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New study spotlights Africa cities’ natural ability to absorb rainfall

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Lucy Chakaodza Lucy Chakaodza UKIMEA Press Office,London
14 November 2022

  • Study authors at Arup say green and blue infrastructure – such as parks, gardens, ponds and lakes – hold key to cities’ ability to manage heavy rainfall and other impacts of climate change, such as extreme heat events


  • Call comes as COP27 in Sharm el-Sheikh enters its second week and focuses on how urban populations can increase resilience against effects of climate change

A new study published today by Arup has revealed the “sponginess”, or natural ability to absorb rainwater, of five cities’ urban centres across Africa. This comes as leaders at COP27 in Sharm el-Sheikh turn their attention to Water Day.

Arup’s first-of-its-kind Africa Sponge Cities Snapshot studies a sample area of approximately 150 square kilometres in the urban centres of five diverse African cities – Cairo, Durban, Kigali, Lagos and Nairobi. Cairo’s urban centre emerges as 20% spongy – compared to Kigali (43%), Lagos (39%), Durban (40%) and Nairobi (34%).

Authors of the study are calling on African cities to protect their existing natural assets and to deploy these strategically to help become more resilient to climate-related challenges, such as increasing heavy rainfall events. This is an opportunity to leapfrog developing nations, particularly in the West – who during times of rapid growth failed to preserve their natural assets, instead too heavily relying on carbon intensive concrete solutions. Overall, the African cities studied were far spongier than those analysed in Arup’s global snapshot undertaken at the beginning of the year – with London 22% “spongy” and Sydney 18% “spongy”.

The call comes as Africa’s urban population is expected to triple to almost[i] 1.5 billion by 20501. Meanwhile, nearly half (44%) of all “disaster events” globally have been flood-related, with 676 floods across Africa between 2001 and 2018, which have caused $6.3 billion worth of damage.[ii] In many African cities, more than half of the population live in informal settlements, which brings unique planning and design challenges and a significant population whose homes are especially vulnerable to floods.[iii]

To create the calculations, Arup’s team used an advanced digital tool, Terrain, which applies machine learning and artificial intelligence techniques to accurately quantify the amount of green infrastructure (e.g., grass, trees) and blue (e.g., ponds, lakes), versus the amount of grey (e.g., buildings and hard surfaces). The snapshot was based on detailed satellite imagery covering an area of approximately 150 square kilometres of the city’s main urban centre. Authors supplemented this analysis with insight on soil types and vegetation, enabling them to estimate how much rainwater would be absorbed in a defined heavy rainfall event.

“

Millions across Africa are at risk of flooding due to the combination of climate change and rapid, unplanned urbanisation. The precious remnants of natural habitats found in many African cities are valuable assets and need to be preserved and enhanced. Nature-based solutions need to be deployed strategically to help cities become more resilient – this capacity has been lost in many western cities that have become concrete jungles. ” Matthew Phillips Matthew Phillips Senior Engineer

The term “Sponge City” was coined in 2013 by Professor Kongjian Yu of Peking University, describing cities that work with nature to absorb rainwater, instead of using concrete to channel it away. Natural infrastructure is not only extremely effective in managing flood water, but also on average 50% more cost-effective than man-made alternatives, delivering 28% more added value.

Cairo
Durban
Kigali
Lagos
Nairobi

Cairo

Egypt’s capital has a population of 21.75 million, making it one of the largest cities in Africa. Cairo experiences very little rainfall throughout the year – an average of 24mm – with only December and January experiencing more than 5mm of rainfall per month. As such, Cairo would not be prepared for higher rainfall intensities which, if only increased by a few millimetres, could result in significant flooding. A flood event in March 2020 resulted in a number of fatalities.

Cairo’s “sponginess rating” of 20% is primarily due to its low concentration of blue-green (permeable) surfaces – which cover 29% of the snapshot area. Almost 40% of the study area was found to be ‘buildings’, with a small proportion of green infrastructure located in small, scattered pockets including low vegetation agriculture in the west.  There are only a small number of parks across the city, with the study area containing less than 5% of trees, far lower than Durban which had 31%. Its moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributes to its overall ‘sponginess’.

