Subbasin LID Structures Command

Last updated on August 19, 2026

Low Impact Development (LID) is a modern stormwater management approach that supports natural hydrologic processes rather than disrupting them. Instead of routing runoff quickly into pipes and channels, LID structures slow, spread, infiltrate, and reuse stormwater close to where it falls. This allows developed sites to more closely resemble pre-development conditions, helping reduce downstream flood peaks, improve water quality, and recharge groundwater.

The Subbasin LID Structures command allows the user to create Low Impact Design (LID) structures within the defined subbasins.

Follow the steps below to use the Subbasin LID Structures command:

From the Input ribbon menu, click the Drainage Subbasins dropdown menu and select the Subbasin LID Structures command.
The Subbasin LID Structures dialog box will be displayed, as shown below.

The following sections describe how to use the Subbasin LID Structures command and interact with the above dialog box.

Selecting LID Structure

The Select LID Structure section allows the user to select the LID structure for which LID structure data will be defined. In this section, the user can create, delete, copy existing LID structure data to a new LID structure, and select the type of LID structure. In addition, the user can navigate between LID structures and view a description for each LID structure type.
Subbasin LID Structures Command Img 3

The following entries are available in this section:

LID structure ID
This dropdown combo box lists all the LID structures defined in the current scenario. Click on the edit option (i.e., pencil icon) to edit the LID structure name. The user can navigate between the previous and next LID structure using the Up and Down arrow buttons. Note that the Up and Down arrow buttons will be disabled (i.e., grayed out) if the current scenario contains a single LID structure.
LID structure type
This dropdown combo box allows the user to select the type of LID structure to be defined. The following LID structure types are available:
- Bioretention Cell
- Bioswale
- Green Roof
- Infiltration Trench
- Permeable Pavement
- Rain Barrel
- Rain Garden
- Rooftop Capture
Description
This field shows additional information that describes the selected LID structure type.
New
The [New] button allows the user to create a new LID structure on the Map View. The ID of every newly created LID structure must be unique.
Copy
The [Copy] button allows the user to copy an existing LID structure along with its associated data to a new LID structure. The software automatically provides a unique ID to the copied LID structure.
Delete
The [Delete] button allows the user to delete the selected LID structure from the current scenario.
LID element picture
The image available to view in this section displays the preview of the selected LID element. Clicking the LID element image displays a dialog box with a detailed view of the selected LID element type.

LID Structure Specifications

The dropdown combo box at the LID Structure Specifications section header contains the following data panel entries, which allow the user to define LID structure data:

Layer Specifications
Discharge Specifications

These data panel entries will be enabled or disabled depending on the selected type of LID structure. The following table shows which data panel entries are enabled or disabled for each LID structure type:

LID Structure Type	Layer Specifications	Discharge Specifications
Bioretention Cell	Enabled	Enabled
Bioswale	Enabled	Disabled (i.e., Grayed Out)
Green Roof	Enabled	Disabled (i.e., Grayed Out)
Infiltration Trench	Enabled	Enabled
Permeable Pavement	Enabled	Enabled
Rain Barrel	Enabled	Enabled
Rain Garden	Enabled	Disabled (i.e., Grayed Out)
Rooftop Capture	Enabled	Enabled

LID Drain System

The drain system in an LID unit is performance-based rather than design-based. Rather than specifying physical pipe dimensions, the user controls how the drain behaves by specifying its height above the bottom of the storage layer and how the drain flow rate (per unit area) changes based on the height of saturated media above it.

When defining the parameters of an LID drain system, consider the following guidelines:

If the storage layer has an impermeable bottom, place the drain at the bottom with zero offset. If the goal is to allow the storage volume to fill completely before drainage begins, place the drain at the top of the storage layer.
If the storage layer does not contain a drain, set the drain coefficient to 0.
If the drain can handle all inflow into the storage layer up to a specific maximum limit, set the drain coefficient equal to that flow limit and set the drain exponent to 0.
If the drain consists of slotted pipes (where slots function like orifices), set the drain exponent to 0.5. The drain coefficient can then be estimated as:
Drain Coefficient = 60,000 × (Total Slot Area ÷ LID Area).
For example, drain pipes with five 1/4 inch diameter holes per foot spaced 50 feet apart produce an area ratio of 0.000035, which results in a drain coefficient of approximately 2.
If the goal is to drain a fully saturated unit within a specified time, set the drain exponent to 0.5 (for orifice flow) and calculate the drain coefficient.
Drain Coefficient = 2D^1/2
where:
- D = Distance from the drain to the surface plus any berm height (in inches or mm)
- o T = Required drain time (hours)

For example, to drain a depth of 36 inches of water in 12 hours, the drain coefficient would be 1. If this drain uses the slotted pipes described in the previous example (with a coefficient = 2), then a flow regulator (such as a capped orifice) must be placed at the discharge outlet to achieve the reduced flow rate.

The following sections describe each type of LID structure and how to define its associated data.

Bioretention Cell

A bioretention cell is an LID structure that consists of a depression filled with vegetation and engineered soil above a gravel drainage bed, designed to store, infiltrate, and evaporate rainwater and runoff. Bioretention cells are among the most versatile LID controls available, performing well in a wide range of climates and site conditions.
Subbasin LID Structures Command Img 6

Layer Specifications

This panel allows the user to define the layer specifications of the Bioretention Cell LID structure.

