Floods

Most weather events descend upon us. Floods rise up to meet us from below – sometimes quietly, sometimes ferociously. They're one of the few weather-related hazards that can be made far worse by the way we choose to live: where we pave the land, build houses, fell trees and dam rivers. And floods may be the most underrated natural disaster known to man. Aside from heat and cold, flooding is the biggest weather-connected killer in the world. Unless you've seen a trickle of a creek become a torrent, you may have no idea of the sheer, brute power that mere water can exert Experts divide flooding into river floods and flash floods, two very different creatures. Flash flooding – the more mercurial and lethal kind – can send cars tumbling down the engorged canal of a hillside town in Italy, or leave a forlorn house marooned on a freshly carved island in the midst of a rampaging Canadian river.

Except for the rare spectacle of a dam break, floods aren't solo events. They're tied to other headline-grabbing weather features, whether it be a hurricane, a series of severe thunderstorms or a melting field of snow. Often the storm steals the spotlight while the flood is treated as an afterthought, perhaps not getting the attention it deserves until many people have suffered.

What makes it flood?

Rain is the first and last word in flooding, but not the only one, of course. Everything depends on how the rain falls and how the water behaves once it's on the ground. A large amount of rain spread out over days of gentle pitterpattering might not cause a problem, while the same rainfall packed into an hour or two could devastate a town. The key factor is whether the landscape can absorb the rain as it falls. This depends on a host of things:

• Urbanization Water flows more quickly and efficiently across pavement than grass. Storm drains can easily clog.
• Soil type Rainfall percolates through sandy soil more easily than through brick-like clay.
• Recent weather All else being equal, the more it has rained in the past few days or weeks, the more likely a flood becomes. Even if the rain puts an end to drought, it might be falling onto soil that's so sun-baked it can't absorb much water at first
• Topography Mountains are great floodmakers – they can wring water out of moist winds blowing into them, and they can channel rains into steep channels in virtually no time at all. Most of the world's worst flash floods have been in mountainous areas. Even arid or semi-arid regions can experience destructive flash floods if the terrain is rugged enough (or if urbanization has paved enough of it).

A river flood builds slowly, often from a winter's worth of snowmelt or a summer of incessant rain over a large area, perhaps more than a metre's worth (39in). As smaller tributaries feed into larger mainstems, the power of the water can multiply to astounding levels, especially in large basins like the Mississippi, which flows south through the central US, or the Yangtze, running through the heart of eastern China. These floods can destroy huge amounts of property and disrupt vast areas for weeks, but they often play out on a long enough time scale that relatively few deaths and injuries occur. (“Relatively” is the key word along China's mountain-cradled Yangtze, which is prone to landslides and dam breaks that exacerbate river floods. More than 3500 died in the Yangtze flooding of 1998, while the floods of 1931 may have killed as many as one million people.)

As the name implies, a flash flood happens quickly. It's often a product of one or more slow-moving thunderstorms that park over a flood-prone area, perhaps one as small as a city neighbourhood, and dump huge amounts of rain in an hour or two. The “wall of water” that flash-flood survivors often describe can raise the water height by a metre or more in less than a minute, although flash floods can easily kill without such a dramatic visual cue. Preceding rains that saturate the soil make a flash flood more likely, and the landscape makes a huge difference as well. There's some overlap between river and flash floods: the latter can occur on a small scale in a region that's already experiencing a river flood.

What can we do?

The age-old dream of controlling rivers has triggered some of the most ambitious engineering projects in history. Chinas Three Gorges project aims to tame the Yangtze with a series of dams that could end up displacing a million people and costing upwards of US$30 billion.

The United States alone has more than 50,000 dams along its rivers and creeks. Without them, Las Vegas, as we know it, could never have thrived in its arid landscape. Yet damming a river can eradicate fish and tamper in many other ways with local ecosystems. In the US, the expansionist mindset of river control ran headfirst into the growing environmental movement of the 1960s and 1970s. Since then, no major new dams have been built in the US, and the Three Gorges project has been roundly criticized by environmentalists.

Of course, there have been many motives for damming: recreation, water supply and hydroelectric power, among others. Still, even in their role as flood-control devices, dams and riverside levees now have a somewhat compromised reputation. A river with levees might flood less often, but once the levees are breached, the resulting flood spreads out more quickly – and sometimes catastrophically – than it otherwise would have. Bottled-up rivers also tend to collect silt at their bases, adding to the potential flood risk years down the line. Dams and levees are now being supplemented by a more variegated approach. Instead of jailing a flood, the idea is to coax it into behaving. Trees and other natural breaks can help slow and channel flood waters. Setting aside open areas along river courses gives a flood room to grow safely. Retention ponds capture flood water for constructive use.

Anyone can spot the kind of torrential, long-lasting rains that act as fuel for flooding. However, predicting floods with precision may be the toughest job in meteorology. River floods can be well anticipated several days ahead (even weeks ahead, if snowmelt is involved), with the help of automated streamflow gauges and computer models that project the water in time and space. The specific height of a major river's flood crest is hard to pin down: it depends on the inflow from myriad smaller, harder-to-diagnose streams, as well as whatever rain or snow might fall between forecast and crest. In 1997, a difference of 1.5m/5ft between the predicted spring crest and the actual one at Grand Forks, North Dakota, made the difference between a serious flood and an epochal one that destroyed much of this city of 50,000 residents.

Short-fuse floods are a forecaster's nightmare. Few national weather services around the world even try to issue formal warnings for these rapid-fire events, which might come and go before word gets to officials, much less to the public. The most sophisticated weather radars can now estimate rainfall amounts minute by minute over wide areas, but knowing the local topography well enough to pinpoint the status of a tiny mountain creek is much more difficult. Even in flat country like Houston, Texas, predicting just how much rain will fall isn't easy. Forecasters had issued a flash-flood warning on the night of June 8,2001, as the remnants of Tropical Storm Allison lingered near the coast. Computer models indicated that the system could dump as much as 13cm/5in of rain by the next day, but nobody expected 64cm/25in to fall overnight across parts of town. The deluge inflicted some US$3 billion in damage and killed more than twenty people. The potential for dam or levee failure, as in Hurricane Katrina, is another hard-to-assess wild card in gauging flood risk.

Seeking safety from a flood has its own risk. Many flood victims die not from drowning but from being tossed into and among debris in a raging current. Cars are a particular hazard: most flash-flood deaths in the US, and many elsewhere, occur in motor vehicles. Time and again, people try to drive through a flooded roadway that looks navigable, only to lose traction and float downstream. Surprisingly, it takes only about 50cm/20in of water to float most vehicles – even large ones. Once a car's wheels have left the pavement, the vehicle floats toward the most rapid flow, which typically is close to the deepest part of the channel. By that point, there's no hope of driving away. The only reliable way to escape a flash flood is to move to higher ground as soon as possible, even if that means leaving one's vehicle and setting out on foot. Most flash floods play out in an hour or two, a small investment of time to save one's life.