A powerful storm hit Libya’s north-east coast on Sunday. Two dams burst upstream from the port city of Derna, causing a torrential flood that has killed more than 11,300 people, according to the Libyan Red Crescent.

The flooding in Derna is a harrowing example of how built-up infrastructure can collide with 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 professor of environmental science and policy Katharine Mach 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 added.

The natural environment is one of the ingredients.

Libya has a dry climate and rarely experiences such heavy rainfall. Storm Daniel dumped a record 414mm of rain on the area around Derna on Sunday, according to Libya’s National Meteorological Centre.

Normally, the region sees just 1.5mm of rain during the whole of September.

Because of climate change, “we are unambiguously in circumstances where rainfall is happening more intensely”, Prof 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 such as Florida, where Prof 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 Dr 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 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-up 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 centres, commercial centres – in the path of harm, in areas that are prone to flooding,” Prof Mach said.

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 are not maintained and managed properly, they can fail and cause catastrophe.

“It is one thing to build a flood control structure,” Prof Mach said. “It is another to make sure it is 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, such as 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 any more.

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