Permeable Pavement Design Spreadsheet

What is a Permeable Pavement?

Permeable pavement (sometimes referred to as pervious pavement or porous pavement) is a type of surface designed to allow water to pass through it, rather than shedding rainfall as runoff. Instead of acting like a sealed surface, it lets stormwater infiltrate into the layers below, where it can be stored, filtered, and slowly released into the ground or a drainage system. This reduces pressure on stormwater networks, lowers flood risk, and improves water quality.

These systems are commonly used in car parks, driveways, footpaths, low‑traffic roads, and urban developments where managing runoff is a priority. A typical permeable pavement includes a porous surface layer (such as permeable concrete, porous asphalt, or modular pavers), a stone sub‑base that stores water, and a geotextile layer that helps filter sediments.

Key benefits include:

• Reduced surface runoff and local flooding
• Improved groundwater recharge
• Better water quality through natural filtration
• Compliance with sustainable drainage (SuDS) requirements
• Longer pavement life due to reduced freeze–thaw stress

Typical Pervious Pavement Detail.

Common Permeable Pavement Design Methods

Designing permeable pavement involves balancing structural strength with hydraulic performance. Several established methods are used depending on site conditions, traffic loads, and regulatory requirements.

Infiltration‑Only Systems — Water infiltrates directly into the native soil beneath the pavement. This method works best where soils have good permeability and groundwater levels are low.

Partial‑Infiltration Systems — Some water infiltrates into the ground, while excess flows through an underdrain. This approach is used where soils drain slowly or where infiltration alone cannot manage peak rainfall.

No‑Infiltration (Tank) Systems — Water is stored temporarily in the sub‑base and discharged through a controlled outlet. This is used where soils are impermeable or where infiltration is not permitted due to contamination risks.

Modular Permeable Pavers — Concrete or plastic grid systems filled with gravel or grass. These are popular for driveways and parking areas and offer good structural performance with high infiltration rates.

Porous Asphalt or Permeable Concrete — Monolithic surfaces that allow water to pass through the material itself. These are used for larger paved areas and provide a smooth, durable finish.

Each method requires careful calculation of:

• Sub‑base storage volume
• Infiltration rate of the underlying soil
• Expected rainfall intensity and storm duration
• Traffic loading and pavement strength
• Overflow or underdrain requirements

How Does the Permeable Pavement Design Spreadsheet Work?

Firstly the spreadsheet calculates the quantity of water that the pavement will be expected to handle. The user simply inputs the pavement location, catchment area, runoff coefficient, return period and climate change allowance. The spreadsheet then automatically calculates the rainfall expected.

Next the user inputs the infiltration rate for the underlying soils. Care should be taken that this value is accurate and measured onsite as close to the pavement location as possible.

Finally the user inputs the proposed porosity of the pavement or sub-pavement layers where the rainfall will be stored prior to infiltration into the underlying ground.

The spreadsheet then uses the CIRIA C753 SUDS Manual methodology to calculate the critical design storm duration and to calculate the minimum required depth of the pavement storage layers.

Why Use a Permeable Pavement Design Spreadsheet?

A permeable pavement design spreadsheet helps engineers and designers manage the many variables involved in sizing and checking the system. Because these pavements must meet both hydraulic and structural requirements, a spreadsheet ensures calculations are consistent, accurate, and easy to update.

Advantages include:

• Automated storage calculations for sub‑base depth and void ratio
• Quick comparison of design storms and infiltration scenarios
• Built‑in checks for soil permeability, groundwater levels, and structural loading
• Scenario testing for different pavement types or rainfall events
• Clear documentation for planning approvals and SuDS compliance

For developers, engineers, and local authorities, a spreadsheet becomes a practical tool that reduces design time and improves confidence that the pavement will perform effectively in real‑world conditions.

Get Started Today

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