Hurricane Harvey and its remnants have managed to dump likely record-setting amounts of rainfall across Texas. The Weather Channel expects that some locations could see accumulation totals of 127cm before the weather finally lets up. Some locations around and outside of Houston have already have seen floods higher than 4.5m.
You might feel naive asking it, but reporting on events such as this tends to leave out one of the most basic questions: How 60cm of rain becomes 3m of flooding. On one level the answer is simple, that water flows from higher areas to lower areas. But complex hydrodynamics and Houston's geography can lead to even more extremes, which helps explain why things are looking so terrible — and will likely remain that way for a while.
"This water could be hanging around there literally for weeks," Barry Keim, Louisiana State Climatologist at Louisiana State University, told Gizmodo.
Flooding occurs when water accumulates in an area faster than it can drain, from rain, oceans or even broken dams, according to the NOAA's National Severe Storms Laboratory. That water moves to the lowest lying areas in the region, such as rivers and creeks, and then flows downhill. You've also probably heard that Houston is quite flat — that means that the water can't drain as quickly, and stays stagnant in already-flooded areas.
Keim explained that this stagnation can lead to a pileup from other factors. Normally, smaller streams trickle into larger and larger rivers, like a network of branches combining into a large trunk. But when the main trunk accumulates too much water, these tributaries can begin to back up, as happened last year in Baton Rouge, Louisiana. "When you have a glut of water in all the main arteries, it has nowhere to go," he said. "It just sits there and might back up and create high water like that."
Houston specifically drains via a bayou system, a flat system of slow-moving rivers or water on flooded land, John Nielsen-Gammon, the Texas State Climatologist at Texas A&M University, explained to Gizmodo. This leads to additional challenges, on top of the fact that it's a city full of concrete and pavement, impervious surfaces that water can't seep into. Porous soil would normally let the water drain more quickly than concrete. As ProPublica reported last year, Houston already floods frequently, as its lightning-fast, fairly un-zoned economic development replaced prairie with pavement. That report found that even areas outside of floodplains, which are supposedly unlikely to flood, have faced flooding in 2009, 2015 and 2016.
Storm surge is often responsible for coastal flooding, but is not a major contributor to Houston's flood problems from Harvey, Nielsen-Gammon told Gizmodo. (Although the National Hurricane Center has reported some surges in Galveston Bay.) And the rainfall is still historic. "In terms of its impact on individual locations, it's probably something like one in a 500 to one in 1000 year event," he said, and added that it could potentially be the largest amount of rainfall over such a large area.
The moral is, 10 to 25cm of rain might mean only a little flooding for higher areas, but anyone on lower ground is getting everyone else's rain, too. And once the water has arrived on flat land, it's there to stay. "There's nothing to do at this stage but wait it out," Keim said.