News Detail
Navigating the complexities of Blue-Green Infrastructure
July 17, 2026 |
Lauren Cook, to begin, how would you define Blue-Green Infrastructure (BGI)?
At Eawag, we define BGI as nature-based solutions implemented by humans that aim to restore the natural water balance. The core objective is to shift away from the traditional model of sending water directly to the sewer; instead, we want to infiltrate it into the ground, allow it to evaporate, and retain it on-site. Practical examples include detention ponds, green roofs, swales, and renaturalised rivers.
However, the definition can get a bit fuzzy depending on who you ask. I would classify permeable pavement as blue-green infrastructure – though ecologists might disagree, since it is not necessarily a habitat for species. Conversely, my engineering colleagues would never include things like a park or a cemetery in the definition, whereas ecologists consider them ecological infrastructures or nature-based solutions. This boundary is where the complexity begins.
What is it about this complexity that fascinates you?
Precisely. It seems simple at first glance: it’s just grass and soil; how can it be that complicated? But the systems are incredibly intricate. For example, take a retention basin: a vegetated basin where water permeates into the ground. Sounds straightforward at first glance, but, for example, the soil must be engineered with precision. It needs to let the water flow through – but not so quickly that it fails to cool the environment or capture all the pollutants.
BGI aims to fulfill many different goals simultaneously: heat mitigation, biodiversity, flood management, improving water quality, erosion control, and so on. You cannot look at these in isolation; you must consider the whole system and bring together disparate actors, from ecologists to urban planners. It is about understanding how these different elements are intertwined on a system level. That challenge is exactly what draws me to this field.
Is BGI a new concept?
We have observed that we have been altering the water balance by paving cities, putting water into pipes, deteriorating water quality, and causing flooding for quite a while. However, the concept that we want to restore this water balance is roughly 30 years old, gaining significant traction around the turn of the millennium. We realized that we didn’t have to rush water out of the city as quickly as possible, but we could store it on-site and let it infiltrate to gain co-benefits like reduced loads on wastewater treatment plants and cleaner rivers.
In Switzerland, the concept has only taken hold more recently. We are behind the curve compared to other nations.
What is the reason for this delay? Is it because we have such abundant water resources that we didn't feel the urgency?
It’s not always about water scarcity. In fact, a main driver for investments in blue-green infrastructure is too much water, specifically flood water. Historically, Switzerland’s climate has been so mild that the worries about flooding or water scarcity were not major issues. The current main driver here, as I understand it, is heat. Overheating cities have been a critical topic in recent years. And that is where BGI can offer relief, even if the impact is hard to quantify precisely, since Swiss cities are already quite green compared to others.
This brings up a critical concern regarding water quality. If we infiltrate water into the ground, for instance in a detention pond, and that water is polluted, what happens to the soil?
That depends on the pollutant. If it’s nutrients, the plants and the soil will just absorb them and the plants will grow. However, heavy metals and micropollutants are a different story. We assume they stay in the soil, but much remains unknown regarding how much actually leaches into the groundwater.
The pollution must go somewhere, and we have to decide where we prefer it to go: into the rivers or into the ground? For highly polluted water, the soil and vegetation may need to be replaced periodically. This creates a fundamental conflict with the goal of biodiversity: The principle of biodiversity is to let a habitat grow and remain resilient for as long as possible, but if we have to replace the soil frequently to manage pollution, we disrupt that cycle. A pond designed for water quality could, in fact, become an ecological trap.
What do you mean by an ecological trap?
The pond will seem attractive to many species. Some will use it as a source for drinking water; others may choose it as a habitat and lay eggs. But if the water is polluted, it affects their health, their reproduction, and they might die from it. It is a classic example of conflicting goals: water retention, flood removal, pollution control, biodiversity, and heat mitigation are often in competition.
How do you optimize between these competing goals? There is no "perfect" solution that satisfies everyone.
