SWMM - LID Control

Contains definitions of low-impact controls modeled in SWMM

This node allows a series of actions, such as:

• Create - Creates a new LID control
• Purge - Deletes unused LID controls
• Properties - Provides a tabular view of existing LID controls

LID controls are low-impact development practices designed to capture surface runoff and provide some combination of detention, infiltration, and evapotranspiration. They are considered to be properties of a given sub-catchment, similar to how aquifers and snow packs are treated.

Bio-retention cells, Infiltration Trenches and permeable pavement systems may contain optional drainage systems in their gravel storage beds to transport captured excessive runoff off-site and prevent the unit from flooding. They may also have an impermeable floor or lining that prevents any infiltration into the native soil from occurring. Infiltration trenches and permeable pavement systems can also be subject to a decrease in hydraulic Conductivity over time due to clogging.

While some LID practices can also provide significant benefits in reducing pollutants, 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 in SWMM, see the following topics:

• Representation
• Utilization
• Location
• Results

SWMM can explicitly model the following generic types of LID controls:

• Bio-Retention Cell - These are depressions containing vegetation grown in an engineered soil mix placed above a gravel drainage bed
• Rain Garden - are a type of bio-retention cell consisting only of the engineered soil layer, with no gravel bed below it
• Green Roof - are another variation of a bio-retention cell that has a layer of soil over a special drainage mat that transports excess rain percolated from the roof
• Infiltration Trench - These are narrow ditches filled with gravel that intercept runoff from impermeable areas of the slope
• Pervious Pavement - These are excavated areas filled with gravel and paved with a mixture of concrete or porous asphalt
• Rain Barrel - These are containers that collect runoff from the roof during storm events and can release or reuse rainwater during periods of drought
• Rooftop Disconnection - Discharges to front landscaped areas and lawns rather than directly into culverts
• Vegetative Swale - These are channels or depressed areas with sloping sides covered with grass and other vegetation

Representation

Lid controls are represented by a combination of vertical layers whose properties are defined per unit area. This allows LIDs with the same design but different area coverage are easily placed in different Subcatchments within a study area.

During a simulation, SWMM performs a moisture balance that tracks the amount of water that moves and is stored within each LID layer. As an example, the layers used to model a bioretention cell and the flow paths between them are shown below:

The following table indicates which layer combination applies to each LID type (x means mandatory, o means optional):

Type of LID	Surface	Floor	Soil	Storage	drainage	Draining Material
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

Usage

Using LID controls in 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. assign any desired combination and Designing of these controls to the selected subcatchments

Keep in mind that when LIDs are added to a subcatchment, the Subcatchment Area property is the total area of the subcatchment (both non-LID and LID parts), while the Percent Waterproof and Width parameters apply only to the non-LID part of the subcatchment.

To implement the first phase, select Hydrology | Project Browser LID Controls category for adding, editing, or deleting individual LID control objects. The Control Editor and LID is used to edit the properties of the various layers of components that make up each LID control object.

For the second phase, for each subcatchment that will use LIDs, select the LID Controls property in the subcatchment Property Editor to launch the LID Group Editor. This editor is used to add or delete individual subcatchment LID controls. For each control added, the LID Usage Editor is used to specify the size of the control and what fraction of the impermeable and permeable areas of the subcatchment it captures.

Location

There are two different approaches to placing LID controls in a subcatchment:

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

The first approach allows a mixture of LIDs to be placed in a subcatchment, each treating a different portion of the runoff generated from the non-LID fraction of the subcatchment. Note that, in this option, the subcatchment LIDs act in parallel - it is not possible to make them act in series (i.e. make the outflow of one LID control become the inflow to another LID) . Also, after placement of the LID, the Percentage Impervious and Width properties of the subcatchment may require adjustments to compensate for the amount of original subcatchment area that has now been replaced by LIDs (see figure below).

For example, suppose a subcatchment that is 40% impermeable has 75% of that area converted to permeable pavement. After the LID is added, the subcatchment's percent impermeability should be changed to the percent impermeable area remaining divided by the percent non-LID area remaining. This results in (1 - 0.75) * 40 / (100 - 0.75 * 40) 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 redirection of runoff (eg, impermeable to permeable) has been done). Also note that Green Roofs and Rooftop Disconnections only handle precipitation that falls directly on them and do not capture runoff from other impermeable areas in your subcatchment.

The second approach allows the LID controls to be placed in series and also allows runoff from several different upstream subcatchments to be routed to the LID subcatchment. If these single LID subcatchments are carved out of existing subcatchments then again some adjustments to Percentage Impermeable, Width as well as the Area properties of the latter may be required. Furthermore, whenever a LID occupies the entire subcatchment, the values assigned to the subcatchment's default surface properties (such as impermeability, slope, roughness, etc.) are replaced by those belonging to the LID unit.

Typically, surface and drain outlets from LID units are routed to the same outlet location assigned to the main subcatchment. However, one may choose to return the entire outflow from the LID to the permeable area of the parent subcatchment and/or direct the outflow from the drain to a separate designated outlet. (When both options are chosen, only Surface Runoff is returned to the permeable subarea.)

Results

The performance of LID controls placed in a subcatchment is reflected in the overall runoff, infiltration, and calculated evaporation rates for the subcatchment, typically reported by the SWMM. The SWMM summary report also contains a section titled LID Performance Summary that provides an overall water balance for each LID control placed in each subcatchment. Components of this water balance include total inflow, infiltration, evaporation, runoff, runoff flow, and initial and final stored volumes, all expressed in inches (or mm) over the LID area.

Optionally, the entire time series of flow 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.