15 October 2010

Population trends, not climate, causing increased flood fatalities in Africa

Posted by Michael McFadden

Floodingoutside Khartoum, Sudan 2009 (image by sidelife, used under creative commons)

Receding flood waters outside Khartoum, Sudan, August 2009 (image by sidelife on Flickr, used under a Creative Commons license)

Guest post by Anne Jefferson, assistant professor at the Department of Geography and Earth Sciences, UNC Charlotte, and science blogger at Highly Allochthonous

In August 2009, unusually heavy rains lasting six hours deluged the city of Khartoum in Sudan. Higher, wealthier parts of the city were relatively unaffected, but in the vast shanty towns that surround the city center, people lost their possessions, homes, and lives. Twenty-seven people died, and over 20,000 homes were damaged or destroyed.

Sadly, the flooding in Khartoum is not an isolated incident. The number of flood deaths in Africa has gone from fewer than 2000 in 1950-1969 to more than 15,000 between 1990 and 2009. In addition, the number of lives affected and the amount of economic damage have dramatically increased.

What’s behind this trend? Is it a reflection of climatically-driven increases in the intensity of precipitation and magnitude of floods? Is it simply that there are more people in Africa now to be affected by floods? Or are there socio-economic patterns at work that have magnified flood risks?

According to a paper in press in Geophysical Research Letters, climate is not to blame for the increase of flood deaths in Africa.

In the new study, Giuliano Di Baldassarre and co-authors examine the hydrologic data on African floods in the 20th century and tease out the causes and consequences of flooding, while pointing to ways that future flood risks could be lessened. Watersheds with long streamflow records and little land-use change can be good indicators of climatic influences on the water cycle. So that’s where the authors started their research.

They identified 79 rivers around the African continent with decades’ worth of good data on flood discharges and minimal changes to watershed land-use, including the Niger, Congo, and Nile. Using two different methods, the authors looked for trends in the flood discharge data. When the data were aggregated, there were no statistically significant trends in flood discharge. When each river was examined individually, 65 rivers showed no statistically significant trend, 10 rivers trended toward smaller floods, and only four rivers trended toward larger floods. Based on these results, the researchers conclude the climatic influences cannot explain the troubling rise in African flood fatalities.

After eliminating a climatic cause for flood fatality increases, the authors turned to population patterns. The total population of the African continent has increased by about a factor of 4 four since 1950, but flood fatalities have increased much more dramatically. When the authors compared the increase in flood deaths to the growth in urban populations, the numbers began to match .

In the last 50 years, Africa has undergone intense urbanization. Cities and large towns are now 10 times more populated than they were half a century ago, and flood fatalities have risen just as fast. The authors also show the geographic association of population growth and deadly floods. While floods occurred in many parts of the African continent between 1985 and 2009, the floods that killed people are strongly concentrated in areas with high population growth. Floods over the past few years in places like Dakar (Senegal), Lusaka (Zambia), and Ouagadougou (Burkina Faso), illustrate the problem. Poor migrants from rural areas to cities end up living in unplanned and informal shanty towns and slums, often in the lowest-lying and most flood-prone areas. These settlements frequently have poor drainage, so flood waters pond, turn stagnant, and become breeding places for disease, increasing the number of deaths that result from the floods (although those numbers would not be counted directly as flood fatalities).

Given that the urban population of Africa is still rapidly growing, what can be done to lessen future flood risks? Di Baldassarre and his colleagues point to examples from around the world where the development of flood forecasting and early warning systems have saved many lives by getting people out of flood-prone areas before the waters rise. But, the researchers caution, getting people out of harm’s way when the rains come should be accompanied by building of effective local institutional capacities for urban planning, and discouraging settlements in areas at high risk of floods. Di Baldassarre’s team concludes that these are “effective and socially sustainable actions that should be pursued with priority in the African continent.”

– Anne Jefferson is an assistant professor at the Department of Geography and Earth Science, UNC Charlotte. She blogs at Highly Allochthnous.


Giuliano Di Baldassarre, Alberto Montanari, Harry Lins, Demetris Koutsoyiannis,, & Luigia Brandimarte, Günter Blöschl (2010). Flood fatalities in Africa: from diagnosis to mitigation Geophysical Research Letters