Nine Trillion Gallons of Water

Hurricane Harvey was not a surprise. But as it slammed into the coast of Texas, it dumped 9 trillion gallons of water on the greater Houston area. Allianz Re’s natural catastrophe (NatCat) risk experts Katherine Wenigmann and Jonathan Meagher explain why this is an unusual extreme.

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Harvey came expected. By the U.S. hurricane forecasters who saw the storm forming days before landfall in Texas and did their best to adjust their forecast as it moved along. By the residents of the Texas coast, especially around Houston because the area is known to be prone to storms and resulting floods. And none the least by NatCat risk experts like Allianz Re’s hydrology and water resources expert Kathrine Wenigmann, who follow floods around the world, and atmospheric science expert Jonathan Meagher, who analyzes
non-European storms and hurricanes.

This time, the guest brought an unexpected gift in the form of a record-breaking rainfall. Harvey brought incredible 51 inches (around 1.3 meters per square meter) of rainfall, breaking the national record and forcing the U.S. National Weather Service to add two more colors to their maps to be able to display the amount correctly.

Although quite a few storms had hit the Gulf region in the past years - like Wilma and Katrina in 2005 and Carla back in the 1960s - Harvey was special. Why?

Jonathan: The difference to other storms is that Harvey stalled when it made landfall. The storm stopped directly at the coast and came out back into the Gulf a  little. Its path was blocked by two high pressure systems, which prevented it from moving. Usually, the storms move inland and lose their energy quite quickly. Harvey did weaken in a sense that its wind speed dropped but it was still able suck warm water from the Gulf to sustain the storm and rainfall.

Jonathan Meagher

So the amount of rainfall might be the same for other storms but might not be experienced in such extreme ways?

Jonathan: Exactly. Once the storms move over land, they lose access to their main source of energy and moisture, which causes the storm to rapidly weaken. Also a moving storm’s rainfall will be spread out over a large area. In addition, the storms move more rapidly over land as they are picked up by mid-latitude winds. Normally, the leftovers end up in the UK within three to four days. Crossing the Atlantic, they lose all their destructive power but it might get somewhat rainier in Northern Europe because of such storms.

How well did the forecasts pan out?

Katherine: Unfortunately, Harvey developed very quickly. The National Hurricane Center (NHC) only identified that Harvey would strengthen in the night from Wednesday to Thursday. Landfall occurred on Friday night. That did not provide a lot of time for a large-scale evacuation. Most meteorologists even believed that this extreme situation could become reality, that Harvey could stop above Houston and dump a lot of rain. But eventually, the rainfall even exceeded initial forecasts, which were 15-25 inches, with isolated amounts as high as 35 inches.

Jonathan: The NHC had forecast Harvey’s track pretty well, including the storm stalling over the Texas coast. However, it’s much more difficult to predict the intensity of a storm than its path.

Katherine Wenigmann

What is going to happen next? Will it get worse?

Jonathan: The blocking flow is changing, so Harvey should start to move (which it did), from Texas to Tennessee and then further up, dissipating along the way.

Katherine: Whenever a large area is flooded, it takes a long road to recovery. It’ll take a long time for many people to move back into their houses. The effects of Harvey are still being analyzed and will be huge. A big concern is that a lot of people do not have flood insurance. Government officials and residents will need to make hard decisions on whether to allow largely unregulated development to continue or whether expensive infrastructure investments and buyouts of flood prone properties should be pursued.

Where does all the water go?

Katherine: Back into the Gulf. Most of the rivers in Houston are starting to go down already. One concern during Harvey was that there was a small storm surge, meaning that the ocean level is higher than usual due to the winds pushing water up against the coast. This prevents runoff from flowing back. It was not as bad as it could have been but still needed to be considered.

Are there any other areas that could be affected in the same way as the U.S.?

Jonathan: A number of Asian countries. Bangladesh is always at risk because of its location - in the middle of the Ganges Delta.

Katherine: Recently, one-third of Bangladesh was under water. This monsoon driven flood in Bangladesh, India and Nepal got muted press coverage compared to the flooding in Houston. Many areas in Florida, such as the Miami and Tampa areas, are also susceptible to flooding and storm surges from hurricanes.

Did Harvey keep you extraordinarily busy?

Katherine: Quite but not overly extreme. We work together with our colleagues from the AGCS catastrophe team, trying to estimate what Allianz’s losses could be even before we get calls from our customers.

Jonathan: Fortunately, we have good computer models that we use for pricing and risk management. Especially for the United States, there are very sophisticated Hurricane models doing a great job. Modeling the flood is not so easy and that's why we were a bit more alert than usual.

Harvey is an interesting phenomenon because of the large amount of rain. The storm itself is not surprising. We are well prepared. As a global business, we deal with large NatCat events around the world almost on a daily basis.

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