What should be done to stop floods in metro stations

Klaus Hans Jacob, a geophysicist and flood expert who analyzed New York’s subway system before and after 2012’s Hurricane Sandy, discusses the continued – and growing – flood risk of coastal underground transportation systems and what planners can do to prepare and protect themselves.

Senator Chuck Schumer is calling for climate action after the Ida flood in Chelsea in September. (Photo by Ron Adar/Shutterstock)

Are cases of major flooding in the metro increasing? And if so, why ?

In New York City, in the last month or so, we’ve had three subway floods — first due to a heavy downpour, then Tropical Storm Henri, and now Hurricane Ida. Meanwhile, we’ve seen similar flooding in cities across America and the world.

I think the message should be pretty clear by now: climate change is not about the future; its effects are happening right now. Warmer oceans mean more humidity in the atmosphere, and as that humidity meets cold air, it all trickles down to cities like the proverbial cats and dogs.

This is not necessarily a problem only for coastal cities. Ida, for example, wreaked havoc throughout the eastern interior of the United States. But, of course, many large metros – from London to Amsterdam, from Marseilles to New York – have been built alongside large rivers or on the coast. This makes them vulnerable to excess water from rising tides or heavy rains. In the last case in New York, it was from above, but the Sandy flood came from a coastal surge.

How does the age of some of these metro systems affect the risk of flooding?

When the subway was built in New York starting in 1904, no one thought of rising sea levels or torrential rains. And so the fundamental design of the underground system did not take these phenomena into account.

We know better now. Over the past 20 years, it has become clear that more severe storms are an inevitable consequence of human-induced climate change.

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But despite decades of addressing it, we are still in reactive rather than proactive mode. Essentially, city officials are cleaning up the damage after the storm, rather than taking action such as moving infrastructure or protecting it.

So what can cities do to better protect aging metro systems?

In the case of older subway systems, it cannot reasonably be expected that they will be displaced in the next few decades. Instead, we need to fix them.

As strange as it may sound, the water itself is not the problem. Rather, it’s a mismatch between the amount of precipitation we see and the location of openings in our subway systems – not just where people enter and leave, but also the ventilation grilles where air enters. and exits and where the electrical cables enter. the system. All of these openings allow water to flow out of the streets and into the subway.

These are known engineering issues that can be resolved. In New York, the Metropolitan Transportation Authority has solved much of the problem caused by coastal storm surges by installing features such as gates and barriers – some permanently installed, others that must be inserted before the water does not appear. These prevent water from entering the subway system. When operating as intended, they can result in a 98% reduction in coastal flooding potential, according to my calculations.

But these measures work for coastal flooding. The issue we found on September 1, 2021 was the result of street runoff entering the system. With coastal storm surges, water only enters the subway system at low levels – perhaps at entrances a few feet above sea level. With rain, even at elevations higher in a city, subways can flood.

How do you solve this problem of street water runoff?

You need to approach it in two ways: prevent street flooding in the first place and protect subway entrances.

Street flooding can be avoided by increasing the capacity of gutters and the sewer system to absorb street runoff. This can be done by widening or adding new gutters, but also by having larger diameter sewer pipes in the roads.

And then you can make the soil more absorbent by planting more trees in the streets and putting permeable surfaces. For example, instead of concrete parking lots, put in gravel, which is a permeable surface that allows the ground to absorb water.

Individual homeowners can, if they have a flat or near-flat roof, put gardens on their roofs rather than having gutters. Green roofs can absorb water falling from the sky; and cesspools – devices that collect rainwater – then release that water slowly over the days, for each house; they can help ensure that sewage systems do not become overwhelmed. These measures work best in areas with lots of single family homes.

It is cheaper to fix the problem before extreme events than to have to fix the problem after the damage is done. Klaus Hans-Jacob, Colombia University

Garbage on the streets can amplify the problem by clogging drainage, but that’s not the systemic problem. It only makes a bad situation worse.

When it comes to protecting existing subway entrances, you can build berms – mini dikes or raised embankments – of several feet at each entrance. This makes it more difficult for people with disabilities, so you also have to modify elevators to get people down.

All it takes is good engineering – there’s no mystery. Well, it’s engineering, political will and money.

Do we see this engineering in new metro systems?

These are not new problems; the fact that water flows downwards has been known since the beginning of mankind. But the new underground systems handle this better. Tokyo takes care of the floods, Taipei the same. They have had problems in the past but are quicker to adapt. For example, transport officials in Tokyo have installed sliding doors in underground passages that can withstand the pressure of 15m-deep floods.

Newer subway systems also tend to have entrances at high points relative to their surroundings. The key is not to let water pool near entrances in the first place – so don’t place subway entrances near low points on a street.

You talked about political will and money…

It’s not cheap. Effectively protecting a city’s subway system from flooding costs tens of billions of dollars. But it is cheaper to fix the problem before extreme events than to have to fix the problem after the damage is done.

Unfortunately, the current trillion-dollar infrastructure bill going through Congress has a grossly insufficient amount for subways — a far greater portion, about $110 billion, goes to bridges and roads than modes of transportation. public, which are expected to receive approximately $39 billion.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bonny J. Streater