While the article obliquely makes reference to this, I wonder if this is somewhat similar to the "birthday" problem in statistics. Any given person has ~1 in 365 chance of being born on a particular day. Yet, if you have 30 people in a room, there is a greater than 50 percent chance that at least one PAIR has the same birthday. Given the hundreds of cities (thousands?) in the US that may be affected by a flood, it's actually quite likely that at least one of them could have a pretty high probability of experiencing one in 500 chance of something each year. I haven't run the numbers, of course, but we're looking at Houston in isolation - it needs to be seen in the broader context of all possible cities. We're focusing on the individual probability of Houston having this improbable string of events, rather than looking at Houston only being the city that experienced this phenomenon.
Of course, the "once in 500 years flood" may also be an inaccurate probability as well, but if its not, then I'm not surprised this is happening SOMEWHERE.
Of course, the "once in 500 years flood" may also be an inaccurate probability as well, but if its not, then I'm not surprised this is happening SOMEWHERE.