Pervious surfaces

Permeable driveway. Photo by Stormwater 360.

Pervious or permeable surfaces, unsealing, depaving, and porous paving are all methods of creating urban surfaces that allow water to pass through them and into the soil below. When rain hits hard, impermeable surfaces, it has nowhere to go, so gathers and flows in momentum until it can be dispersed typically through pipe infrastructure. Permeable surfaces allow water to be filtered into the soil at the source. This is a simple solution that significantly reduces demand on stormwater infrastructure, can reduce flooding risk, and is especially useful in extreme weather events. Here are common strategies to increase ground permeability:

‘Pervious or permeable surfaces’ are both words used to describe a surface’s ability to be penetrated by water. 

‘Unsealing’ refers to leaving surfaces in an unsealed condition, or removing impervious caps (unsealing), so that they remain or again become permeable.

‘Depaving’ is the process of removing impermeable surfaces like concrete to replace them with permeable surfaces or green infrastructure.

Porous paving’ is a paved surface that allows water to pass through it. It can be used as a replacement for traditional impervious surfaces in an urban context. The paving allows runoff to infiltrate into the underlying base course where it is temporarily stored and slowly released either into the subgrade or underdrain (Crossland et al., 2016). 

Some other strategies for increasing the permeability of urban landscapes are:

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Name of NbS

Pervious surfaces

Type of NbS

Hybrid living/engineered interventions

Location

  • urban
  • periurban
Permeable pavements come in different shapes and styles. Photos by Pavecraft.

Relationship to Indigenous knowledge

While this nature-based solution doesn’t specifically integrate Indigenous knowledge systems, it does address the issue of cities prioritising hard surfaces over soil and vegetation. 

As cities expand, people have distanced themselves from Papatūānuku (the living earth) with concrete and other impermeable surfaces. A crucial aspect of our future involves re-establishing connections between built and natural environments, utilising nature to help mitigate the impacts of climate change.

Interestingly, one of the most effective methods for reducing disaster risks, is to literally turn to the earth and soil for assistance. Reintroducing soil and vegetation into our urban areas is essential. By incorporating permeable surfaces, we can reforge links between people and cities with the natural ground state. In doing so, we have the opportunity to embrace Indigenous worldviews and knowledge systems, enhancing urban resilience against extreme weather events and creating safer, more sustainable communities.

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Climate change benefits
  • Changes in rainfall
  • Coastal inundation
  • Increased temperatures
  • Sea level rise
  • Storm surge 
  • Urban heat island effect

Stormwater management is vital in Te Moananui Oceania nations experiencing an increase in extreme weather events due to climate change. Te Moananui Oceania is on the front lines of climate change, already experiencing more intense cyclones, rain storms, and loss of coastal infrastructure. The Secretariat of the Pacific Environmental Programme (SPREP) notes that in the last decade, there have been three times more weather-related natural catastrophes – mostly floods – than 60 years ago. 

Without intervention, increased rainfall induced flooding will cause damage to island nation cities across the Pacific. Flooding of lowland and coastal areas causes erosion, damaging coastal infrastructure, and allows contaminated water to flow into residential areas, and oceans. This causes an increase in water-bourne diseases (Ronneberg, 2008), and damage to reefs. In cities, overloaded stormwater systems cause significant damage – proven in the effects of 2023’s Cyclone Gabrielle, which caused significant damage to homes and infrastructure, and killed 11 people in Aotearoa New Zealand.

In cities, replacing existing impermeable surfaces with permeable ones is a simple and effective way to assist stormwater systems in dealing with increased rainfall. In island nations, permeable surfaces can also be used to adress storm surges and coastal inundation in part. 

Future development in Te Moananui Oceania must consider both the built and natural environments together, including water-sensitive design right from the beginning.  Stormwater management makes our cities more resilient and contributes to livability, connectivity and sustainability.

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Societal / socio-cultural benefits
  • Climate change adaptation
  • Disaster risk reduction

The incorporation of porous surfaces in urban landscapes fosters a multitude of social and cultural benefits. Primarily, the adoption of porous surfaces supports climate resilience by mitigating urban heat island effects and managing stormwater runoff, safeguarding both infrastructure and ecosystems. 

Pourous surfaces facilitate community interaction by providing inviting spaces for outdoor activities and social gatherings, nurturing a sense of belonging among residents. The integration of green spaces through permeable surfaces promotes physical activity and mental well-being, contributing to overall healthier lifestyles. Culturally, porous surfaces can be designed to reflect and preserve local heritage, celebrating Indigenous landscapes or historical landmarks, thereby fostering pride and identity within communities.

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Ecological and biodiversity benefits
  • Climate regulation
  • Disturbance prevention

Extreme flooding negatively impacts ecologies. Flood water and increased runoff from impervious surfaces cause dangerous floods, severe erosion, diminished recharge of groundwater, and degraded habitat for fisheries (Selbig, 2019). Urban water runoff transports pollutants, sediment, bacteria, and pesticides which can be harmful to terrestrial and water ecoogies.

Managing stormwater through nature-based solutions contributes to disturbance prevention; rain gardens and swales remove pollutants and naturally filter water, and wetlands hold stormwater for examples.

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Technical requirements

Porous paving is relatively simple to lay on a small scale. The typical construction sequence for laying permeable pavement is, as per the Auckland Council’s Permeable Pavement Construction Guide (Auckland Council, 2024):

  1. Prepare the ground including erosion and sediment control measures.
  2. Lay liners or drains as needed
  3. Fit the geotextile
  4. Place the basecourse
  5. Lay clean, bedding material over basecourse
  6. Lay pavers

Porous paving needs to be cleaned and inspected every two years (Auckland Design Manual, 2024) to clear any sediment or build-up, especially after extreme weather events. If maintenance is carried out correctly, the paving should have a similar life span to impermeable alternatives like concrete. 

Other methods of creating permeable surfaces have differing technical requirements, depending on their location, purpose and existing condition. Stormwater solutions must consider existing pipework, stormwater solutions, catchments, flood plains etc.

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Garden streets, designed to create permeable surfaces in a new neighbourhood development. Image by Auckland Design Manual.

Issues and Barriers

The Auckland Design Manual (2024) notes some minor challenges for the implementation of porous pavement. Some Oceanic nations have low permeability soils, which may require under-drains. Pavers may move on sloped areas if not correctly constructed. This can be remedied with herringbone patterns or pervious strips.

Opportunities

There is a significant opportunity in Te Moananaui Oceania to replace impervious surfaces with natural, permeable surfaces, in order to improve stormwater resilience. This is a strategy that can be implemented on small scales, or in large urban contexts. As climate change continues Oceanic nations must include water-sensitive design in all urban contexts.

Financial case

Conventional stormwater systems are designed to move stormwater as quickly as possible away from urban areas to the nearest body of water typically, which involves significant infrastructure such as piping, underground drains, and paved channels. They typically have higher labour and maintenance costs (Crile, Moore & Venu, 2014). Implementation of nature-based solutions that increase permeability requires less piping and infrastructure to be built or maintained. 

Nature-based solutions allow for savings during construction phases because there is less cost for site grading and preparation (Crile, Moore & Venu, 2014). Additionally, if permeable surfaces can stop or reduce the negative impacts of extreme weather, there are savings in remediation and clean up.

Typical porous paving layers. Image by Melbourne Water.
Permeable pavement systems. Image by Fairfax County
References

Further resources: