The catastrophic and life-threatening floods that have drenched Houston in Texas, have once again demonstrated how Mother Nature is ramping up the severity of weather events to unprecedented levels.
Hurricane Harvey has broken the US record for rain from a single tropical storm. Roads in the USA’s fourth-largest metro area became rivers. Thousands have been rescued from flooding homes, with more than 30,000 people needing temporary shelter and, according to FEMA, an estimated 450,000 people are likely to seek federal disaster aid. Cleanup and rebuilding could take years. In order to understand how the flooding became so extreme, it is vital to comprehend the dynamics of what made this a particularly severe storm and why Houston is so susceptible to flooding.
Harvey landed off the Gulf coast near Rockport, Texas as a Category 4 storm with 130 mph winds. Over numerous days, the storm has barely moved, although the winds were downgraded to tropical storm strength.
Normally, upper-level winds in the atmosphere steer big hurricanes and keep them moving after they make landfall. With Harvey, those steering winds broke down, and a high pressure system to the northwest kept Harvey locked in place.
This is why Harvey has become such a catastrophic rainfall disaster. Extremely heavy rainfall persisted across a very wide area around Houston. 43 inches of rain (or 138cm) have been recorded in some areas. The corresponding issue is then, coupled with coastal storm surges, how the waters are going to drain – and it’s clear that the hydrogeology cannot deal with it.
Another way to think about the scale of Harvey’s rainfall was that, according to US meteorologists WeatherBell, within 48 hours around 14-15 trillion gallons (68.2 trillion litres) of water had fallen on Houston and its surrounding areas. The calculation is depth of rain multiplied by square miles covered. At the time of writing, 5 trillion more gallons (22 trillion litres) are still expected to come. Climate change is thought to have been a key contributory factor to the severity of the flooding. Sea-level rise in the Gulf, increased summer heat, and increased water temperature all likely conspired to make this storm slightly worse. Indeed, sea level rise in the Gulf of Mexico has accelerated and is now 4.5cm higher than it was in 1993. This is on top of global sea level rise. This is charging the power of hurricane events and storing up even greater levels of moisture and summer humidity.
Houston is essentially swampland and is therefore prone to flooding. Underpinning Houston’s land surface are unconsolidated clay, clay shales and poorly cemented sands up to several miles deep. The region’s geology developed from river deposits formed from the erosion of the Rocky Mountains. These sediments consist of a series of sands and clays deposited on decaying organic marine matter that over time, transformed into oil and natural gas.
Because Houston’s terrain is so flat, drainage away from the urban area is difficult. Urban sprawl has also limited the city’s natural drainage capacity.
As millions have flocked to the metropolitan area in recent decades, building regulations have allowed developers to pave over prairie land that once absorbed huge amounts of rainwater. More concrete hard standing creates flashy run off into the sewers or pooling surface water flooding. That has led to an excess of floodwater during storms that chokes the city’s vast bayou network, drainage systems and two huge federally owned reservoirs, endangering many nearby homes.
Sustainable drainage measures, where vegetation and soil can help soak up the water like a sponge, have not had a look in. The near-miss of Hurricane Ike in 2008 prompted discussions about building additional coastal flooding protections, like dykes and levees, but these didn’t deliver new schemes.
To account for the certainty of flooding, Houston has built drainage channels, sewers, outfalls, on- and off-road ditches, and detention ponds to hold or move water away from local areas. When they fill, the roadways provide overrun. This presented a challenge for the federal and city disaster response teams. If the order to evacuate up to 4 million people had been carried out in a short space of time, it is possible that many would have been stuck in traffic as the flood water started to rise, creating a huge humanitarian disaster.
Harvey is the third 500-year flood to occur in Houston in the last three years. Described by the American media as “Houston’s Katrina”, the emergency response has, to date, been thankfully better.
It begs the question whether this is the new normal, with all the climate change factors in place, and whether there is the political will to make the changes needed to mitigate even more severe events in the future.