Porous Pavement LID Control in #SWMM5 and #InfoSWMM for Sustain

This is an introduction along with images of the LID Control and data require3ments in InfoSWMM. Every InfoSWMM Control uses from one to 5 layers of data - each with different data requirements. You can use the Siting Manger of InfoSWMM Sustain to find LID locations and the LID Optimizer to find the optimized number of units, cost of units, area and thickness of the LID layers based on your runoff and water quality control objectives.

The Porous Pavement LID Control has five components or Process Layers: Surface, Pavement, Storage and Underdrain.

The Surface Process Layer consists of:

· The Storage Depth in inches or millimeters

· The Vegetative Cover Fraction (0 to 1),

· Surface Roughness (Manning’s n)

· Surface Slope (percent).

The Soil Process Layer consists of:

· Soil Thickness in inches or millimeters,

· Porosity as a Fraction (0 to 1),

· Field Capacity as a Fraction (0 to 1),

· Wilting Point as a Fraction (0 to 1),

· Conductivity (inches/hour or millimeters/hour),

· Conductivity Slope,

· Suction Head (inches/hour or millimeters/hour)

The Pavement Process Layer consists of:

· The Pavement Thickness in inches or millimeters

· The Void Ratio (Voids/Solids) (0 to 1),

· Impervious Surface Fraction,

· Pavement Permeability in inches/hour or millimeters per hour,

· Clogging Factor (fraction).

The Storage Process Layer consists of:

· The Storage Height in inches or millimeters,

· The Void Ratio as a Fraction (0 to 1),

· Surface Conductivity in inches/hour or millimeters/hour,

· Clogging Factor

The Underdrain Process Layer consists of:

· Drain Coefficient (inches/hour or millimeters/hour)

· The Void Ratio as a Fraction (0 to 1),

· Drain Exponent

· Drain Offset Height (inches or millimeters)

An example of a Porous Pavement Driveway.

The four layers used in a simulation for a Permeable Pavement LID are shown in the following image.

Low Impact Development Pavement Layer

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

Thickness

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

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 it takes to fully clog the system (Yclog), the Clogging Factor can be computed as: Yclog * Pa * CR * (1 + VR) * (1 - ISF) / (T * VR) 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 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, then the Clogging Factor is 5 x 36 x (1 + 0.2) / 6 / 0.2 = 180.