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A rain garden in Elmwood Park, Philadelphia

Green versus grey infrastructure – what's best?

Senior project engineer Kathryn DePippo weighs up the costs and benefits of investing in green and grey infrastructure to reduce flooding and pollution from overflowing sewers.

After many years’ absence the Monarch butterfly has made a comeback in Queens, New York’s largest borough, to the delight of the people who live there. Making urban spaces more wildlife-friendly is one of the co-benefits of the city’s ongoing investment in green infrastructure to reduce pollution from combined sewer overflows (CSOs).

Like other cities in the developed world, New York’s ageing infrastructure is heavily reliant on combined sewers that convey both sewage and surface water. Heavy rain or melting snow can cause these combined sewers to overflow, resulting in untreated sewage flowing into rivers.

Increasingly stringent federal regulations to improve water quality, and the expectation of more frequent and severe flood events as a result of climate change, have galvanised cities across the US to take concerted action to increase resilience and reduce CSO volumes.

Faced with the high cost of upgrading or building conventional infrastructure, several large US cities are turning to green infrastructure, a collective term for a wide variety of sustainable urban drainage systems, as part of integrated management solutions to collect and treat stormwater run-off at source.

'Cookie cutter’ technologies

New York is renewing existing infrastructure, such as expanding the wet-weather capacity of wastewater treatment plants, but it is also implementing green infrastructure on a broad scale after analysis revealed that approximately US$1.5bn could be saved compared to investing in only grey infrastructure upgrades.

The city is capturing rainfall from impervious surfaces in CSO areas using a range of ‘cookie cutter’ technologies that can be retrofitted relatively quickly on roadways, sidewalks and other public property. To date it has installed thousands of bioswales (rain gardens), planters, tree trenches, pervious pavements and other stormwater collection devices.

The primary goal of the programme is to improve water quality by reducing CSOs, which will decrease bacterial levels and increase dissolved oxygen in watercourses, and in a cost-effective way. It will also deliver community and environmental benefits: more urban green spaces, more attractive streetscapes and reduction of the urban heat island effect, which in turn will improve the quality of life for residents and create more habitats for birds and pollinators.

Overflows in New York have been greatly reduced but a number of challenges have prevented the city from reaching its goal of improving CSO capture rate to over 75%.

These include population and infrastructure density limiting the available space within rights-of-way to locate these systems, siting constraints due to public transport clearance requirements, underground utility conflicts and unfavorable soil conditions.

Improving community cohesion

In 2011 Philadelphia implemented its Green City, Clean Waters programme to use mostly green infrastructure technologies to reduce stormwater pollution entering watercourses by 85%.

Even though Philadelphia is currently on track to achieve its CSO reduction target, the city has encountered challenges similar to those in New York, notably, co-ordination and approval with utility agencies.

But it is already reaping the environmental co-benefits as the programme is supporting wider efforts to redevelop Philadelphia’s parks and playgrounds, creating more green spaces to support community cohesion, education and play.

The approach in Cleveland is to introduce a handful of large-scale systems – wetlands, infiltration basins, underground storage facilities – that manage millions of litres of water on an annual basis.

But with an ambitious target to cut overflows from 80 to just four per year, equivalent to 98% capture of all combined sewage, Cleveland has found it still needs to invest heavily in grey infrastructure and is planning to build no less than seven large-scale storage tunnels.

The triple bottom line

Whether you’re a public authority or a private utility, how do you decide whether to opt for green or grey, or both? This requires cost-benefit analysis to evaluate the triple bottom line (financial, social and environmental) impacts of alternative green and grey infrastructure projects.

This will build a complete picture of a project in terms of engineering and lifecycle costs, and positive and negative impacts on the local community, demonstrating the difference in net benefits between green and traditional infrastructure solutions and thus informing investment decisions.

The simple answer is that there is a need for both green and grey solutions.

For certain cities we can see that an integrated approach can be the most cost-effective option in improving water quality standards and increasing the resilience of infrastructure in the face of changing weather patterns.

What combination of green and grey a city finds to be the optimum one depends on a range of factors: state of existing infrastructure, urban development plans, regulatory pressures, available open space and community needs.

For cities in the US, the main goal of reducing CSOs is more to improve water quality, driven by legislation to reduce discharges, and less to reduce the risk of flooding. In the UK, by contrast, towns and cities are more focused on reducing disruptive flooding.

Advantages and disadvantages

Although retrofit sustainable urban drainage systems are being successfully implemented at a large scale across the US to enhance the performance of existing infrastructure, it is important to understand their limitations.

They can be very effective at preventing overflows during smaller storms, controlling up to 2.5cm of run-off.

For depths above that caused by more extreme weather events, traditional infrastructure remains critically important for hydraulic control.

Perhaps the apogee of this approach is Thames Tideway in London, a 25km ‘super sewer’ currently being constructed under the River Thames. (In parts of London, permeable paving and storage structures are also being retrofitted to attenuate surface water flows during rainfall events.)

In addition, the lifecycle costs of going green must be considered: thousands of individual capture systems installed across a city will require a significant maintenance effort.

On the other hand, the green approach can provide a wide array of important environmental benefits that cannot generally be achieved through traditional methods.

It could also help to improve social equity because climate challenges such as the heat island effect, stormwater run-off and riverine flooding often have the most impact on the poorest residential areas of a city.

Another advantage is that green infrastructure allows for improved trust and awareness between those organisations who own and operate water networks and their stakeholders and consumers.

Nearly all water and wastewater infrastructure is underground, out of sight and out of mind. Green features to capture CSOs are nearly all above ground, so bill-payers can see service providers are taking action to clean up their home cities and tackle climate change, and, importantly, how they are spending their money.

This article is based on a paper Kathryn DePippo has presented at major water industry conferences including WEFTEC, the largest annual water quality conference and exhibition in the world.

Kathryn DePippo

Senior project engineer

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