EPA SWMM LID Controls: Modeling Low Impact Development

Low Impact Development (LID) practices have become essential components of modern stormwater management. EPA SWMM 5.2 provides comprehensive LID modeling capabilities that allow engineers to simulate how green infrastructure performs under various rainfall conditions. This tutorial covers the seven LID types available in SWMM and provides guidance for accurate model setup.

Understanding LID Controls in SWMM

SWMM models LID controls as specialized units that can be deployed across subcatchments. Each LID unit consists of multiple vertical layers that interact to capture, store, infiltrate, and/or drain stormwater runoff. The model tracks water movement through these layers using a combination of surface hydrology, unsaturated zone flow, and groundwater interactions.

The seven LID control types available in SWMM 5.2 are:

1. Bio-Retention Cells - Vegetated depressions with engineered soil media
2. Rain Gardens - Simplified bio-retention without underdrains
3. Green Roofs - Vegetated roof systems with drainage mats
4. Infiltration Trenches - Gravel-filled trenches for infiltration
5. Permeable Pavement - Porous surfaces with aggregate storage
6. Rain Barrels - Simple storage containers
7. Rooftop Disconnection - Routing roof runoff to pervious areas

Bio-Retention Cell Modeling

Bio-retention cells are among the most effective LID practices, combining surface ponding, soil filtration, and optional underdrain systems. In SWMM, bio-retention uses all five layers.

Surface Layer Parameters

The surface layer controls ponding and evaporation:

Soil Layer Parameters

The soil layer is critical for pollutant removal and flow attenuation:

Where K is the effective hydraulic conductivity, K_sat is saturated conductivity, and K_r is the relative permeability as a function of moisture content theta.

Key soil parameters include:

Storage Layer and Underdrain

The storage layer represents the gravel reservoir below the soil media:

For underdrains, SWMM uses an orifice equation:

Drain parameters include the drain coefficient, drain exponent (typically 0.5), and drain offset height from the bottom of the storage layer.

Example Bio-Retention Setup

For a 500 ft² bio-retention cell treating 5,000 ft² of impervious drainage area:

[LID_CONTROLS]
;;Name           Type
BioRetention     BC

[LID_USAGE]
;;Subcatchment   LID Process      Number  Area    Width   InitSat  FromImp  ToPerv  RptFile
S1               BioRetention     1       500     20      0        50       0       *

Permeable Pavement Systems

Permeable pavement combines a porous surface with aggregate storage to infiltrate and detain runoff. SWMM models three distinct layers: surface, pavement, and storage.

Pavement Layer Properties

The pavement layer has unique parameters:

Design Infiltration Rate

The effective infiltration rate through permeable pavement systems is governed by the most restrictive layer:

For sites with poor native soils (Hydrologic Soil Group C or D), underdrains are typically required.

Green Roof Modeling

Green roofs in SWMM consist of three layers: surface (vegetation), soil (growing media), and drainage mat. They differ from ground-level LIDs in that there is no infiltration to native soil.

Key Parameters

The drainage mat captures excess water and conveys it to roof drains. SWMM assumes this layer drains freely and does not restrict outflow.

Evapotranspiration Considerations

Green roofs can significantly reduce runoff through evapotranspiration (ET). SWMM calculates ET from the soil layer using:

Where f_1 is a moisture availability factor, K_c is the crop coefficient, and ET_0 is the reference evapotranspiration from climate data.

Rain Barrels and Cisterns

Rain barrels are the simplest LID type in SWMM, consisting only of a storage layer. They capture roof runoff and release it through a drain or overflow.

Barrel Configuration

Key parameters for rain barrel modeling:

The drain delay parameter is particularly important for rain barrels, as it simulates time required for active use (irrigation) between storms.

Infiltration Trenches

Infiltration trenches are linear features that capture and infiltrate runoff through gravel-filled excavations. SWMM models them with surface and storage layers.

Trench Parameters

The seepage rate should be based on field infiltration tests, not literature values, as native soil conditions vary significantly.

LID Placement and Sizing

Treatment Train Approach

Multiple LID types can be combined in series by routing outflow from one LID to another subcatchment:

1. Rooftop drains to Rain Barrel overflow to Rain Garden
2. Parking lot sheet flows to Permeable Pavement with underdrain to Bioswale
3. Sidewalk drains to Infiltration Trench overflow to storm sewer

Sizing Guidelines

The LID-to-impervious area ratio depends on the LID type and performance goals:

Model Validation Considerations

Monitoring Data

Validating LID models requires field monitoring data including:

• Inflow volume and peak rate
• Outflow (overflow + underdrain) volume and peak rate
• Soil moisture content (if available)
• Water levels in storage layers

Common Calibration Parameters

When calibrating LID models, focus on these sensitive parameters:

1. Hydraulic conductivity - Controls infiltration rate
2. Storage layer seepage rate - Controls exfiltration
3. Soil thickness and porosity - Controls storage volume
4. Drain coefficient - Controls underdrain discharge

Performance Metrics

Report LID performance using standard metrics:

Limitations and Best Practices

Known SWMM LID Limitations

1. No horizontal flow - Each LID unit is modeled as a vertical column with no lateral flow between units
2. Simplified clogging - Clogging is modeled as a gradual decrease in permeability, not particle transport
3. Limited winter performance - Frozen ground and snow storage are not explicitly modeled
4. No vegetation growth - Plant establishment and seasonal changes are not simulated

Best Practices

1. Verify parameters with local data - Use field-measured infiltration rates, not literature values
2. Include pre-treatment - Model forebays and sediment chambers as separate units
3. Consider maintenance - Reduce long-term performance expectations for unmaintained systems
4. Use continuous simulation - Single-event models do not capture antecedent moisture conditions
5. Validate with monitoring - Calibrate models against measured performance data when available

References

1. Rossman, L.A. (2022). Storm Water Management Model Reference Manual Volume I - Hydrology (Revised). EPA/600/R-15/162A. U.S. Environmental Protection Agency.

2. Rossman, L.A. & Huber, W.C. (2016). Storm Water Management Model Reference Manual Volume III - Water Quality. EPA/600/R-16/093. U.S. Environmental Protection Agency.

3. Prince George’s County, Maryland. (2007). Bioretention Manual. Department of Environmental Resources.

4. American Society of Civil Engineers. (2018). Permeable Pavements. ASCE/T&DI/ASCE-G-IP-02-18.

5. National Academies of Sciences, Engineering, and Medicine. (2006). Evaluation of Best Management Practices for Highway Runoff Control. NCHRP Report 565.