Hurricanes over shallow water bring more bad news

When Hurricane Harvey roared off the Gulf of Mexico to pummel Houston, Texas, on 25 August 2017, it looked like a peculiarly American problem. Harvey wasn’t the first storm to find its way into Gulf, and Houston wasn’t the first American city to suffer the consequences. 

It was also an unusually extreme event. Parts of the Houston metropolitan area got an astounding 1.5 metres of rain. At the height of the storm, water was falling on Houston at a rate comparable to the discharge of the Amazon River, says Arnoldo Valle-Levinson of the University of Florida, Gainesville.

But extreme as the storm was, it was by no means one-off. Rather, say Valle-Levinson and other scientists studying the storm, it carries valuable lessons for coastal cities, worldwide.

One of these lessons, Valle-Levinson says, comes from understanding that the reason Houston was flooded so badly stemmed in part from the fact that drainage patters conspired to hit it with a 1-2-3 punch. 

It began with Buffalo Bayou, a normally sluggish channel that drains downtown Houston and much of the terrain to the west. When Harvey hit, the bayou suddenly had to absorb fantastic amounts of runoff. 

“It became a river, not a bayou,” Valle Levinson says. “That was the first punch.”

The second punch came from the San Jacinto River, which flows in from the north and meets Buffalo Bayou in a T-intersection just upstream from where both meet the ocean in Galveston Bay. When the two sets of floodwaters collided, they backed up rather than running out into the bay, exacerbating flooding, especially in Buffalo Bayou. 

The third punch came from the ocean. Not only is the exit from Galveston Bay narrow, but the storm had driven a lot of water shoreward. In addition, much of the Gulf Coast had been experiencing slightly higher than normal sea levels for reasons that had nothing to do with Harvey. 

“That hinders anything coming out of Galveston Bay,” Valle-Levinson says. “It’s a 1-2-3 punch that knocked out Houston for several days.” 

Other cities are at risk for similar disasters. “Whenever you have a system with two rivers sort of competing to flush out, one will block the other and cause flooding,” Valle-Levinson says.

If the ocean is closed off, he adds, that will also jam up the works. 

“You don’t need a hurricane,” he explains. “Just an anomalously large pulse [of water].” Heavy runoff from distant mountains could have the same effect. 

Other scientists note that big rainstorms can also produce a backlash into the sea, possibly even to places as far offshore as Australia’s Great Barrier Reef.

When Harvey hit Houston, says Steven DiMarco of Texas A&M University, College Station, Texas, the runoff wasn’t just water. It also picked up contaminants, ranging from gasoline from 500,000 flooded cars to household chemicals and industrial compounds swept up from abandoned waste sites.

One worry, DiMarco says, was that these chemicals could have reached the Flower Garden Banks National Marine Sanctuary, a federally protected coral reef located about 185 kilometres offshore. 

Luckily, he found, the run-off plume went elsewhere. But that didn’t have to be the case. Any large city, if hit by a massive flood, is at risk of producing a surge of toxic contaminants washed out into the ocean, toward whatever sensitive environments might exist nearby. 

“In Harvey, there were dioxins,” says Rosanna Neuhausler, a PhD student in geography at the University of California, Berkeley, who is working to compile data on which hazardous waste sites on the US Superfund list pose the greatest dangers if hit by tropical storms. So far, her work is preliminary, but climate models show an increase in extreme storms, she says, “[and] we don’t know where these storms are going to hit.” 

Still more research shows that weather models may underestimate the risk of storms intensifying rapidly as they make their final approaches to the coast. 

Traditionally, one of the ways this has been assessed is via a metric called the tropical cyclone heat potential (TCHP), says Henry Potter, also of Texas A&M.{%recommended 5715%}

TCHP is measure of the total amount of energy available to fuel the storm’s growth, calculated by summing up the water’s heat energy down to the depth where its temperature drops to 26 degrees Celsius, thought to mark the temperature divide between hurricane-fueling and hurricane-neutral. 

TCHPs above 90 kilojoules per square centimetre are associated with very rapid intensification of hurricanes, Potter says. But offshore from Houston, a survey conducted only two weeks before Harvey’s arrival showed that while the waters were very warm, their shallow depth limited their total heat energy. The TCHP was only 36, well below the alarm threshold. But despite the relatively low level, when Harvey moved into this region, it intensified rapidly. 

Why this happened, Potter says, isn’t fully clear, but it’s an indication that the metric may not work in shallow waters. 

And if it didn’t work in Texas, it may also underestimate the risk for tropical storms moving over warm, shallow waters, everywhere on the globe. “Anywhere there’s a shallow shelf would have the same problems,” he says.