Surface Layer Definition

This section allows the user to define the surface properties of the LID structure. The following entries are available in this section:

Berm height
This entry field defines the maximum depth to which water can pond above the surface of the LID structure before overflow occurs. Clicking the [...] button displays the Berm Height lookup dialog box, showing depression storage values based upon land surface type.
Vegetative volume fraction
This spin control entry field represents the fraction of the volume within the storage depth that is filled with vegetation. This is the volume occupied by stems and leaves, not their surface area coverage. Normally, this volume can be ignored, but it may be as high as 20% for very dense vegetative growth. By default, the software uses a value of 5. However, the user can enter a different value ranging from 0 to 100.
Manning’s overland flow roughness
This entry field defines Manning’s roughness coefficient for bioretention cell overland flow. Clicking the […] button displays the Overland Flow Roughness lookup dialog box, showing Manning’s roughness values for overland flow.
Surface slope (H:V)
This entry field defines the slope of the surface layer of the bioretention cell.

Soil Layer Definition

This section allows the user to define the soil properties of the LID structure. The following entries are provided in this section:

Soil layer thickness
This entry field defines the thickness of the soil layer.
Soil porosity
This entry field represents the volume of pore space relative to the total soil volume. Clicking the […] button displays the Soil Porosity lookup dialog box, showing soil porosity values for typical soils.
Field capacity
This entry field represents the volume of pore water relative to the total soil volume after the soil has been allowed to fully drain. Below this level, vertical drainage of water through the soil layer does not occur. Clicking the […] button displays the Field Capacity lookup dialog box, showing field capacity values for typical soils.
Wilting point
This entry field represents the volume of pore water relative to the total volume of well-dried soil, where only bound water remains. The soil moisture content cannot fall below this limit. Clicking the […] button displays the Wilting Point lookup dialog box, showing wilting point values for typical soils.
Hydraulic conductivity
This entry field represents the hydraulic conductivity for fully saturated soil. Clicking the […] button displays the Hydraulic Conductivity lookup dialog box, showing hydraulic conductivity values for typical soils.
Conductivity slope
This entry field represents the average slope of the curve of log (hydraulic conductivity) versus soil moisture deficit (i.e., porosity minus moisture content).
Suction head
This entry field represents the average soil capillary suction along the wetting front. Clicking the […] button displays the Suction Head lookup dialog box, showing suction head values for typical soils.

Storage Layer Definition

This section allows the user to define the storage properties of the crushed stone or gravel layer used in bioretention cells. The following entries are provided in this section:

Storage layer thickness
This entry field defines the thickness of the bottom gravel layer. Crushed stone and gravel layers are typically 6 to 18 inches (15 to 45 cm) thick.
Void ratio
This spin control entry field represents the volume of void space relative to the volume of solids in the layer. By default, the software uses a value of 40. However, the user can enter a different value ranging from 1 to 100.
Seepage rate
This entry field represents the rate at which water seeps into the native soil below the layer. Clicking the […] button displays the Soil Saturated Hydraulic Conductivity lookup dialog box, showing saturated hydraulic conductivity values for typical soils.
Clogging factor
This checkbox entry field represents the total volume of runoff processed by the LID structure to completely clog the storage layer, divided by the void volume of the layer. The user can use a value of 0 to ignore clogging. Clogging progressively reduces the infiltration rate in direct proportion to the cumulative volume of runoff processed by the LID structure and may only be of concern for infiltration trenches with permeable bottoms and no underdrains. By default, this checkbox is unchecked, and the entry field is disabled (i.e., grayed out).

Discharge Specifications

This optional panel allows the user to define discharge specifications of the Bioretention Cell LID structure.
Subbasin LID Structures Command Img 15

Storage Layer Discharge Specifications

This section is used in a situation where the bioretention cell does not directly discharge any of the stored rainwater. By default, the checkbox at the section header is unchecked, and the contents are disabled (i.e., grayed out). Check the checkbox to enable the contents of this section.

The following entries are available in this section:

Discharge flow coefficient
This entry field defines the discharge flow coefficient “C.” This coefficient value is applied to the following equation that computes the flow rate (per LID unit area) through an outlet as a function of the height of stored water above the discharge outlet’s offset. The user can use a discharge flow coefficient of 0 if the layer has no drain.
Q = C x h^e
Where:
Q = Outflow per unit area
C = Coefficient, dependent upon unit system and assigned exponent
h = Height of saturated media above the discharge outlet
e = Exponent, typically 0.5 to act as an orifice
Discharge flow exponent
This entry field defines the discharge flow exponent “e.” This exponent value is applied to the previous entry’s equation that computes the flow rate (per LID unit area).
Discharge outlet height
This entry field represents the height of the outlet above the bottom of the bioretention cell.
Open discharge outlet at height
This checkbox entry field defines the height of stormwater contained in the bioretention cell’s storage layer at which the discharge outlet automatically opens when the water level rises above it. By default, this checkbox is unchecked, and the entry field is disabled (i.e., grayed out).
Close Discharge outlet at height
This entry field defines the height of the stormwater in the bioretention cell’s storage layer at which the discharge outlet automatically closes when the water level falls below it. Note that this entry field is only enabled when the Open discharge outlet at height checkbox is checked.

Storage Layer Discharge Adjustment

This section allows the user to define the control curve that adjusts the computed outflow based on the water head above the discharge outlet. By default, the checkbox at the section header is unchecked, and the contents are disabled (i.e., grayed out). Check the checkbox to enable the contents of this section.

This section contains a table with Water Head Above Outlet and Adjustment Factor columns. The Water Head Above Outlet column specifies the water head above the discharge outlet. The Adjustment Factor column specifies the adjustment factor applied to the computed outflow. Once the control curve data is entered, the corresponding graphical plot is displayed next to this table.

Bioswale

A bioswale is an LID structure consisting of a shallow channel or depression with sloped, vegetated sides. It is designed to slow runoff and promote infiltration into the native soil. Bioswales are commonly used along roadways and parking lots to intercept sheet flow before it reaches storm drains.
Subbasin LID Structures Command Img 16