What Factors Make for a Deadly Flood?

A powerful storm hit Libya’s northeast coast on Sunday. Two dams burst upstream from the port city of Derna, causing a torrential flood that has killed more than 5,000 people, according to local officials.

The flooding in Derna is a harrowing example of how built infrastructure can collide with the climate and geography to turn a storm into a disaster.

“Floods are the most damaging natural hazard when it comes to destruction of property and lives lost,” said Katharine Mach, a professor of environmental science and policy at the University of Miami. But the danger and damage can vary widely, and a whole “recipe” of factors determines a given location’s flood effects, she said.

The natural environment is one of the ingredients. Libya has a dry climate and rarely experiences such heavy rainfall. A storm dumped a record 15 inches, or 400 millimeters, of rain on the area around Derna on Sunday, according to Libya’s National Meteorological Center. Normally, the region sees just 1.5 millimeters, or less than a tenth of an inch, of rain during the whole month of September.

Because of climate change, “We are unambiguously in circumstances where rainfall is happening more intensely,” Dr. Mach said. Last week, similar extreme flooding from the same Mediterranean storm hit Greece and Turkey.

In dry regions, rain tends to stay on the surface rather than seep into the ground and often creates fast-moving flash floods. Wetter regions like Florida, where Dr. Mach is based, have more spongy soil that can absorb water and mitigate some of the flooding danger.

Derna is also built on top of an alluvial fan, a type of landscape formed at the base of mountain ranges by loose sediment washing down rivers and streams. These landscapes are known to be at risk of “ultrahazardous flooding,” said Brett Sanders, a professor of civil and environmental engineering at the University of California Irvine. When alluvial fans experience heavy rainfall, floods tend to strike suddenly, travel very quickly, and carry a lot of sediment and debris that can bulldoze whatever is in the way.

But on top of these natural factors, the built environment is the determining factor in how much people suffer from a given storm and subsequent flooding.

Historically, cities have been built where there is water. “People around the world have put stuff — infrastructure, houses, industrial centers, commercial centers — in the path of harm, in areas that are prone to flooding,” Dr. Mach pointed out.

Often people have also built flood control and water supply infrastructure, like Derna’s dams, alongside cities. This infrastructure, however, can sometimes inadvertently increase risk. Once a dam or similar flood-control structure is in place, people often believe an area is safe and build accordingly. But if those structures aren’t properly maintained and managed, they can fail and cause catastrophe.

“It’s one thing to build a flood control structure,” Dr. Mach said. “It’s another to make sure it’s maintained through time.” She pointed out that governments are typically far less politically motivated to maintain infrastructure than to build it in the first place.

Compounding the problem, existing infrastructure, like the dams above Derna, was designed and built for a past climate, to now-outdated specifications for rainfall, among other hazards. Early warning systems and emergency management protocols have similarly been tailored according to historical experience that might not apply anymore.

The Intergovernmental Panel on Climate Change forecasts that Mediterranean storms like the one that hit Libya could become less frequent in the future, but the storms that do form will become stronger and more extreme.