Future perspectives - Give water somewhere else to go
Nov 08, 2023
Extreme rainfall, flooded streets, destroyed houses and infrastructure, people fighting for their lives amidst the floods. 2021 was also marked by such tragic events. It´s time to ask how flood disasters happen, and what can be done about them.
Water – curse and blessing
Chronic sea level rise is a particular concern to low-lying coastal cities, which has led many to invest in protective infrastructure – building seawalls, raising roads, creating artificial barriers, dikes and reefs. These ‘hard’ structures are so often considered the go-to in flood defence technology that in 2017, 14 percent of the total U.S. coastline had been armored in this manner. That percentage continues to grow. But seawalls and barriers are more of a blunt instrument than a finely-tuned solution. They’re expensive to build and require constant, vigilant maintenance. And their ecological impact is stark – countless coastal habitats have been destroyed or displaced to make space for a seawall. They’re also not a truly permanent solution. In cities like Miami, where sea level rise is happening especially quickly, waterfront residents are being asked to make their existing seawalls taller, at a significant financial cost. And that’s just one part of an ambitious state-wide effort to keep water inundation at bay, or at least, to minimize its impact on the city. Of course, the sea is not the only potential source of floodwater in urban areas. Climate change-induced extreme rainfall events are becoming more common in many parts of the world, bringing huge quantities of water into the densely-populated heart of the metropolis, and to their rivers, canals and lakes. The effects of these changing weather patterns are already being felt. Unpublished research from the Potsdam Institute for Climate Impact Research suggests that along the Danube, so-called ‘50-year floods’ now happen twice as often. And a 2018 paper in the journal Environmental Research Letters says that, even with relatively minor changes to the climate, rivers in the majority (264 of 571) of European cities will see a significant increase in their ‘10 year high flow mark’. “It’s important to remember that there are different types of flood, and they all bring their own challenges,” says Prof. Dr. Hannah Cloke, a physical geographer, natural hazards researcher and hydrologist at the UK’s University of Reading. “We know from our climate models that many of them are going to get worse with climate change. But what’s very clear is that our cities are not adequately prepared for floods happening now, let alone what we think is going to happen in the future.”
EFAS sent their first bulletin to the relevant national agencies on Saturday, July 10. The data continued to roll in, and the forecast only became more worrying. Independently, the German weather service forecast that more than 200 millimeters of rain would fall on some areas in less than 48 hours. The bulletins were upgraded to warnings; issued to ensure that authorities could get people to safety before the inevitable floods began. Despite this, the events of those days would go on to claim hundreds of lives.
Flood protection is complex
There’s no easy solution to this. Established cities like London and New Orleans can’t be rebuilt from scratch, or picked up and moved elsewhere. Their critical infrastructure is woven into the landscape. Their people too. Rerouting and uprooting are not a practical option. But both Cloke and Tortajada agree that we could be doing much better when it comes to planning new buildings and developments, and retrofitting existing structures. “The first thing we must do is acknowledge that the risk exists, and then design our systems based on that risk,” says Tortajada. “Resilience is less about flood control and more about flood management.” Cloke concurs, saying, “We’re going to see these heavy rainfalls again and again in our cities, so we have to think slightly differently about how they’re built. When you assume something is going to flood, that changes its design parameters.” A challenge facing all major cities is the amount of impervious surfaces used in their construction. Chosen for their robustness, concrete and asphalt are everywhere in our urban centres; in roads and parking lots, footpaths and roofs. Rain falling on these surfaces can’t penetrate into them. With nowhere else to go, it flows along them, before entering a culvert or gutter that eventually directs the water into the municipal drains. In some cities, there are dedicated stormwater systems to manage this runoff, carrying it directly to rivers or streams without treatment. Other urban areas – especially those that grew rapidly during the Industrial Revolution – tend to rely on combined sewers. They collect both runoff and wastewater (from domestic, commercial and sometimes, industrial sources) within the same system of pipes and tunnels. This waste is then carried to a treatment plant to be processed until it meets strict environmental standards, before being discharged into a nearby body of water. At least, that’s how they work most of the time. The problem arises when the volume of wastewater that enters a combined sewer system exceeds its capacity, as in times of heavy rain or snowmelt. When that happens, wastewater skips the treatment plant, and is instead discharged directly to nearby streams or waterways, through relief points (known as combined sewer overflows, or CSOs). Numerous cities are working to upgrade their sewers, and reduce the frequency and volume of discharges entering and polluting their precious waterways. London’s approach has been to construct a 25 km long super sewer, the Thames Tideway tunnel, using three Herrenknecht tunnel boring machines (TBMs). The project, which started in 2016, will eventually intercept, store and divert millions of cubic meters of this polluting overflow, reducing the number of discharges from an average of 60 per year, to just four. In Washington D.C., CSOs discharge more than 11 billion liters of sewage into Rock Creek and the Anacostia and Potomac Rivers annually. The city is investing in a similar solution – three tunnels, bored with Herrenknecht TBMs. A fourth tunnel is already being planned. When completed, this network is expected to prevent 98 percent of all overflow into the rivers.
A variety of solutions
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