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Why Harvey’s devastation is so severe

HOUSTON -- We saw it coming, but couldn't quite take it in. And it will be days more before we really know the full extent of what Harvey has done. We are watching an enormous disaster in what feels like slow motion.

Given rains of the magnitude we are seeing, and the existence of a major metropolitan area in Houston's location, this would have been a horrific event no matter what. But actions can always make a difference in just how bad a disaster gets, so it's worth looking at what led up to this event, in both the short and long term.

Harvey first gained the attention of meteorologists and Texans last Wednesday. But the risk has been there for decades, at least, in the development patterns of the city. In neither case were the warning signs enough to prevent the disaster, though for very different reasons.

The National Hurricane Center (NHC) first predicted that the storm, newly reformed in the Gulf of Mexico after having dissipated a few days earlier, had the potential to become a tropical storm threatening Texas. On Thursday Harvey began to make national news, as it quickly strengthened and new forecasts predicted that it could make landfall as a Category 3 hurricane, the first to strike the US since 2005.

The most immediate hazard was to the coastline west of Houston, from intense winds and storm surge. But the forecast tracks showed Harvey's center dawdling after landfall, and many computer models predicted enormous rainfall totals for large areas on and near the coast -- including Houston -- as they envisioned the storm lingering into the middle of this week.

Statements from the NHC and other official sources, mirrored in online chatter among scientists, recognized that the rain could well turn out to be the real threat, and that the metropolis of Houston was very vulnerable. The storm total rainfall numbers likely to be realized now are at the high end of the range indicated by the models, but the potential for a major flood in the city was apparent.

No evacuation of Houston was ordered, though. The governor suggested it might be a good idea, but the mayor contradicted him. Now that we see what's happening, it looks, on the face of it, like it would be a lot better if people had left.

But this was a difficult judgment call, and it isn't obvious what the right one would have been. The Houston metro area has 6.5 million people, and the last time a large fraction of them tried to get out at once, for Hurricane Rita in 2005, the result was awful gridlock. It would have made a bad situation much worse if the roads had still been full of cars on Saturday night when the heavy rains began.

And before the rains fell, when it might otherwise have been possible for Houston residents to evacuate, the more imminent threat was still to the coast, where many mandatory evacuations had been ordered, so the roads needed to be kept clear for those people. The multiple enormities of this event -- the magnitudes of the populations involved, the multiple threats (wind, surge and rain) posed by the storm at landfall, the volume of rain falling after landfall, and the enormous area covered -- closed off options that might have been available in lesser storms.

Now, in any case, it's impossible for most people to leave, with highways inundated.

The situation is in the hands of state and local first responders, FEMA, and the National Guard. As in the first days after the levees failed in New Orleans in 2005, the rest of the country is stuck watching helplessly as we see a catastrophe that has the potential to rival Katrina -- or possibly even exceed it, as Houston's population is several times what New Orleans' was in 2005 -- unfolding, slowly, over days.

As agonizing as this feels, though, the groundwork for Harvey -- as with most "natural" disasters -- has been laid over decades, as the city has grown without enough consideration of flood risk. And unlike with the rainfall forecast, the signs here have been clear.

Houston has always been flood-prone. Tropical Storm Allison, in 2001, caused the last flood comparable to this one; Harvey looks on track to surpass it. The decades just before and after that have seen repeated "100-year" and "500-year" floods, with the most recent in 2009, 2015, and 2016. But with money to be made, development has proceeded with little restraint, in defiance of scientists' warnings of increasing exposure to future floods and residents' repeated experiences of past and present ones. Wetlands have been paved over and neighborhoods have sprawled into floodplains, with little accommodation for floods on the scale these recent events have reached.

Developers have shown little tolerance for regulation, and state and local government have not fought hard to impose much. Federal rules aren't up to the challenge, either. Many of the vulnerable areas in Houston are outside 100- and 500-year flood zones on FEMA's maps, zones which recent events (even before Harvey) have shown to be almost certainly too small.

And, of course, the recurrence of 100- and 500-year floods every few years is unlikely to be entirely accidental. Though Houston developers, many state and local officials, and the occupant of the White House don't want to hear it, it's likely that human-induced climate change has played a role in the increasing frequency of these events. There is more water vapor in a warmer atmosphere, and many other extreme rainfall events both nationwide and worldwide (including, very nearby, the Louisiana flood of just a year ago) have been shown to be amplified as a consequence.

But Harvey's relationship to global warming is a topic for another day.

Today, the disaster is still very much ongoing, slowly -- too slowly. For most of us, there is little to do but send thoughts and prayers to those in danger and those rushing in to help, and watch as the awful predictions come true.