Reducing Erosion in Public Open Space Using Contour Swale Principles

Slow, Spread and Sink as a Slope Stabilisation Strategy

Erosion in Australian public open space is rarely a planting failure alone. More often, it is a hydrological design issue. Across sloping reserves, road verges and linear corridors, concentrated surface flow accelerates downslope. Traditional drainage systems are designed to move water away quickly.

While this approach satisfies hydraulic conveyance requirements, it frequently increases scouring, sediment transport and long-term maintenance costs. And it treats rainwater as a waste product rather than a valuable resource.

Contour swale principles reverse that logic. Instead of accelerating runoff, they intercept it early, redistribute it laterally and encourage infiltration. The objective is not to remove water as quickly as possible, but to manage it upslope before it becomes destructive.

For landscape architects, this is fundamentally about slope length, steepness, velocity and soil stability.

ChatGPT’s interpretation of a permaculture-style contour swale using Australian plants similar to Ozbreed’s highest WSUD performers. Of course, this is a work of fiction created by AI and doesn’t perfectly follow the logic of working on contour. Scroll to the bottom of this article for detailed plant selection resources for specifying for wet/dry-tolerant WSUD plants.

Understanding the Mechanism of Erosion on Slopes

On unbroken slopes, runoff gains velocity rapidly. Once sheet flow transitions to concentrated flow, erosion risk increases significantly. Typical failure patterns in public open space include:

• Rilling and shallow gully formation
• Undermining of turf edges and pathways
• Scour at pipe outfalls
• Loss of topsoil from batters
• Sediment deposition in low points and drainage pits

Australian rainfall patterns intensify this cycle. Extended dry periods reduce soil cohesion and biological activity. Intense rainfall events then deliver high-energy runoff onto hydrophobic or compacted surfaces.

Without contour interruption, runoff progressively concentrates and gains velocity as it travels downslope, increasing erosion risk over time. The design solution is to interrupt slope length and dissipate energy.

Vegetated Swale Erosion Control

By aligning shallow swales on contour, designers interrupt the gravitational pathway of runoff.

When properly designed and detailed:

• Flow velocity is reduced through increased surface roughness
• Water is distributed laterally rather than concentrated downslope
• Sediment deposition occurs upslope
• Batters remain stable due to reduced hydraulic stress
• Peak erosive force is broken into smaller, manageable increments

This approach is particularly effective in:

• Sloping public reserves
• Roadside verges subject to sheet flow
• Golf course roughs and embankments
• Linear open space corridors
• Residential interface zones above retaining structures

Rather than one large drainage intervention at the base of a slope, multiple shallow contour-aligned swales break the slope into hydraulic segments.

Each segment slows and attenuates water before it can gain destructive momentum. These systems can scale down to tiny micro bunds upslope of a singular tree, to giant agricultural systems spanning vast distances along contour lines.

Importantly, these systems must remain legible and maintainable. Swales should have clearly defined cross-sections, stable batter gradients and robust overflow pathways that reconnect safely to formal drainage systems during extreme events.

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.

Integrating Swales with Public Realm Design

For landscape architects, swales should not read as leftover infrastructure. When integrated early in master planning, they can become:

• Edge-defining landscape elements
• Tree planting zones with passive irrigation
• Biodiverse habitat strips
• Informal pedestrian edges within parkland

In linear reserves, contour swales can follow natural landform rather than imposed orthogonal layouts. This reinforces topographic logic and reduces long-term maintenance stress.

While inspired by rural contour systems, urban applications require engineered precision. Batter stability, compaction regimes and overflow detailing must be resolved through documentation, not left to site interpretation.

Stabilising Batters Through Planting

Hydrology alone will not stabilise slopes. Vegetation is a structural component of erosion control.

Deep and fibrous root systems:

• Increase soil cohesion
• Improve aggregate stability
• Reinforce batter surfaces against shear stress
• Enhance infiltration by maintaining pore structure
• Reduce surface crusting and runoff initiation

On infiltration mounds or the downslope edges of swales, planting becomes particularly critical. Root architecture acts as a living geotextile, binding soil particles and resisting displacement during storm events.

Species selection must respond to cyclical wet and dry conditions. In public open space, plantings are often exposed to:

• Periodic saturation during heavy rainfall
• Extended dry periods
• Compacted or dispersive subsoils
• Low organic matter content

Plants that tolerate only free-draining soils, as well as trees that require consistently moist soils, frequently fail in these transitional zones.

Selection should prioritise proven performance in alternating moisture regimes and adequate structural density to increase surface roughness.

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: https://www.ozbreed.com.au/summary-plants-in-various-wet-feet-conditions/ and our bioswale plant palette which you can find here: https://www.ozbreed.com.au/plant-palettes/

Establishment density is equally important. Under-planting leaves soil exposed during the most vulnerable period, the first 12 to 24 months after installation. Early canopy closure significantly improves erosion resistance.

Design Detailing for Long-Term Performance

Contour-based systems must still satisfy engineering and asset management requirements. Critical considerations include:

• Confirmed soil classification and dispersiveness
• Defined swale cross-sections with stable side slopes
• Check dams or level spreaders where gradients increase
• Controlled overflow points designed to resist scour
• Clear maintenance access for sediment removal

Where local standards default to piped drainage, early collaboration with civil engineers is essential. Distributed contour systems can often reduce downstream infrastructure sizing and protect high-value assets from sediment load.

Importantly, slow-spread-sink strategies do not eliminate conventional drainage. They complement it by reducing the volume and velocity entering pipes and basins.

Lifecycle and Asset Management Considerations

Erosion repair in public open space is expensive and highly visible. Reinstating batters, replacing turf and addressing sedimentation in waterways diverts operational budgets away from proactive improvements.

Preventative contour-based design delivers measurable lifecycle benefits:

• Reduced sediment loads entering downstream infrastructure
• Protection of paths, retaining walls and utilities
• Lower frequency of reactive maintenance
• Improved plant establishment success
• Greater long-term slope stability

By reducing concentrated discharge, designers reduce the likelihood of major failure events. Distributed systems provide redundancy. If one swale experiences localised damage, the entire slope does not fail.

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.

From Rural Concept to Urban Strategy

Slow-spread-sink is often dismissed as a rural or permaculture concept that fails to move water fast enough for urban requirements.

In reality, it is a hydrological principle directly applicable to urban public realm design that complements other drainage systems while providing a certain amount of redundancy for them.

Contour-aligned swales shorten slopes, dissipate velocity and stabilise soils. When integrated with disciplined planting design and sound detailing, they transform erosion-prone reserves into resilient, functioning landscapes.

The objective is not aesthetic mimicry of rural systems. It is the application of contour logic within an engineered urban framework.

To view Ozbreed’s flood tolerance trial data, click here, and to view our bioswale plant palette and other functional palettes, click here.