Water Safe Cities

People knee-deep in water. Houses flooded. Cars submerged.

For many in Africa’s second largest city, Lagos, this is the reality of the rainy season.

Rains cause much of the flooding in Nigeria’s economic hub, but the city also sits on the west coast of Africa at the confluence of several rivers, which creates the perfect conditions for a triple flood threat – riverine, coastal and stormwater.

Moreover, the city has developed rapidly in recent decades, in part because Lagos is home to a high number of people who seek greener pastures primarily for survival of their families left in the hinterlands, as well as those displaced by dramatic weather-related events. In 2018 alone, 50,000 people moved to the capital from elsewhere in Nigeria due to climate migration – the largest such movement of people in Africa. All of this has an impact on the city’s economy, as the city will have to recover from the damage inflicted on its infrastructure and services. Our analysis shows that Lagos will be the most impacted C40 city in Africa.

What's happening in Lagos is familiar to many cities.

In the following maps, you will discover how C40 cities are set to see an increase in the different types of flooding that occur in urban areas and what this means for people and public funds. You will also see, however, how cities are working hard to keep their populations water safe.

Cities around the world are no strangers to riverine flooding.  Venice ,  São Paulo ,  Sydney  and  Kuala Lumpur  are just a few examples of cities that have experienced inundation that has brought them to a standstill over the past three years.

In 2021, the Tiete and Pinheiros rivers in São Paulo burst their banks after parts of the city saw 10 cm of rain in just three hours, causing gridlock on transport routes across the city. In December 2021, the four main rivers running through Kuala Lumpur hit dangerous levels, causing nearby rail lines to shut down and halting mobility across the city. In March 2022, storms prompted flood warnings along Australia’s east coast, including in Sydney. Thousands of residents had to flee their homes.

By 2050, C40 cities will experience combined riverine flood volumes of 10.5 million m ³  annually.

That's equivalent to four-and-a-half times the volume of the Great Pyramid of Giza.

Here’s how C40 cities will experience riverine flooding by 2050 if we maintain a business-as-usual scenario.

By 2050, more than 7.4 million people living in at-risk areas of the 97 C40 cities will be exposed to riverine flooding annually.

The five C40 cities with the largest amounts of riverine flooding are in South and South East Asia, but nearly all C40 cities are set to see a dramatic increase between now and 2050. Cities in Latin America see some of the most dramatic increases by 2050.

The five cities with the highest population exposure to riverine flooding are all in South and Southeast Asia. These densely populated cities mean people face greater exposure to flooding and a larger risk to human life. While the populations in these cities are significant today, climate migration projections show that they are also set to see significant increases by 2050. Dhaka, alone,  receives 2,000 migrants a day .

What’s more, the impact on populations in Global South C40 cities will be 10 times that on populations in Global North cities.

Those living in informal settlements face a greater risk of damage to their homes and livelihoods. In addition, migrants inside cities, often face greater vulnerability, as they trade one set of risks for another when they leave their homes, then settle into vulnerable housing in informal settlements made of fragile materials, often in high-hazard-risk areas. For others, flooding is a significant disruption  rather than a livelihood threat, as much of a city can be brought to a standstill and people cannot go about their daily lives.

Urban damages from riverine flooding will also more than treble to US$64bn every year and impact US$136bn of GDP production annually.

Equivalent to the annual GDP of Croatia and Kuwait, respectively.

Damage costs are highest in North American and Asian cities These regions are where infrastructure and urban property are most at risk and where the damage is the most expensive, owing to their high valued assets. Although certain regions will experience greater impacts from flooding than others, cities in Europe, Latin America and Africa will see large increases in the number of people exposed to flooding and damages. Across all regions, several cities see the costs of urban damages more than double by 2050.

Green and grey: Riverine flooding solutions Cities are tackling the issues associated with riverine flooding by implementing river catchment management programmes. Oslo has developed a river basin plan, for instance, while Singapore has undertaken the  downstream naturalisation of the Kallang river , which runs through the city’s central Bishan Mio Park, preventing the frequent flooding that nearby roads used to experience.

In November 2021, children waded through murky brown water on the streets of Bengaluru and elderly residents were trapped in their apartments. The city was hit by more than 118mm of rainfall in 13 days, double the typical amount for that time of year. Four lakes in the city overflowed, causing flooding. Water poured into the streets and surrounding neighbourhoods. Dwellings were waterlogged for three days and more than 200 people were evacuated.

The map shows how often intense rainfall events will increase in your city by 2050.  

By 2050, 39% of C40 cities will face a severe increase in stormwater flooding under a business-as-usual scenario. That compares with 18% under  a moderate climate scenario . The C40 cities facing the greatest increase in stormwater flooding lie across Africa, South Asia and Latin America.