I work a lot with decision tools called multi-criteria decision analysis (MCDA). It’s an elegant way to optimize: You assign weights to the different objectives.
When you communicate with stakeholders, you can’t ask: "Do you prefer heat mitigation or biodiversity?" Nobody can answer that rationally. People are typically biased and not fully rational when faced with complex trade-offs.
By using these tools, we aim to make the decision-making process more rational. We uncover the trade-offs so that decisions are informed. Ultimately, it comes down to the decision-makers and the people impacted:
"Do you care more about your basement being flooded or being cool on a summer day?" or "Is having cleaner water more important than having more butterflies in your city?"
There is no perfect solution, but we must ensure these difficult choices are made transparently.
What are the biggest challenges when working with municipalities to implement these solutions?
For the engineers, going from conventional “grey” infrastructures to BGI requires a major paradigm shift. They are trained to build pipes and inlets to move water through the system quickly. Asking them to change that mindset and let water infiltrate slowly on-site is a significant hurdle that requires convincing.
However, acceptance is growing. For example, we worked with the community of Fehraltorf, a small town of 30,000 people surrounded by farmland. They had never heard of BGI, but once we explained the principles, they were immediately interested. I believe acceptance of nature-based solutions is generally quite high, which is why I’ve been giving more presentations recently. The topic is gaining significant attention, especially among stakeholders.
Once implemented, how do we measure the performance of BGI elements? It seems harder to quantify than concrete structures.
That is one of the basic issues of BGI. It offers so many co-benefits, but they are notoriously hard to quantify. In traditional concrete solutions, like a dam or a retention wall, we can reliably measure exactly how much water it will hold or how much resistance it will provide.
Nature-based solutions are far less predictable. A retaining forest or a natural slope might be cheaper and more resilient, but you cannot guarantee it will perform to a specific level. There is no "strain-stress curve" for a forest. Performance varies based on soil saturation, tree types, growth stages, or storm damage. We need better metrics to know how to improve designs and identify synergies or trade-offs between different elements.
Where is future research in BGI heading?
Several critical areas are emerging. First, water scarcity. As we plant trees and vegetation, and as drought periods increase, we must ask if we need to irrigate these systems to ensure they keep performing. Simultaneously, extreme rainfall events are becoming more common. We need to build resilient systems that can survive the swing from extreme drought to extreme flood.
Spatial planning is another massive challenge. With so much demand for space and so many stakeholders, it is an extremely complicated optimization problem. We must address flooding, biodiversity, and heat all at once.
Finally, there is a crucial area of research regarding developing countries and the social science behind it. How do we realize these infrastructures in dense mega-cities in places like Uganda or India, where flooding and heat are already extreme? Do the current designs even work in those conditions, or do we need to rethink them entirely? The challenges there are much bigger, but so are the potential benefits.
Dr. Lauren Cook has a background in hydrologic engineering, energy, and climate impacts modeling. She is interested in research questions that employ stormwater and energy infrastructure to mitigate and adapt to the impacts of climate change.
Lauren Cook leads the research group on Multifunctional Blue-Green Design at Eawag’s Department of Urban Water Management. Her research focus lies on performance-based design of blue-green infrastructure.
Blue-green infrastructure
Blue-green infrastructure (BGI) is an integrated, nature-based approach to urban planning. The concept combines the natural water cycle (water bodies, infiltration areas) with green elements such as green spaces, green roofs and trees, in order to manage water sustainably and promote biodiversity. In contrast to traditional, grey infrastructure (concrete channels and pipes), BGI utilises natural processes to absorb, store and filter rainwater. This reduces the risk of flooding, improves water quality and, at the same time, cools the city. Researchers at Eawag are conducting interdisciplinary projects to investigate how effective blue-green infrastructure can make our cities more climate-resilient and liveable.
Cover picture: Lauren Cook in Glattpark, Opfikon (ZH). Various BGI elements have been implemented in the park – such as an artificial lake that serves as a retention basin. (Photo: Julian Salinas/ETH Board)