How can nature-based approaches help prevent flood disasters?

Reports estimate riverine flooding will cause up to $1.2 trillion damages in capital stock in Asia in a given year by 2050. Traditional flood protections can prevent such damages, but without the right engineering, construction, and maintenance, they can prematurely degrade. How can this be avoided? 

Dams and levees are reassuring structures. But not all of them are created equal…

There’s something reassuring about concrete. It’s sturdy, durable, and that’s likely why so many believe static, grey flood protections like dams and levees will keep them safe from flooding.   And in most cases, those structures will. But when dikes, embankments or levees are developed without the right materials, resources, or engineering expertise, these flood protections can prematurely degrade. 

I recently found this myself when I was conducting site visits across Asia, for example in the Philippines and India, where my team and I discovered several flood protections in a degraded state. There are many ways this can happen: 

• Soil, sand, or clay used in construction may not be compacted well enough, leading to holes in the core. 
• Steel or reinforced concrete sheet piles might be installed at an angle that leaves cracks and openings (see figure 2 below) or are not placed deep enough leading to holes in the toe structure, drainage, and further hollowing of the core. 
• And while concrete looks robust, placing concrete embankments too near to the river, within the dynamic zone, can lead to high flow velocities (up to 7 metres per second during floods), increasing erosion at the toe of embankments and further threatening the stability of embankments (see figure 1 below). 

Either way, while some of these structures still have a concrete cover, they may no longer serve their purpose as effective flood mitigation measures – especially not in the long term. The two main reasons for this are the location of the embankments (being too near the river) and the way they are constructed (as discussed above).

An additional problem is that to the untrained eye, these structures might still seem fit for purpose, as they have a smooth concrete cover. However, they offer a false sense of security, given that any future flood may result in the collapse of a hollow embankment, which remains undetected due to the concrete cover. 

As climate change leads to more extreme rainfall and bigger floods, I consider these structures unsustainable in both their capacity to mitigate floods and the significant amounts of steel and concrete they demand. We know these structures will have to offer even more resilience in the future, which means we need adaptable solutions. So even when those flood defences aren’t degrading, steel and concrete structures are much more difficult to adapt to future needs.

Adopting a more nature-based approach leads to solutions that are more sustainable and adaptable over time, while offering a range of other benefits. However, they come with their own challenges. In this blog, we’ll be looking at how these approaches can work in practice and where they’re most appropriate. 

The benefits of a nature-based approach to flood mitigation

With a nature-based approach, you place nature at the centre of your design process. This can come in several different forms. We’ll first explore why we will have to give our rivers more space, whether the embankment is grey or green, then look at how grey solutions can be replaced by a greener solution. And finally, we’ll look at additional measures to reduce floods. 

• Reduced flow velocity due to the river having more space
• Less erosion and scouring of the bed, improving stability of embankments
• Lower flood water levels allow for lower embankments, reducing the required materials and costs
• Improved groundwater replenishment and soil fertility around rivers

Of course, this approach demands more space and land than traditional flood defences to allow for additional channels and retention areas. However, this space is not lost, as we will explore in one of the future blogs.

Making room for the river is a nature-based approach that focuses on the riverine environment and the location of flood protections therein. But there are other components, including approaches that seek to create embankments from natural materials as much as possible. The room for the river type of approach sets the right boundary conditions that allow for a wider selection of green solutions for flood protection.

My colleagues from the Netherlands, Michel van Heereveld and Michel Tonneijck would even argue that a concrete cover for embankments is completely unnecessary and impractical in maintenance. “There are suitable alternatives that are more appropriate for green embankments, which are equally strong and much more easily adaptable and maintainable,” Michel explains. “Examples include a top layer of well-kept grass cover and/or dynamically stable rock.” 

Those more nature-based solutions are easier to expand, and offer biodiversity benefits like improved habitats for a wider array of plants, insects, and animals.

3: A solution at the root of the problem: reducing runoff to the river 

If there’s still too much water in too confined a space within the riverbed, then the best approach may be to reduce the amount of rainwater reaching the river. This can be done through approaches like reforestation, or improving rainwater infiltration upstream to minimise runoff to the river. We’ve seen several projects that demonstrate how this can be a cost-effective approach.

Preserving and restoring wetlands, along with other water retention measures, can also lower flood levels. These measures allow for lower levees downstream and avoid the expensive measures needed to create more room for the river – reducing overall flood control costs. However, we also know that creating new retention basins is often a difficult and expensive practice, but when previously connected wetlands can be reconnected with the river and act as a buffer, it can be a cost-effective solution.