Designing Infiltration Swales on Reactive Clay Soils in Australia

Practical WSUD Design for Marginal Infiltration Contexts

Reactive clay soils are often cited as a reason to avoid infiltration systems. Low permeability, shrink-swell behaviour and saturation risk raise legitimate engineering concerns.

However, dismissing swales on clay sites overlooks an important distinction: infiltration does not have to mean rapid vertical drainage. It can mean controlled lateral hydration.

As water becomes more precious and the cost of planting failures during drought are examined, especially older established trees which can often benefit the most from occasional deep watering, passive irrigation becomes a more attractive option than pushing all stormwater out to sea as “waste.”

In the amenity sector, we can gain inspiration from permaculturists who are obsessed with working on contour to “slow the flow” of water down the landscape. They will generally avoid encouraging water to rush downhill like many bioswales seen in commercial projects. If a swale must go downhill, they will periodically dam it with gabbions or boulders to give it the best chance to sink into the soil.

For specifiers working in clay-dominant regions across Australia, the question is not whether infiltration is possible. It is how to design for it safely.

Note: ChatGPT’s interpretation of a permaculture-style contour swale using clay-tolerant Australian plants similar to Ozbreed’s highest performers. Scroll to the bottom of this article for detailed plant selection resources for specifying for clay-tolerant WSUD plants. 

Understanding Infiltration Rates in Clay Australia

Clay soils typically exhibit low saturated hydraulic conductivity. Standard infiltration rates may be insufficient to support deep, rapid vertical drainage. However:

• Surface detention combined with slow lateral seepage can still reduce peak flow.
• Organic matter incorporation significantly improves aggregate stability (obtained sustainably through intelligent planting, rather than calendar application of costly inputs).
• Deep-rooted and shallow-rooted planting progressively enhances macroporosity.

A swale in clay soil should not be designed as a soakwell. It should be designed as a controlled spreader of water.

Note: This is Ozbreed’s bioswale plant for general Australian conditions, best for areas that don’t receive frost or extreme humidity. You can view our frost-tolerant and tropical versions of this palette and different palettes for other functions here.

Addressing Common Engineering Objections

Prolonged Saturation Risk

On reactive soils, prolonged ponding can lead to instability. The solution is not abandoning swales but refining their detail:

• Shallow profiles with wide bases
• Gentle batters for safety
• Limited temporary ponding depth
• Underdrain swale detail where infiltration is demonstrably constrained

An underdrain does not negate infiltration intent. It provides hydraulic control while allowing partial soil hydration.

Asset Protection

In clay contexts, setback from footings and pavements is critical. Geotechnical advice should inform:

• Offset distances
• Subsoil moisture modelling
• Controlled discharge points

Contour-aligned swales reduce concentration of flow compared to fall-line drains. By distributing water laterally, they reduce erosive force and localised saturation risk.

Compliance Under WSUD Clay Soil Design

WSUD clay soil design requires clear performance logic:

• Peak flow attenuation
• Controlled discharge
• Demonstrated pollutant treatment where required

Biofiltration swales with engineered media over clay subgrades can meet treatment targets while still applying contour-based spreading principles.

Designing for Lateral Hydration

On clay sites, performance often improves when:

• Swales are aligned on contour rather than steep fall
• Soil is amended with organics (initially with composted material, after establishment plants can sustainably provide exudates and mulch using chop n drop maintenance)
• Tree root zones are integrated into infiltration corridors

Even limited infiltration improves subsoil moisture buffering, particularly important for urban canopy performance.

Plants Suitable for Reactive Clay WSUD

Ozbreed have been testing for wet feet and drought tolerance for over two decades. Two resources that are helpful are our Wet Feet Trial data, which you can find here, and our bioswale plant palette which you can find here.

Note: Some of the plants that have performed the best in our wet feet trials are surprising. These westringias are incredible in wet and dry cycles, unlike almost every other westringia you will ever see. Westringia fruticosa‘WES04’ PBR Trade Name Grey Box™

The Strategic Advantage

Contour-informed swales on clay soils:

• Reduce velocity and erosion (slow the flow)
• Lower peak discharge
• Improve soil moisture resilience
• Support vegetation and ecological development

They are not high-rate infiltration systems. They are controlled water distribution systems.

When detailed correctly, infiltration swales on reactive clay soils can meet WSUD objectives while managing geotechnical risk.