Lesson 16 – Site Permeability in Natural Building

What is Site Permeability?

Permeable sites minimise stormwater run-off by permitting rain water to be absorbed into the soil.

A lack of permeability increases flooding in urban areas during storm events affecting not only infrastructure, but our homes as well.

Many simple measures can be taken to counter-act this, and good building design should always consider ways in which site permeability can be enhanced or maintained.

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How will site permeability benefit me?

Providing a contribution to your overall Water Sensitive Urban Design (WSUD) aims, enhancing or maintaining permeability on site can also:

  • Reduce the volume of stormwater runoff which can cause localised flooding. Localised flooding can damage homes and property and greatly increase insurance premiums in certain areas;
  • Reduce pollution of waterways and habitats;
  • Reduce the need for expensive upgrades to local stormwater infrastructure;
  • Increase infiltration to sub-soil and allow groundwater recharge. This will not only help maintain groundwater supplies, but also aid local site ecology by ensuring sufficient water reaches tree root zones;
  • Reduce downstream flooding and stream-bank erosion.


How can I increase site permeability?

The simplest way to increase your site permeability is to maximise areas where natural drainage can occur.

These will include garden beds and lawn areas.

However, where areas such as paving or driveways are required, the type of surface or sub-surface construction can greatly affect the overall permeability outcome.

The diagram below lists the most permeable surfaces to the least.

Keep it in mind when selecting building materials for your project.

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Permeable Paving

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Whilst most increases in site permeability can be achieved without specialist treatment, in some situations measures such as porous/permeable paving may be required.

For example, where paving is required to carry a certain load capacity with a high frequency of use, such as a car park.

Porous or permeable pavements supporting load bearing structures are comprised of a pervious base and sub-base.

These allow infiltration of water, and in some cases, retain polluting particles.

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Design Considerations

Whilst use of permeable paving can have many beneficial outcomes, not all sites are suitable for its installation.

In addition, lack of maintenance can lead to clogging which in turn reduces infiltration levels.


Therefore, to ensure effectiveness of permeable paving, the following site conditions must be considered:

  • What is the primary design purpose? Flood mitigation, water quality improvement, water conservation;
  • Depth to groundwater table;
  • Soil type;
  • Soil depth;
  • Type of traffic (weight and volume);
  • Slope;
  • Design life;
  • Maintenance and clogging;
  • Local regulations;
  • Adjacent infrastructure.

It is advisable to seek specialist advice when selecting and installing permeable paving to ensure efficacy.


Garden Beds and Swales

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To improve permeability of semi and non porous surfaces consider sloping small area surfaces to a garden bed.

For larger sites such as car parks, an engineered swale with a drainage component can be utilised to reduce and treat discharge to the stormwater system.

Alternative Design Options

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In situations where it is not possible to improve your site’s ground level permeability, there are other options available to reduce stormwater quality and flow.

This may include inner urban sites which have been built on boundary to boundary.

Such measures may include raingardens, rainwater tanks, green walls and roofs.

The latter may provide additional benefits such as open space amenity to building occupants, an increased ecological contribution, increased insulation and reduction in urban heat island effect.


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