Hydraulics of porous pavement systems

Porous pavements are a Low Impact Development (LID) technology for reducing runoff from paved surfaces. The pavement system is designed to include pores in the top layer so that rainfall and runoff can percolate down into the aggregate layers below. The water can then accumulate in the aggregate layer and infiltrate into the underlying soil. A useful website with an overview of porous pavement systems can be found here.

Benefits of porous pavements include:

  1. Reduced runoff from paved surfaces
  2. Capture of pollutants within the runoff on site rather than allowing them to flow downstream
  3. Increased ground water recharge and reduced runoff volume from a land development

However, porous pavement systems have limitations that need to be considered before deciding on whether to use them in design. These include:

  1. Weaker pavement structure – The introduction of pores into the pavement is achieved by removing fine aggregate from the mix design. The pores reduce the load bearing capacity of the pavement limiting its application to parking lots, and low use roads that do not carry heavy vehicles
  2. Increased maintenance requirements – Sediment particles can become trapped in the pores of the pavement preventing runoff from percolating through the surface layer and reducing the efficacy of the pavement. Regular flushing or cleaning is required or additional sediment control structures need to be installed around the pavement.
  3. More complex design – There are many cases in which the infiltration capacity of the soil is inadequate and drainage pipes are required in the pavement aggregate layer. This means that a hydraulic design as well as a structural design is required.
  4. More complex installation – The process of pavement construction is more complex as care needs to be taken to prevent surface sealing and ensure that the pavement porosity is adequate to meet the design needs.

This page focuses on the hydraulic behavior of porous pavement systems. For a more detailed overview of such systems see chapter 14 of the recently published Handbook of Environmental Engineering.

Porous pavement systems fundamentally behave like detention ponds with the pond volume being the volume of the pores within the pavement and aggregate layers. As such, porous pavements can be analyzed using a pond routing model. The pond has three potential outflows: infiltration into the soil below the pavement, outflow through porous pipe under-drains, and overflow along the pavement surface if the pores become fully flooded. This model was formalized by Schwartz (2010) who defined an effective curve number for a given porous pavement. This was later extended into a preliminary design tool by Martin and Kaye (2014) who developed design charts for sizing porous pavements given a desired effective curve number.

The effective curve number can be used for a preliminary assessment of the viability of porous pavement systems, however full hydraulic design requires more analysis. The primary goal of hydraulic design is to eliminate surface runoff and maximize infiltration into the subsoil and resulting ground water recharge. If the underlying soil has a high infiltration capacity then the pavement may not need an under-drain. In this case the surface pavement layer should be designed for its mechanical properties and the depth of the sub-base layer of aggregate should be sized to ensure that the pavement does not fully flood. This may require routing the appropriate design storm through the system for different aggregate depths until an acceptable/optimal depth can be established.

In the case of a system in which the sub-soil has low infiltration capacity, that is, an infiltration capacity that results in the need for an unacceptably deep sub-base aggregate depth, one or more under-drains is needed. An under-drain behaves like an orifice. See Murphy et al. (2014). In this case the total under-drain effective outflow area should be sized and located vertically so that the outflow rate through the under-drains when the pavement is full is greater than the peak inflow due to rainfall on the pavement surface for the appropriate design storm.