this massive inflatable ball was designed to plug a New York City subway tunnel, and it's just one of the many options New York's Metr Transit Authority has explored to try and keep the subway and its dense network of porous tunnels dry after Hurricane Sandy. The MTA spent about 7.6 billion, and yet this is New York Subway last year and the year before, and the year before that. The reason? Well, it turns out that hurricanes aren't the only thing keeping the subway soaked. Rainfall flooding poses a different kind of flood risk. So now, Engineers like the MTA's Eric Wilson are looking to invest up to an additional billion dollars into a whole new raft of measures to try and keep some 4 million daily Subway Riders moving. But New York's Subway is old, densely populated, and right on the coast. If you can stop water from getting in here, you could potentially flood-proof any underground Network in the world. Even when it's not raining, keeping the subway dry enough to be functional is a huge challenge that's partly

because like many other metros around the world, some of the Subways tunnels go under the water table, a level where groundwater organically sits under New York. If you've ever been on the New York City subway, you'll sometimes see water in the tracks, and that's often because of groundwater that's seeping into those subway tunnels. There's so much groundwater down there that a network of 254 pumps pump out 10 million gallons of mostly groundwater every single day. It goes into a pump room um that moves that water up back into the city sewer before it's released into um New York City Harbor, and it's that very location on the Harbor that makes keeping water out even more challenging. New York City is a coastal city, and the exposure is the perimeter of the city rather than just a single river through the city's core. Since the subway was built in 1904, sea levels around Battery Park here have risen by about a foot, and while Subways around the world World face issues of

storm surges, especially in Asia where tsunamis are prevalent. Their networks are often newer and were designed with flooding and storm surges in mind. Outside of Tokyo, for example, the Japanese government has constructed this enormous metropolitan area outer underground discharge channel designed to capture flood waters and store them until the drainage system is able to handle them. But in New York, the subway system was designed at a time when it didn't need to hold this level of storm water. The potential flaws in the system were laid bare in October 2012 when Hurricane Sandy hit New York. A 14t storm surge left 44 people dead and caused around $19 billion in damages and lost economic activity. The fortifications at the time were pretty rudimentary, including sort of sandbags, and plywood fences. You know, the subway system was never constructed to withstand the magnitude of surge um that we experienced during Sandy. The event really served as a wakeup call to all of us to try and make the subway hurricane-proof. The MTA invested $7.6

billion into a range of fixers across the subway roads and tunnels. At first, a wide range of options were considered, like the inflatable tunnel plug, which the MTA decided not to go with as it wasn't as reliable or maintainable as it needed to be. Instead, The Authority chose to pursue other methods like these flood locks, which have been installed at around 50 stations across the network. MTA staff will go and assemble these flood logs one on top of the other, and the protection that these provide will hold back the storm surge. A similar system is also used in Paris, where around 70 stations along the banks of the river Seine are at risk of flooding. There, they take things a step further and actually try and raise the banks of the river itself, something that's much harder for New York to do. So in other places, the MTA has invested in storm doors as opposed to flood logs. One person can easily close the door and bolt it into position. The MTA says that these have been installed at around

150 stations, but not all of the entrances to Subways are vertical doorways. Many are horizontal, and that's where a device called a flex gate can come in. The flex gate lives in a box. A worker will retract the flex gate along this track that coincides with the entrance of the subway. The MTA said Flex Gates have been installed at around 75 live stations, and it isn't aware of any other Subway network using this system. But one of the hardest spaces to protect from Storm surges isn't the Subways entrances; it's its ventilation system, the greatest you see on the sidewalks. The New York City subway system was designed to have natural ventilation. When you're walking above these, you'll get a blast of air. So, in the coastal storm surge risk area, some 2,230 vents have this mechanical closure device or MCD installed. This element is a door that opens up and creates a seal across the subway vent. All of these measures are Deployable rather than passive, which means that people have to

physically activate the measures a few hours ahead of a flood, but it does require a full system shut down. Nobody can move in and out. The problem is not all storm events are worthy of shutting the entire network down, and so when tropical storm ID hitting in 2021, the Subway still got soaked. It exposed the subway system to the dangers of Inland flooding events. The MTA says the storm B more than 7 Ines of rain, smashing records for the most rainfall ever to hit New York in an hour and causing $128 million of damage to MTA systems. But it also taught the MTA that its storm surge Protection Systems wouldn't save the network from rainwater flooding. So here we have a map of where torrential rain is forecast to impact across New York City, and you'll note that this is quite a different map. With torrential rain, floods can occur throughout the city. To reduce the risk of flooding in those areas, the MTA needed a passive system that would require no advanced

notice to be deployed, like raising the entrances to some of the subway stations by a few inches. They prevent dorm water that is pooling on the street from overtopping the curb and then traveling down into the subway station. The MTA says it's raising the entrance to numerous stations as a proactive measure, and the strategy isn't new. It can also be found in Taipei where entrances are raised around 2 feet to avoid tsunami flooding. The MTA has used that same tactic on the ventilation grates around 19 different stations, and just this element of height is sufficient to protect these Subway vents. But New York sidewalks can be notoriously crowded. Elevating vents isn't always an option, so in some locations, the MTA is deploying those same mechanical closure devices, except now they automatically begin to close up when water is present. To implement these measures and others across the network, the MTA is seeking an additional $6 billion on top of the 7.6 already spent on post-Sandy measures. And even one minor break in these measures could

allow water into the system, but the MTA says it's not trying to stop all water from getting in. If water does get into the system, the strategy is to get it out into the city's sewer system. But as sea levels rise and storms worsen, these measures will only protect the network for so long. The image on the left shows the projected storm surge over Manhattan's Whitehall station today at 4 feet high. By 2080, it's set to be double that depth at over 9 feet. And while the MTA says it hopes that the measures it's installing will provide protections for the next century, it's also looking to outside players above ground to help make a difference. There are several parallel initiatives underway to protect the city. In Manhattan, local projects are looking to elevate the areas here and here. Also underway is a study that would look at a series of larger protections across New York City Harbor. Another tactic is to install permeable paving and rain gardens instead of concreting over large surfaces where water could be.

absorbed something that's currently being deployed in London, and one of the simplest measures the city can take is to keep the drains clear. However, of the initial $7.6 billion allocated to post-Sandy resilience, the MTA says only $4.8 billion worth of projects have been completed, with some 1.2 billion still in the design or planning phases. To make matters worse, the recent scrapping of New York's congestion charge plan has left a $16 billion hole in the MTA's budget. So, will the MTA truly be able to manage flooding on the network? I think the proof will be in the pudding. The extent to which all these tools are working behind the scenes to prevent those service impacts, that will really be the sign of success.

Why $7.6B of Hurricane-Proofing Didn’t Stop This Subway From Flooding | WSJ Pro Perfected

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