But other resiliency changes on which the M.T.A. spent big are not fixing the city’s current threat of heavy rain; the Sandy-era fabric gate for the A-train tunnel has never been used outside of drills, and that kind of technology isn’t very effective at closing subway openings off from quick, shifting cloudbursts. M.T.A. leaders talk often about “multihazards,” how a host of climate horrors will shape the subway. This summer, some of the city’s flash floods occurred after extreme heat or days of wildfire smoke from Canada that hovered uneasily over New York. Those forces test the subway, too. Covering sidewalk vents with heavy mats may keep water out, but should be “temporary,” Wilson says, while people like him work toward more complicated architectural solutions. Covered vents mean less circulation — and more heat.
I think there are also cities with underground transport systems which are at extreme risk from flash floods, even more than Valencia/Spain.
You might remember these pictures from Henan, China. This is possible not only in China.
For example Munich. Munich is crossed by the Isar river, a rapid mountain river in a narrow valley which is in turn embedded in the Alpine Foreland or Molasse basin, a landscape shaped by the end of an ice age, with wide valleys which could collect considerable amounts of rain. In high waters that occur perhaps every decade, it can look like that. The subway of Munich crosses the river in tunnels which are many floors below the surface, I’d gues some 30 to 40 meters. And it has entrances close to the river.
In case of a large once-in-a-century flash flood, I think this is a really dangerous initial situation. (But be careful and use your own critical thinking - I am no specialist in this).
We have the general problem that we are largely not prepared for such situations, caused by weather conditions which were almost unthinkable as little as 15 years ago.