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    Rapid, unplanned urbanisation, combined with climate change is putting millions across Africa at risk of flooding. As cities across the continent expand, they have an opportunity to harness the power of their natural assets – including precious remnants of natural habitats – to build their resilience to extreme weather events.

    The first step to maximising the benefits of natural infrastructure is to understand what is already in place. We’ve used our land use analysis tool, Terrain, to better understand the “sponginess” or existing natural ability of cities around the world to absorb rainfall. Our new Africa Sponge Cities Snapshot looks at the urban centres of five cities across the continent: Cairo, Durban, Kigali, Lagos and Nairobi.

    x3 Expected growth in Africa's urban population by 2050

    +50% of many African cities' populations live in informal settlements

    $6.3bn of flood damage across Africa between 2001 and 2018

    Creating the snapshot

    Using Terrain, we calculated the amount of green and blue areas in the urban centres of each city. We then factored in the impact of soil types and vegetation and calculated the rainfall runoff potential. We then used this to produce a calculation of a city’s natural absorbency from its green and blue spaces – and the greater the natural absorbency, the higher the sponge ranking.

    • Blue, green and grey infrastructure
    • +
    • Soil types and vegetation
    • +
    • The water runoff potential
    • =
    • City's sponge snapshot

    Terrain

    Terrain helps cities rapidly understand how land is being used. It harnesses the power of data analytics, machine learning and automation to accurately digest large quantities of data and satellite imagery.

    Find out more

    • 20,000m2 Area of land analysed per second
    • 5x Quicker than manual approach

    Africa sponge city profiles

    This survey is not intended as a scorecard or an assessment of risk. For example, some cities may be less “spongy”, but have fewer heavy rain events to cope with. The snapshot is aimed at getting cities thinking more about nature as an asset and as infrastructure – to be retained, enhanced and deployed to help better cope with some of the greatest challenges facing us.

    Cairo
    Durban
    Kigali
    Lagos
    Nairobi

    Cairo - 20% spongy

    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 – mostly during the months of November through to March, 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 – 40% spongy

    A city on the east coast of South Africa, 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, built-up eastern central business district contrasts with dense tree coverage representing almost a third of the area across the steeper inland sections to the west. The city’s moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributed to its overall ‘sponginess’.

    Kigali – 43% spongy

    Kigali, the capital of Rwanda with a population of over 1.2 million, is located in an area known as the Central Plateau. Its 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 – 39% spongy

    Lagos, on the south-western coast of Nigeria, has a reported population of over 20 million. The city 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 – 34% spongy

    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 - 20% spongy

    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 – mostly during the months of November through to March, 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 – 40% spongy

    A city on the east coast of South Africa, 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, built-up eastern central business district contrasts with dense tree coverage representing almost a third of the area across the steeper inland sections to the west. The city’s moderately-high runoff soil classification, composed of less than 50% sand and between 20-40% clay, further contributed to its overall ‘sponginess’.

    Kigali

    Kigali – 43% spongy

    Kigali, the capital of Rwanda with a population of over 1.2 million, is located in an area known as the Central Plateau. Its 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 – 39% spongy

    Lagos, on the south-western coast of Nigeria, has a reported population of over 20 million. The city 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 – 34% spongy

    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’.

    Discover more

    Publication

    Global Sponge Cities Snapshot

    How spongy is my city? Why does it matter? Our snapshot is aimed at getting people talking. How can we move beyond concrete solutions to help cities cope with increasingly heavy rainfall and other impacts of climate change?