Durban

Durban has a population of just over 3 million. Its rainy season runs from October through to March, with a yearly average rainfall of 1,009mm. The city has experienced severe and unprecedented flooding, with a national state of disaster declared in April 2022. Extreme heavy rain resulted in the deaths of hundreds of people and saw 40,000 driven from their homes.

Durban’s “sponginess rating” of 40% is primarily a result of its high concentration of blue-green (permeable) surfaces – covering two thirds of the snapshot area. Relatively equal proportions of roads and buildings together comprise 40% of the overall study area. The flat central business district contrasts with dense tree coverage representing almost a third of the area across the steeper inland sections. Its moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributed to its overall ‘sponginess’.

Kigali

Kigali’s rainy season runs from September through to May, with a yearly average rainfall of 1,137 mm. The country has seen the frequency of flooding rise throughout the 2000s. Kigali is increasingly seeing severe rains through the traditionally drier period. Over two days in early February 2000, up to 500mm of rain was recorded in Kigali, resulting in a number of fatalities.

Kigali’s “sponginess rating” of 43% is primarily due to its high concentration of blue-green (permeable) surfaces – which cover 67% of the snapshot area. Characterised by rolling hills, valleys and ridges, natural vegetation represents a third of the study area, spread around the residential clusters. It had more low vegetation cover than any other urban centre studied, at almost one third. Its high runoff soil classification, composed of less than 50% sand and more than 40% clay, further contributes to its overall ‘sponginess’.

Lagos

Lagos, on the south-western coast of Nigeria, has a reported population of over 20 million. The city frequently experiences annual flooding during the wet season that runs from April to October, with an average rainfall of 1,689mm a year. In recent years, Lagos has been experiencing more frequent flooding, making it one of the country’s most vulnerable cities. The total economic losses due to flooding across the city is estimated at $4 billion per year. Its residents perceive flooding as the second most significant hazard, after crime.

Lagos’s “sponginess rating” of 39% is primarily due to the high concentration of blue-green (permeable) surfaces – which cover 58% of the snapshot area. The study area contains 34% low vegetation, undeveloped marshland. Large green expanses within the central residential areas in Lagos Island contrast its densely populated western region where there is little green space. Lagos’s high runoff soil classification, composed of less than 50% sand and between 20 and 40% clay, further contributes to its overall ‘sponginess’.

Nairobi

Kenya’s capital Nairobi is an inland city elevated at 1,795 metres above sea level with a population of just over 5 million. The long rainy season runs between March and May with a shorter rainy season between October and December. Its yearly average rainfall is 1,061mm. In April 2016, inhabitants of Nairobi were caught up in flash flooding after a storm brought heavy rainfall that lasted for nearly three hours. In May 2021, floods struck Nairobi which led to several deaths and considerable material damages in parts of the capital.

Nairobi’s “sponginess rating” of 34% is primarily a result of its high concentration of blue-green (permeable) surfaces – which cover just over half of the snapshot area. The city benefits from a large quantity of green infrastructure, particularly grasslands. This is driven by parkland as well as urban backyards spread across the study area. However, a high proportion of the study area also featured high-density development with almost no green space. Its high runoff soil classification, composed of less than 50% sand and more than 40% clay, further contributes to its overall ‘sponginess’.

Cairo

Cairo

Egypt’s capital has a population of 21.75 million, making it one of the largest cities in Africa. Cairo experiences very little rainfall throughout the year – an average of 24mm – with only December and January experiencing more than 5mm of rainfall per month. As such, Cairo would not be prepared for higher rainfall intensities which, if only increased by a few millimetres, could result in significant flooding. A flood event in March 2020 resulted in a number of fatalities.

Cairo’s “sponginess rating” of 20% is primarily due to its low concentration of blue-green (permeable) surfaces – which cover 29% of the snapshot area. Almost 40% of the study area was found to be ‘buildings’, with a small proportion of green infrastructure located in small, scattered pockets including low vegetation agriculture in the west.  There are only a small number of parks across the city, with the study area containing less than 5% of trees, far lower than Durban which had 31%. Its moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributes to its overall ‘sponginess’.

