Permeable Pavement
Permeable pavement is one of the most effective low impact development (LID) strategies available in modern stormwater design. Unlike traditional pavement, which sheds nearly all rainfall as runoff, permeable pavement allows water to infiltrate directly through the surface, fundamentally changing how developed sites interact with stormwater.
In this article, we’ll explain what permeable pavement is, how it works, its benefits and limitations, and how it is modeled in tools like EPA SWMM and WWHM 2012.
What Is Permeable Pavement?
Permeable pavement refers to a category of engineered pavement systems designed to allow rainfall and runoff to pass through the surface rather than flow across it.
These systems are constructed with open-graded materials, creating void spaces that allow water to move downward.
Typical Structure:
Surface layer (porous asphalt, concrete, or pavers)
Storage layer (gravel or crushed stone reservoir)
Underlying soil (native subgrade or amended soil)
Water infiltrates through the surface, is temporarily stored in the base layer, and then:
infiltrates into native soils, or
is conveyed through an underdrain system
Unlike conventional pavement, which behaves like a sealed barrier, permeable pavement functions more like a sponge—capturing, storing, and slowly releasing water.
Types of Permeable Pavement
Several types of permeable pavement are commonly used, each with different applications:
1. Porous Asphalt
Similar to traditional asphalt but without fine aggregates
Creates voids for infiltration
Flexible and commonly used in parking lots
2. Pervious Concrete
Rigid structure with interconnected pores
Suitable for areas requiring structural strength
3. Interlocking Concrete Pavers
Individual blocks with joints filled with gravel or sand
Provide both strength and infiltration pathways
4. Plastic Grid Systems
Reinforced grids filled with grass or gravel
Ideal for low-impact or aesthetic applications
Choosing the Right Type
Selection depends on:
load requirements
site conditions
cost
aesthetics
Benefits of Permeable Pavement
Permeable pavement provides multiple hydrologic and environmental benefits:
1. Reduces Runoff Volume and Peak Flow
Decreases stress on stormwater systems
Reduces flooding and erosion risks
2. Improves Water Quality
Filters pollutants such as:
oils
heavy metals
nutrients
3. Promotes Groundwater Recharge
Encourages infiltration into native soils
Helps sustain baseflows in streams
4. Supports Regulatory Compliance
Recognized as a Low Impact Development (LID) practice
Helps meet stormwater permit requirements
5. Mitigates Urban Heat Island Effects
Stores less heat than traditional asphalt
Can reflect more solar radiation
Limitations and Design Challenges
Despite its benefits, permeable pavement has important constraints:
Maintenance Requirements
Requires routine vacuum sweeping or cleaning
Clogging from sediment can reduce performance
Higher Initial Cost
Typically more expensive than conventional pavement
Structural Limitations
Not suitable for:
heavy truck traffic
high-speed roadways
Site Constraints
Requires adequate soil infiltration capacity
In clay soils or shallow groundwater conditions:
underdrains are often required
Careful site evaluation is critical before implementation.
How Permeable Pavement Works
During a rainfall event:
Water infiltrates through the surface layer
It enters the gravel storage layer
Water is temporarily stored
Then either:
infiltrates into native soil, or
drains through an underdrain
Result:
Significant reduction in surface runoff
Delayed and reduced peak flows
Permeable pavement effectively transforms impervious areas into hydrologically active systems.
Modeling Permeable Pavement in WWHM 2012
In WWHM 2012 (used for stormwater design in Washington State), permeable pavement is modeled as a:
Low Impact Development (LID) Best Management Practice (BMP)
Key Inputs:
pavement type
soil properties
storage layer characteristics
underdrain configuration
Because WWHM uses continuous simulation, it evaluates performance over long-term rainfall records, allowing engineers to:
assess compliance with permit standards
evaluate long-term hydrologic performance
Modeling Permeable Pavement in EPA SWMM
EPA SWMM also includes permeable pavement as an LID control.
Key Parameters:
Surface layer
thickness
void ratio
Storage layer
depth
porosity
Soil layer
hydraulic conductivity
infiltration rate
Underdrain system
diameter
elevation
discharge characteristics
Application area
percent of subcatchment treated
What SWMM Can Simulate:
runoff reduction
peak flow attenuation
pollutant load reduction
This allows direct comparison between:
conventional pavement scenarios
LID-based designs
Common Applications
Permeable pavement is widely used in:
parking lots
sidewalks and plazas
residential driveways
low-traffic streets
retrofit projects
It is particularly effective where large impervious areas can be converted into infiltration-based systems.
Key Design Considerations
Successful implementation requires careful planning:
Soil Evaluation
Verify infiltration capacity
Determine need for underdrains
Maintenance Planning
Prevent clogging through regular cleaning
Sediment Control
Minimize upstream sediment sources
Climate Considerations
Account for freeze-thaw cycles
Plan for snow management
Regulatory Compliance
Follow local stormwater standards
Understand allowable LID credits
Final Thoughts
Permeable pavement is a powerful stormwater management tool that transforms paved surfaces into functional hydrologic systems.
To summarize:
It reduces runoff and peak flows
Improves water quality
Promotes groundwater recharge
Supports regulatory compliance
Can be modeled in both WWHM 2012 and EPA SWMM
However, its success depends on:
proper site selection
thoughtful design
ongoing maintenance
When applied correctly, permeable pavement is one of the most effective strategies for modern stormwater management.
Want to Learn SWMM Faster?
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