x means required, o means optional):

LID Type Surface Pavement Soil Storage Drain Drainage Mat
Bio-Retention Cell x x x o
Rain Garden x x
Green Roof x x x
Infiltration Trench x x o
Permeable Pavement x x o x o
Rain Barrel x x
Rooftop Disconnection x x
Vegetative Swale x

When a user adds a specific type of LID control object to a SWMM project the LID Control Editor is used to set the design properties of each relevant layer (such as thickness, void volume, hydraulic conductivity, drain characteristics, etc.). These LID objects can then be placed within selected subcatchments at any desired sizing (or areal coverage) by editing the subcatchment’s LID Controls property.

LID Results

The performance of the LID controls placed in a subcatchment is reflected in the overall runoff, infiltration, and evaporation rates computed for the subcatchment as normally reported by SWMM. SWMM’s Summary Report also contains a section entitled LID Performance Summary that provides an overall water balance for each LID control placed in each subcatchment. The components of this water balance include total inflow, infiltration, evaporation, surface runoff, drain flow and initial and final stored volumes, all expressed as inches (or mm) over the LID’s area.

Optionally, the entire time series of flux rates and moisture levels for a selected LID control in a given subcatchment can be written to a tab delimited text file for easy viewing and graphing in a spreadsheet program.

LID Usage Editor

The LID Usage Editor is invoked from a subcatchment’s LID Group Editor to specify how a particular LID control will be deployed within the subcatchment. It contains the following data entry fields:

Control Name

The name of a previously defined LID control to be used in the subcatchment. (LID controls are added to a project by using the Project Browser — see LID Utilization.)

LID Occupies Full Subcatchment

Select this checkbox option if the LID control occupies the full subcatchment (i.e., the LID is placed in its own separate subcatchment and accepts runoff from upstream subcatchments).

Area of Each Unit

The surface area devoted to each replicate LID unit (sq. ft or sq. m). If the LID Occupies Full Subcatchmentboxis checked, then this field becomes disabled and will display the total subcatchment area divided by the number of replicate units. (See LID Placement for options on placing LIDs within subcatchments.) The label below this field indicates how much of the total subcatchment area is devoted to the particular LID being deployed and gets updated as changes are made to the number of units and area of each unit.

Number of Replicate Units

The number of equal size units of the LID practice (e.g., the number of rain barrels) deployed within the subcatchment.

Surface Width Per Unit

The width of the outflow face of each identical LID unit (in ft or m). This parameter applies to roofs, pavement, trenches, and swales that use overland flow to convey surface runoff off of the unit. It can be set to 0 for other LID processes, such as bio-retention cells, rain gardens, and rain barrels that simply spill any excess captured runoff over their berms.

% Initially Saturated

For bio-retention cells, rain gardens, and green roofs this is the degree to which the unit’s soil is initially filled with water (0 % saturation corresponds to the wilting point moisture content, 100 % saturation has the moisture content equal to the porosity). The storage zone beneath the soil zone of the cell is assumed to be completely dry. For other types of LIDs it corresponds to the degree to which their storage zone is initially filled with water.

% of Impervious Area Treated

The percent of the impervious portion of the subcatchment’s non-LID area whose runoff is treated by the LID practice. (E.g., if rain barrels are used to capture roof runoff and roofs represent 60% of the impervious area, then the impervious area treated is 60%). If the LID unit treats only direct rainfall, such as with a green roof or roof disconnection, then this value should be 0. If the LID takes up the entire subcatchment then this field is ignored.

% of Pervious Area Treated

The percent of the pervious portion of the subcatchment’s non-LID area whose runoff is treated by the LID practice. If the LID unit treats only direct rainfall, such as with a green roof or roof disconnection, then this value should be 0. If the LID takes up the entire subcatchment then this field is ignored.

Send Drain Flow To

Provide the name of the Node or Subcatchment that receives any drain flow produced by the LID unit. This field can be left blank if this flow goes to the same outlet as the LID unit’s subcatchment.

Return All Outflow To Pervious Area

Select this option if outflow from the LID unit should be routed back onto the pervious area of the subcatchment that contains it. If drain outflow was selected to be routed to a different location than the subcatchment outlet then only surface outflow will be returned. Otherwise both surface and drain flow will be returned. Selecting this option would be a common choice to make for Rain Barrels, Rooftop Disconnection and possibly Green Roofs.

Detailed Report File

The name of an optional file where detailed time series results for the LID will be written. Click the browse button to select a file using the standard Windows File Save dialog or click the delete button to remove any detailed reporting. Consult the LID Results topic to learn more about the contents of this file.

LID Utilization

Utilizing LID controls within a SWMM project is a two phase process that:

1. creates a set of scale-independent LID controls that can be deployed throughout the study area,
2. assigns any desired mix and sizing of these controls to selected subcatchments.

Bear in mind that when LIDs are added to a subcatchment, the subcatchment’s Area property is the total area of the subcatchment (both non-LID and LID portions) while the Percent Imperviousness and Width parameters apply only to the non-LID portion of the subcatchment.

To implement the first phase, one selects the Hydrology | LID Controls category from the Project Browser to add, edit or delete individual LID control objects. The LID Control Editor is used to edit the properties of the various component layers that comprise each LID control object.

For the second phase, for each subcatchment that will utilize LIDs, one selects the LID Controls property in the subcatchment’s Property Editor to launch the LID Group Editor. This editor is used to add or delete individual LID controls from the subcatchment. For each control added the LID Usage Editor is used to specify the size of the control and what fraction of the subcatchment’s impervious and pervious areas it captures.

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