Transport impacts Extreme rainfall events can bring cities to a halt. Cloudburst events, where intense rain falls in short periods of time, cost  Copenhagen DKK 1.3 billion in 2011 , for example, prompting the city to invest in a major cloudburst management plan. Stormwater floods often cause major disruption to the mobility of a city by impacting the transport sector. In recent years,  Washington DC,   Madrid  and  Paris  have experienced intense downpours that flooded their metro stations. Sydney’s simulations of stormwater flooding found that the flooding of one rail corridor alone would  disrupt 70% of the city’s trains . As the rail corridor is linked directly to the central business district, floods could cause huge economic losses, not to mention the cascading impact on substations, the motorway and the airport.

Capturing run-off Flooding is a complex threat that cities are navigating. Cities will have to protect their critical infrastructure from the damage stormwater flooding can wreak. In the United States of America, every US$1 invested in flood resilience actions saves up to US$318 in damage. Adaptation measures can also lead to employment gains and prevent job losses. In Europe, around 500,000 additional jobs will be created by 2050 if adaptation work boosts infrastructure development.

Lagos is another city that is taking action. In addition to developing a stormwater management plan and city-wide drainage network, it is working with local communities that live informally along river channels, educating them so as to prevent improper waste disposal into rivers. The city is also planning nature-based solutions by promoting depaving and encouraging the use of permeable surfaces. This will improve natural drainage and capture rainwater run-off for reuse. The city has ambitious aims to incorporate every household into it’s sustainable drainage plan. ‘If every house is able to store rainwater, we can hold 90% of the rainwater run-off in Lagos,’ Permanent Secretary of Drainage Services in Lagos State, Shodeinde Nurudeen Okalekan.

Other cities are also developing innovative nature-based solutions to tackle flooding from rainfall. Buenos Aires is implementing a depaving strategy to increase the city’s water absorption, while New Orleans and New York are building rain gardens and bioswales.

 While such cities are adopting nature-based solutions, others are strengthening their emergency response plans, like Rio de Janeiro. The city monitors real-time weather data at its Operations Centre and coordinates response and evacuation actions to safeguard people living in areas at high risk from landslides and stormwater flooding.

Coastal cities are also at risk from rising sea levels and storm surges. Ever more frequent and intense weather patterns mean hurricanes and storm surges can lead to sudden flooding events along city coastlines, as seen in New York during Hurricane Sandy and, more recently, in Venice. In 2019, Venice experienced high tides that submerged 85% of the city. Homes, businesses, restaurants and ancient monuments suffered irreparable damage. The total cost to the city was  US$1.1 billion .

Rio de Janeiro is another C40 city with high coastal flooding risk in future, although global data for the city are sparse. 'By the end of the century high tides would see 10% of the city area flooded twice a day,' says Felipe Mandarino from the Instituto Pereira Passos, Rio de Janeiro City Government. The city is taking action, however, partnering with NASA to produce detailed projections of how the city will flood in order to plan for future scenarios.

Urban damages from coastal flooding in C40 cities will reach US$19 billion by 2050, more than four times current levels.

When it comes to coastal flooding, Asian cities once again have the largest exposed populations and are also set to see rapidly increases by 2050, much of it due to climate migration.

Natural and artificial barriers Coastal cities are also taking action. Ho Chi Minh City has implemented coastal nature-based solutions, with the Can Gio mangrove forest protecting the city from coastal flooding. Wetland restoration is a critical part of the Louisiana Coastal Master Plan, protecting the city of New Orleans. During Hurricane Sandy, wetlands helped prevent an estimated US$625 million in property damage. US coastal wetlands provide storm protection worth an estimated US$23.2 billion per year. Rotterdam, in contrast, has erected critical artificial barriers against flooding. The city’s Maeslant barrier protects it from in a 1-in-10,000-year coastal flood.

The above examples of climate risk show how the hydrology of a city is interconnected, often beyond city borders. Many protection systems (levees, dykes, etc.) and reservoirs that provide water to cities are managed at a national or regional level. Mayors will need to form partnerships at various levels to address the water risks their cities can face. But cities can’t do this alone. National governments and regional groups that manage water risks across cities, counties and countries will also have to act to protect cities through integrated water management.

• Develop an emergency response plan to protect vulnerable citizens from flooding.
• Implement river catchment management (for example, river basin plans, infiltrating and retaining water in upper catchment areas, renaturalising rivers and creating buffer protection for rivers).
• Implement green solutions for water permeability and flood protection (SUDs) (such as floodplains or green riverbanks, bioswales, rain gardens or depaving).
• Implement coastal nature-based solution barriers (such as mangroves).
• Implement artificial barriers against flooding (such as seawalls or flood gates).

 Los Angeles County is in the midst of a severe drought.  Between 2012 and 2016, both it and the wider state of California experienced their most severe drought in 1,200 years. In 2022, the state recorded one of its driest Januarys in history with drought conditions impacting the whole City of Los Angeles.

