Permeable Pavements for Source Control Stormwater Management

Urban development often goes hand in hand with an increase in concrete and asphalt surfaces. Impervious surfaces increase the volume of water conveyed into the drainage system. To promote groundwater recharge and reduce the volumes of water to be treated, stormwater management at the source should be encouraged.

In this article, you will find basic information on permeable pavements. Two types of permeable pavements are presented: porous materials and concrete pavers with permeable joints.

What is a permeable pavement?

Permeable pavements allow better water infiltration on surfaces that are normally impervious. There are different types of permeable pavement, including porous asphalt, porous concrete and concrete pavers.

Where can permeable pavements be used?

Permeable pavements are suited to low-traffic areas, since their structural capacity is reduced. They are therefore normally used for:

• Sidewalks
• Bike paths
• Parking areas
• Low-traffic residential streets

Types of permeable pavements

For all types of pavement, water passes through the selected surface material, then infiltrates into the ground through a gravel filter layer. The water can then be intercepted by perforated drains connected to a drainage system.

Porous materials

Porous concrete and porous asphalt are produced by reducing the fine materials in the mix and adding certain supplementary materials to ensure the pavement’s strength.

The voids in the structure allow water to infiltrate. Designing these pavements is complex, since the material’s permeability reduces its structural strength. A balance must therefore be found between permeability and strength.

In winter conditions, resistance to freeze-thaw cycles must also be ensured.

In addition, the pores frequently become clogged, so the pavement must be cleaned with a vacuum or a water jet to maintain a high infiltration rate.

Concrete pavers with permeable joints

Concrete pavers consist of impervious units separated by permeable joints. The infiltration rate varies depending on the spacing between the units and the permeability of the joints.

According to the Guide de gestion des eaux pluviales (Quebec Stormwater Management Guide), this type of pavement is better suited to freeze-thaw cycles, and clogging is less problematic than with porous pavements.

Installing a permeable pavement

In general, systems allow for either:

• Full infiltration
• Partial infiltration with perforated drains
• Partial infiltration with perforated drains and a flow restrictor

Systems with perforated drains generally convey part of the infiltrated stormwater to a retention pond, retention or infiltration chambers or an underground tank. They are preferred when the soil’s infiltration capacity is low or the water table is high.

Systems with perforated drains and a flow restrictor can be used to convey stormwater either to a tank or to the storm sewer system.

Designing porous pavements

Here are some key design considerations according to the Stormwater Source Control Design Guidelines of the Greater Vancouver Sewerage & Drainage District (GVS&DD):

• The impervious area draining to the permeable pavement should be no more than twice the permeable area (2:1 ratio). If the soil surrounding the permeable surface carries a heavy sediment load, another pretreatment structure, such as a grassed swale, may be required.
• A full infiltration system can be used if the soil infiltration rate is greater than 15 mm/h. The Guide de gestion des eaux pluviales, however, recommends an infiltration rate of 12.5 mm/h, based on an earlier GVS&DD study.
• An analysis of the in-situ soil must be carried out to properly assess the site’s potential infiltration rate.
• The surface slope must be greater than 1% to prevent sediment from accumulating on the surface.

In addition, spreading salt or sand on the pavement in winter is prohibited, to prevent clogging and groundwater contamination.

Advantages and disadvantages of permeable pavements

The main advantages are:

• Reduced runoff volume
• Help in resolving downstream overloading in the drainage system
• Contribution to groundwater recharge
• Improved appearance of this type of development

The main disadvantages relate to:

• Pavement maintenance costs
• The limited structural capacity of the pavements
• Functional constraints due to in-situ soil characteristics
• The height of the water table
• Winter conditions

For an overview of other stormwater management facilities, read Nine Types of Stormwater Management Facilities Used in Quebec.

---