Durban

Durban

Durban has a population of just over 3 million. Its rainy season runs from October through to March, with a yearly average rainfall of 1,009mm. The city has experienced severe and unprecedented flooding, with a national state of disaster declared in April 2022. Extreme heavy rain resulted in the deaths of hundreds of people and saw 40,000 driven from their homes.

Durban’s “sponginess rating” of 40% is primarily a result of its high concentration of blue-green (permeable) surfaces – covering two thirds of the snapshot area. Relatively equal proportions of roads and buildings together comprise 40% of the overall study area. The flat central business district contrasts with dense tree coverage representing almost a third of the area across the steeper inland sections. Its moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributed to its overall ‘sponginess’.

Kigali

Kigali

Kigali’s rainy season runs from September through to May, with a yearly average rainfall of 1,137 mm. The country has seen the frequency of flooding rise throughout the 2000s. Kigali is increasingly seeing severe rains through the traditionally drier period. Over two days in early February 2000, up to 500mm of rain was recorded in Kigali, resulting in a number of fatalities.

Kigali’s “sponginess rating” of 43% is primarily due to its high concentration of blue-green (permeable) surfaces – which cover 67% of the snapshot area. Characterised by rolling hills, valleys and ridges, natural vegetation represents a third of the study area, spread around the residential clusters. It had more low vegetation cover than any other urban centre studied, at almost one third. Its high runoff soil classification, composed of less than 50% sand and more than 40% clay, further contributes to its overall ‘sponginess’.

Lagos

Lagos

Lagos, on the south-western coast of Nigeria, has a reported population of over 20 million. The city frequently experiences annual flooding during the wet season that runs from April to October, with an average rainfall of 1,689mm a year. In recent years, Lagos has been experiencing more frequent flooding, making it one of the country’s most vulnerable cities. The total economic losses due to flooding across the city is estimated at $4 billion per year. Its residents perceive flooding as the second most significant hazard, after crime.

Lagos’s “sponginess rating” of 39% is primarily due to the high concentration of blue-green (permeable) surfaces – which cover 58% of the snapshot area. The study area contains 34% low vegetation, undeveloped marshland. Large green expanses within the central residential areas in Lagos Island contrast its densely populated western region where there is little green space. Lagos’s high runoff soil classification, composed of less than 50% sand and between 20 and 40% clay, further contributes to its overall ‘sponginess’.

Nairobi

Nairobi

Kenya’s capital Nairobi is an inland city elevated at 1,795 metres above sea level with a population of just over 5 million. The long rainy season runs between March and May with a shorter rainy season between October and December. Its yearly average rainfall is 1,061mm. In April 2016, inhabitants of Nairobi were caught up in flash flooding after a storm brought heavy rainfall that lasted for nearly three hours. In May 2021, floods struck Nairobi which led to several deaths and considerable material damages in parts of the capital.

Nairobi’s “sponginess rating” of 34% is primarily a result of its high concentration of blue-green (permeable) surfaces – which cover just over half of the snapshot area. The city benefits from a large quantity of green infrastructure, particularly grasslands. This is driven by parkland as well as urban backyards spread across the study area. However, a high proportion of the study area also featured high-density development with almost no green space. Its high runoff soil classification, composed of less than 50% sand and more than 40% clay, further contributes to its overall ‘sponginess’.

Arup Africa Sponge Cities Snapshot: Full results

City

 Overall % "sponginess"

Green-blue area 

Soil Classification and Run Off Potential 

Kigali

43%

67% 

High runoff potential (<50% sand and >40% clay)

Durban

40%

59% 

Moderately-high runoff (<50% sand, 20-40% clay)

Lagos

39%

58% 

Moderately-high runoff (<50% sand, 20-40% clay)

Nairobi 

34%

52% 

High runoff potential (<50% sand and >40% clay)

Cairo

20%

29% 

Moderately-high runoff (<50% sand, 20-40% clay)

[i] https://population.un.org/wup/publications/Files/WUP2018-Report.pdf
[ii] https://www.unesco.org/en/articles/being-able-predict-floods-will-save-lives-west-africa3
[iii] The World Bank’s Population living in slums (% of urban population) – Sub-Saharan Africa report (2018)


Read the Africa Sponge Cities Snapshot