Water is essential to a city’s daily life. Without it, everyday activities such as drinking water, hand washing and cleaning become restricted. Water restrictions have impacts on the health and wellbeing of citizens, particularly low-income communities, and racial and ethnic minorities, making diseases and viruses more spreadable.

For some cities, such as LA, drier conditions also exacerbate wildfires. Forests burn for longer and more intensely, as dry, hot and windy weather, combined with more flammable vegetation, increase the probability of large-scale wildfires. This has knock-on effects on the environment, health and economy – damaging ecosystems, creating health problems and destroying homes, buildings and livelihoods.

Water use is also high in cities for industrial processes and businesses. The average LA resident uses 70 gallons of water a day, but adding business and industry to the mix increases water use to 110 gallons. Restricting water consumption can hinder the city economy. Instead, many cities retrieve water to make up for the loss of supply and the increase in demand that comes with a growing population. ‘We spend somewhere between US$2 billion and US$3 billion dollars per decade on purchasing wholesale water,’ says David Pettijohn, Director of Water Resources at Los Angeles Department of Water and Power. Heavy drought periods, combined with groundwater contamination, environmental requirements, and population growth, means Los Angeles has to purchase around 70% of its water ‒ and it is not the only city that needs to do so.

By 2050, C40 cities will face surface water losses of more than 16.1 billion m ³  a year.

That’s equivalent to Sydney Harbour drying up 30 times over.

The map tells us which C40 cities are at greatest risk of surface water losses by 2050.

The map tells us which C40 cities are at greatest risk of surface water losses by 2050. North American cities see the greatest shortfalls in subsurface water, but across all regions, various cities see large water losses per capita.

For cities that are missing from the map due to insufficient data, we’ve included a qualitative analysis below and have also looked at agricultural drought.

Equivalent to 200,000 olympic swimming pools.

By 2050, C40's African cities will experience the highest levels of agricultural drought relative to their surface area. One-fifth of global food is produced in cities, so agricultural drought has a direct impact on urban farming. It also has an indirect impact, as declining crop yields outside cities increase food prices. Once again, those who are most vulnerable will be affected most. In many Global South cities, lower crop yields will devastate livelihoods in informal markets, while inhabitants will be unable to purchase higher priced food, leading to food insecurity.

Drought is not a sudden event. There are a series of long-term factors leading up to it. Unlike Cape Town, whose extreme drought of 2018 left the world waiting to see whether it would experience a ‘‘day zero’’ event, Johannesburg has not yet experienced a drought. But it is dangerously close.

Between 2014 and 2015, South Africa’s Gauteng province came very close to drought. Just as Johannesburg was about to really feel the impact of drought, the city had some grace. However rains meant consumer habits were back to normal again with high consumption demand. Things will only get worse if the city is caught unprepared. When the risk of drought is high, Johannesburg enforces water restrictions and has to rely on other dams (often outside of the city and greater South Africa) to supplement the water supply to meet the city’s everyday needs. There are huge pumping costs involved in doing this.

Johannesburg is the economic hub of South Africa. A severe drought and restrictions would have knock-on effects on the whole country, similar to those seen during the 2018 Cape Town drought. The Western Cape province, meanwhile, saw an economic loss of ZAR 5.9 billion (US$400 million) due to smaller agricultural yields alone, as water use had to be cut by 60%. Tourism and industry across the country also saw a decline in terms of economic output.

 The cost of drought Drought not only impacts food security, but other key services, including health, sanitation and business.

By 2050, it will cost C40 cities a combined US$111bn a year to replace the water lost from subsurface levels during drought.

Below are the per capita costs for each C40 city

Drought costs are standardised by comparing cost relative to a city’s population. Once again, cities across all regions have large costs as represented by the circles. North American cities face the highest absolute costs of replacing subsurface water losses. The C40 cities with the highest increase in drought costs between now and 2050 are all within different regions (Latin America, Europe and South Asia).

Understanding water scarcity A separate but relevant water issue is that of water scarcity, which can often be exacerbated by drought. Freetown is one city undergoing water scarcity, particularly in its informal communities, home to densely packed and vulnerable populations. Isolated communities in the eastern part of the city often lack sufficient water. Increased migration to cities will also have an impact on the supply-demand balance. Freetown’s population has more than doubled over the past 20 years, primarily due to conflict and climate change. The city’s continued population rise means higher water demand that will not be met by the city’s main water supply, the Guma Valley.

When people cannot get water, there are devastating consequences. ‘We have seen instances of gender-based violence towards women (in Freetown) when they are out collecting water for their households,' says Adams Sesay, Water Coordinator for the Mayor’s Delivery Unit. Women and girls also miss out on education, as they spend significant amounts of time collecting water. There are also impacts on healthcare in such communities. Minimum levels of water access and sanitation mean people are exposed to cholera and other diseases.

