How much is a green bridge?

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What is a green bridge? What is its purpose? Why might their lifecycle cost might be lower than a conventional bridge?

Major infrastructure projects have a significant impact on local society, the environment and the economy. Bridge construction can impact on natural resources and, if this not considered in their design, can be detrimental to the environment.

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Key Terms

A green bridge

I thought a green bridge was a bridge covered with grass that allowed badgers and other small animals to cross a busy motorway without getting hit by a car. Let’s be more refined here with the definition. The COST European Handbook define green bridges as a landscape bridge that connects a population. It’s an artificial structure over road, water or rail which is either vegetated or provides some wildlife function.

Part of Lower Thames Crossing Concept (courtesy of New Civil Engineer)

There are quite a few cool examples of different types of green bridges using this definition. For example there are around 50 purpose built green bridges in the Netherlands (although they call green bridges “Ecoducts”).

Lifecycle cost

There is no set definition that everyone uses for this, so for the purpose of this episode, the lifecycle cost is the total cost for design, construction, operating, maintaining, demolition, and salvage of the bridge. You can debate this definition. I’m keeping it short here as I’ll be doing a future episode on lifecycle cost.

The Lower Thames Project

The Lower Thames Crossing project will be the longest road tunnel in the UK and it has a budget of £6.3 billion. It’s a major new project around London to develop 23km of new roads including 50 new bridges and viaducts requiring 4km of tunnels that will connect Kent to the South of London to Essex to the North, incorporating 7 new green bridges.

The green bridges are primarily a safe passage for cyclists, pedestrians and horse riders, but is also required to act as a green bridge for wildlife.

A quote from the Lower Thames executive director in an article by the New Civil Engineer emphasised the need for the project to have green credentials minimising the carbon footprint, enhancing the natural environment, and providing better connectivity for the landscape, ecology and habitats.

The project to build roads also includes planting woodland, creating habitats including ponds waterways and hedgerows. There is a significant focus on in the requirements of the Lower Thames crossing to not only mitigate the environmental impact of the new infrastructure, but to provide an overall positive impact on the local environment.

Why are green bridges important?

Let’s focus on the issues associated with local wildlife habitats, and consider road bridges specifically. Roads and bridges which bisect natural habitats make it difficult to separate wildlife. Fencing off the roads and bridges is not full proof, and so animal mortalities occur. Road kill is a common term. So isolating and removing wildlife from roadways has not really been successful. Green bridges have been identified as a way of providing access to local wildlife, to provide points of safe transition.

How do you prioritise green?

There are lots of criteria to consider in what makes a bridge a green bridge. There are lots of different standards on sustainability that could be applied, from the Environmental Protection Agency and the Federal Highway Administration, to globally recognised Leadership in Energy and Environmental Design (LEED) and the World Commission on Environment and Development (WCED).

A consistent definition of sustainable development in this context is that the bridge will meet “the needs of the present without compromising the ability of future generations to meet their needs.”

So green bridges are partly about sustainability, and an interesting debate is the relative priority of sustainability criteria when developing a new green bridge. What are the priorities? A paper by Marzouk, Nough and El-Said researches a set of criteria across different standards then ranks the importance of different criteria related to the sustainability of green bridges.

They consider several bridge-specific criteria and then weight them using the Simos method – the paper defines this method, and I recommend that you read it if you can access it. I’ve produced a graphic showing the different criteria in the context of a green bridge.

Cost is only 14th on this list of criteria for a green bridge

What is the cost impact of making a bridge green?

The comparative construction cost of a green bridge and a conventional bridge for the Lublin bypass in Poland was assessed in an article by Karas in the Journal Research in Ecology.

To add the required features of a green bridge Karas estimated that the total construction cost increased by between 15 and 25%.

Interestingly Karas also references several studies which show that, despite the larger capital cost, the lifecycle cost of bridges with more sustainable features is lower. The sustainable bridges also have a lower environmental impact over their lifetime. The least expensive bridge alternative is likely to have the least associated emissions and embodied energy.

The lifecycle costs do not look into the impact of carbon avoided by increasing access for pedestrians and cyclists, or the estimated benefits of greener spaces.

There is a limited amount of information on the cost of green bridges, but Natural England found that retrofitting bridges to make them more green cost between £1.1 million to £10 million. However, it is hard to know how much of that cost was around elements to make the bridge green.

Summary

Green bridges are growing in importance, as the population becomes more aware of the impact that the built environment has on wildlife. Improving the accessibility of bridges for other modes of transport such as cycling and even horse riding increases the value of these bridges.

I will be following the progress of the Lower Thames Crossing project. Such a megaproject in London, in a world which is different to when it was originally planned. Will the willingness to pay for green bridges be greater than in pre-Covid times? Let me know what you think!