Better design thinking to battle floods

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While the devastation brought by Hurricane Harvey grabbed international attention, other U.S. cities began to feel the pangs of nervousness as many of them are becoming more vulnerable to fiercer storms. In fact, New York City is already reviewing its plans for a massive rainstorm as the existing infrastructure cannot handle more than 1.75 inches of rainfall per hour.

High and fast water rising in Bayou River with downtown Houston in background under cloud blue sky. Heavy rains from Hurricane Harvey caused many flooded areas in greater Houston area.

In the U.S., the National Climate Assessment has observed above-average rainfall since 1991 with the greatest increase in the Northeast, Midwest and upper Great Plains regions, which have seen a rise of over 30 percent compared to pre-1960 levels.

Going by recent reports, floods cause greater property damage and more deaths than tornadoes or hurricanes. And Houston’s flood was truly a disaster of proportions: the sky unloaded nine trillion gallons of water on the city within two days!

With cities continuing to grow upwards and outwards, the effects of heavy rainfall been compounded. “The more impermeable surfaces you have, the more you obstruct natural runoff,” says Franz Jenowein, JLL’s Director of Global Sustainability Research. “This translates to greater potential for excess water to negatively affect cities.”

It is equally a design problem as much as it is climate change. In the face of such natural calamities, better urban planning and city design can come to the rescue.

Build to suit?

Urbanization has been blamed for many modern day environmental disasters. However, there can be an exception. “Urbanization can be a problem—but it can also offer solutions,” says Jenowein, citing Singapore as a case in point.

“Singapore city is a great example of how you can take excess water and use it to your advantage.” The small island state uses advanced drainage infrastructure to collect and recycle water, and sensors to monitor water levels across the city in real time, he explains.

Another case in point is that of China, which is battling the twin challenge of rapid city growth and extreme weather by adopting a new tactic: turning its cities into giant sponges.

About 30 pilot cities in China are trying to trap and hold more water to deal with such problems as flooding, drought, extreme heat and pollution.

The country relies on a range of innovations, such as green roofs on buildings and more urban wetlands. It is already being hailed as a bold step to solve some of the environmental problems plaguing the world’s most populous country.

Meanwhile, in coastal cities that are accustomed to heavy rainfall, local authorities are beginning to wake up to the emerging challenges of flooding through urban planning and resilience strategies. Last year Norfolk, Virginia was awarded a government grant of $120 million to implement a suite of innovations, including underground cisterns, permeable pavers and upgraded storm drains, as well as the creation of urban forests and riverfront wetland areas.

In Berlin, which in summer 2017 saw its heaviest rainfall in a century, green spaces with deep soil and swale trenches are being used to turn the city into a sponge to help cope with increased rainfall. As the water evaporates, it also cools the urban environment.

Retrofitting a solution

Individual buildings can be critical blocks to begin with. “Installing features like backflow valves in plumbing and drainage systems and elevated podiums for parking spaces to protect vehicles, as well as locating critical infrastructure away from areas prone to flooding, are all important ways buildings can mitigate flood risks and protect against damage,” says Jenowein.

Further, engineered designs should include drainage and inlet and outlet locations along with elevations. Overflow outlets are needed to prevent flooding.

Green roofs too soak up significant amounts of rainwater, though this puts additional demand on a building’s load-bearing capacity. For flood mitigation, a blue roof is more effective and efficient. “Blue roofs are basically plastic sponges that retain water on flat roofs,” explains Jenowein.

In combination with a retention tank, the system harvests rain for landscaping, irrigation or greywater and can relieve pressure on drainage infrastructure by slowing the flow of water. “Blue roofs are two to three times cheaper per square foot than green roofs, and have more water retention capability,” he describes.

While making a new construction, underground utilities can be placed around and even through suspended pavement systems. However, all underground utilities should be protected from water and root penetration.

Though there can be several solutions, one of the biggest challenges for cities is retrofitting them into existing infrastructure. And while new building developments have an opportunity to integrate mitigation measures from the start, they are also constrained by the surrounding urban environment.

Urbanization being a never-ending phenomenon, impervious surfaces dominating urban cores, existing stormwater and sewer systems are often inadequate to handle peak flows. To reduce pressure on existing systems and increase capacity, cities must consider every available option to help manage stormwater, and hence, flooding.

That said, the climate has changed, so should architecture.


About the author:

Franz Jenowein is a director within JLL’s Global Research team. He is responsible for its sustainable real estate research program publishing analyses on Eco-cities, climate change, flood risk, resilient cities, smart buildings, workplace health and productivity, green districts and sustainability transparency. Franz has a wide experience in carbon management, operational building energy and carbon performance benchmarking and energy efficiency management across European and global real estate investor and occupier portfolios.

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