Medellin, Colombia - Scientists are finding a new suspect to blame for flood disasters around the world, and it’s not climate change: It’s sediment.
Researchers in Spain have found the number of yearly floods and related disasters around the world has jumped more than 9 times since the 1950s - to more than 180 per year - and that the trends in rainfall attributed to climate change are not enough to explain such a rise.
Instead, the dramatic rise of water-related disasters seems to follow GDP, the researchers found. As economies around the world grow, people are clearing more and more forests to make way for cities and farms. Without trees to retain soil, excess dirt and rocks are poured into rivers, damaging their capacity to cope with storms.
"A woman told me that four years ago the lagoon was deep enough to easily cover her, say three metres. Now the depth reaches her belly."
- Juan Darío Restrepo, EAFIT University
Sediments are produced when water, wind, ice or changes in temperature break up rocks and soil into small bits, in a process called erosion. Erosion is a natural process that shapes the landscape and transports nutrients.
However, human impacts on land - especially deforestation - have significantly increased erosion rates all around the world.
A 2010 study by the University of Cantabria in Santander, Spain and the Universidad Nacional de La Plata in Argentina looked at sediments and their connection with economic growth, using the La Plata River in South America as a case study.
The study found while unpopulated Andean regions showed no major increase in erosion, in places near the Sao Paulo metropolitan area sedimentations rates increased “gently” starting in the mid-1980s and then sharply since 2000, showing more than tenfold growth in about 20 years.
A 2012 follow-up study by the Earth Science Department at the University of Cantabria found that as GDP increased in northern Spain, so did the rates of sediment production, and also the number of water-triggered disasters in the region.
Colombia’s Magdalena River
The researchers’ hypothesis is that human contributions to erosion have risen to an extent that the world is now undergoing “global geomorphic change”, which makes floods and landslides worse, regardless of the changes in the weather.
The Magdalena River in Colombia is a dramatic illustration of the changes, and the inter-relationships between economic growth, sediments and flooding.
The Magdalena is born in the Colombian Andes, and flows north into the Caribbean Sea. More than 80 percent of Colombians live in the Magdalena Basin, and about 85 percent of Colombia’s GDP is generated there.
Once it was the swiftest way to move between cities in Colombia’s interior and the Atlantic Ocean. But current navigation on the Magdalena is very limited, partly because of excess sediments accumulated in the riverbed.
The Magdalena flooded in 2010 and 2011 when rainy seasons enhanced by the La Niña phenomenon caused the worst climate-linked tragedy in Colombian history, affecting more than four million Colombians. Two-and-a-half million became climate refugees. One million hectares of cropland were flooded, and more than 800 roads destroyed. The country spent more than $3.9bn in emergency humanitarian aid to cope with the tragedy.
Researchers from EAFIT University in Medellin, Colombia, and University of Colorado at Boulder in the United States have studied the erosion in the Magdalena basin, and found that sediment production is higher than in any other South American river.
Colombia is responsible for five percent of global forest loss, and ranks seventh in the UN Food and Agriculture Organisation’s list of countries undergoing deforestation - a list led by Brazil and Indonesia.
The situation is particularly critical in the Magdalena Basin, where less than 10 percent of the original vegetation cover remains
“This has been a farming and cattle country. We’ve cut down the forest to raise livestock,” said Juan Darío Restrepo, a geology professor at EAFIT University who is passionate about the erosion problems of the Magdalena basin.
In a 2010 study, Restrepo and his team found that deforestation could be blamed for 32 percent of the basin’s sediment production. Add in mining and construction activities and human contributions could account for almost half of the total sediments of the basin, he said.
Most of these sediments end up in a regional system of low-elevation river lagoons and wetlands called the Momposina depression. This region acts a sort of hydraulic shock absorber, with excess water stored in the lagoons, preventing flooding downstream.
Researchers estimate that 20-45 percent of the sediments that arrive are trapped in the depression, slowly clogging the system.
In November 2012, in a trip to the Momposina depression, locals confirmed some of Restrepo’s team findings.
“A woman told me that four years ago the lagoon was deep enough to easily cover her, say three metres. Now the depth reaches her belly,” he said.
|A flooded street in Cali, Colombia in 2011 [AFP]
In 2010 and 2011 flooding expanded across the Colombian Caribbean Coast, destroying the levees of the country’s largest canal and flooding whole towns.
Afterward, the Colombian government created an adaptation fund with a budget of $10.5bn to rebuild the disaster zone and to avoid future tragedies. The spending plan includes new houses and infrastructure, relocation of communities away from risk areas, and the construction of levees and other hydraulic control structures.
This month, the Colombian Ministry of Environment and the UN Development Programme announced a new project to reduce the Momposina depression’s vulnerability to climate change. Actions will include an overhaul of climate information systems, rehabilitation of wetlands, and support for resilient agriculture and institutional capacity-building projects.
However, none of the projects has allocated funds for large-scale erosion control in the upper Magdalena. Restrepo says these actions focus on building protection structures, rather than attacking the source of the problem. Apart from “hard” engineering, an effective solution to the country’s worsening flooding needs “soft engineering and ecosystem management”, he said.
Despite their global significance, the link between sediments, economic activity and disasters remains virtually unknown outside academic circles. If scientists turn out to be right and there is a global soil change, humanity will have to face yet another environmental challenge - one probably as big as climate change itself, experts say.
This article first appeared in the Thomson Reuters Foundation news service