Water sensitive urban design

SESL Australia

With increasing urbanisation around the world, this has meant that the natural land that used to soak up runoff has been replaced with an impermeable surface such as roads and buildings. In heavy rain, this unsurprisingly leads to increased runoff, and the potential for pollutants to enter into our waterways.

Understanding the behaviour of rainfall runoff in these urban settings is an important scientific and practical issue. Stormwater management policies in Australia aim to ensure that stormwater run-off does not adversely impact the quality of receiving waters, including waterways, wetlands.

The following diagram shows how increasing coverage reduces infiltration and increases runoff

Four graphs showing how runoff increases as urbanisation increases

[Used by permission from the Washington Organic Recycling Council, www.soilsforsalmon.org.]

If we look at the natural forest cover example, the graph behind the forest indicates that 50% of all water that arrives as rain is lost again in evapotranspiration (evaporation from surfaces and transpiration by plants). Another 20% flows laterally through the soil. And 30% soaks into the soil to replenish groundwater. There is no runoff.

In contrast, the diagram of the city shows 80% of all rainfall is lost as runoff, only 20% is lost as evapotranspiration, and now there is no replenishment of groundwater.

This situation is detrimental for several reasons:

  • The runoff picks up wastes which pollute waterways and kill aquatic and marine life.
  • The nutrients in the runoff cause toxic algal blooms.
  • The high flows erode soil.
  • It increases sedimentation and choking of waterways.
  • It reduces the infiltration of water into aquifers, on which much of the world’s population depends.

Better urban design

To counter this situation, the concept of water-sensitive urban design (WSUD) was developed in Australia in the 1990s and is now practised in other countries as well.

Melbourne Water defines the key principles of WSUD as:

  • Protecting and enhancing natural water systems within urban developments.
  • Integrating stormwater treatment into the landscape, for example as parklands.
  • Improving the quality of water draining from urban developments through filtration and retention to remove pollutants close to their source.
  • Reducing runoff and peak flows, thus reducing peak flooding.
  • Adding value while minimising development costs.

In Australia’s increasingly arid environment, WSUD effectively increases the availability of clean water while improving waterway health and mitigating damage from stormwater runoff.

Wikipedia’s article on WSUD goes into detail on the many principles, objectives and techniques used in WSUD, and describes many practices, including:

  • retention systems, swales and basins
  • infiltration trenches and systems
  • sand filters
  • porous paving
  • sedimentation basins
  • constructed wetlands
  • ponds and lakes
  • rainwater tanks
  • aquifer storage and recovery.

Structural soil

In this context, we want to focus on the role of structural soil in supporting the aims of WSUD.

As we’ve covered previously, “structural soil” is growing medium formulated to provide both structural support for roads and pavements and physical support for trees in the form of soil for roots to grow in. Perhaps counterintuitively, structural soil looks like a pile of gravel with a bit of soil. In reality, the gravel packs down solid to support the traffic above while retaining a large volume of space, part-filled with soil, part with air. Both water and oxygen infiltrate easily, and roots can range widely without disturbing the paved surface.

The structural soil then contributes to the aims of WSUD by reducing runoff (therefore less pollution clogging drains and waterways, less erosion and less localised flooding) and increasing infiltration (therefore bigger, healthier street trees and more groundwater).

SESL has developed this concept extensively in Australia, as seen for example in the successful planting of thousands of trees at the Sydney Olympic site at Homebush Bay.

Further reading 

Melbourne Water. WSUD Key Principles.

SESL Australia. 2007. Trees and traffic.

SESL Australia. 2011. Structural soil.

Stenn H et al. 2009. Building Soil – Guidelines and Resources for Implementing Soil Quality and DepthBMP T5.13. Department of Ecology, State of Washington, USA.

Wikipedia. Water-sensitive urban design.