Alternative water solutions Cities will need to manage drought through supply and demand solutions, while also assisting the most vulnerable. In Global South cities such as Freetown, city leaders are working with those at greatest risk of water shortage. Mayor of Freetown Yvonne Aki-Sawyerr is working with the city council and the Women4Water initiative to set up community-based water kiosks to increase access in the most vulnerable, low-income areas. The scheme has already received funding for 25 kiosks and plans to scale this up to 83.

Los Angeles has reduced demand for water through several measures. It is encouraging citizens to abandon traditional lawn culture and choose drought-resilient native plants, already replacing more than 51 million square feet of turf with drought-resilient plants (details of which can be found on a city website). The city is promoting conservation by providing rebates to people who install water-efficient devices in their homes. LA also imposes outdoor water-use restrictions to manage demand, with fines and ordinance measures for those who do not comply. The result? Water use is the same as it was 50 years ago, despite the addition of well over a million people.

 Johannesburg is also exploring alternative water sources. A stormwater catchment plan would boost water provision for 650,00 people by capturing the rainfall that drains out of the city due its high geographic position. The city is also looking to increase water supply by accessing groundwater safely in a collaborative scheme that incorporates privately drilled boreholes.

• Implement a water conservation behavioural change programme
• Implement water conservation solutions to increase water availability
• Implement water-system efficiency improvements (for example, reduce non-revenue water) to reduce water supply risks

Almost two years into the Covid-19 pandemic, London’s hospitals continue to work on recovery following the challenges caused by the virus. For some, this has been compounded by the impacts of climate change. July 2021 saw the capital hit by severe flooding after intense downpours, prompting 3,000 calls to the London Fire Brigade in just one day. The main site of the Royal Free Hospital was heavily impacted by flooding. Then Whipps Cross and Newham Hospitals had to divert accident and emergency patients to alternative facilities as three wards were left without power. 43% of London's hospitals are at risk of flooding in future. 'Even in a city like London, there’s still a lot of work to do,' says Shirley Rodrigues, Deputy Mayor for Environment & Energy, City Hall. An important reminder that no city is immune to the impacts of flooding. The pressure on hospitals and healthcare systems from extreme climate events is a familiar scene to many cities and one that others will face in future. In 2012, storm surges from Hurricane Sandy hit several New York hospitals, inundating back-up generators and requiring more than 200 patients to be evacuated.

By 2050, more than 2,400 hospitals and healthcare facilities in C40 cities could be overwhelmed by flooding.

Cities must take disaster events into account when planning their healthcare systems and hospitals. With their highly dense populations and greater need for services, cities are likely to face increasing pressure in future as flooding and drought have a greater impact on healthcare systems. Not only will these floods potentially claim lives, but they will also have direct and indirect healthcare impacts. Cities are directly impacted when hospitals are flooded, as seen in London in 2021. But they are also indirectly impacted when flood waters mix with sewage contamination and cause severe illness.

Hospitals at a standstill Drought can have direct and indirect impacts on a city. A lack of adequate water supply from drought can directly impact hospitals and healthcare facilities in cities. Hospitals need water to function – for cleaning wounds, surgical procedures and, of course, hand washing and sanitising to prevent infection and the spread of disease. Two major hospitals in Johannesburg struggled under the weight of the Covid-19 pandemic in 2021, when nearby reservoirs were unable to serve both sites, making the treatment of patients increasingly difficult. Cities will experience similar circumstances as surface water losses are set in increase through 2050.

To make up the water losses that cities face, groundwater extraction is often a sought-after solution for private water use and industry. Such was the case in Johannesburg, after a local NGO had commissioned a borehole drilling project to supply the hospital with emergency water , indicating the alternative measures private companies and organisations take to supply water. However, the practice is controversial. It can lead to subsidence and cause large areas of land to sink, in turn exacerbating flooding and prompting buildings to collapse.  Nine of the top eleven cities at risk of sinking by 2100 are C40 cities. 

Reduced water supply from drought and scarcity can impact population health and place a burden on city hospitals. When clean water is not available for drinking, hand washing or general hygienic practices, water-borne diseases and viruses can spread easily within dense areas. This is particularly prevalent in informal settlements, where housing and water and sanitation facilities are cramped and often shared and where populations are expected to rise further.

Hospitals and the healthcare sector, in turn, are seeing increased numbers of patients and facing stress on resources.

Some regions will inevitably face more pressure than others, making emergency plans for healthcare facilities and systems critical.

 The impacts on healthcare of flooding and drought have knock-on effects on society more broadly. An inability to access treatment often traps people in a cycle of not being able to live a healthy, happy life, causing emotional stress, but also leaving them in a situation where their livelihoods are jeopardised by not being able to work due to illness. This also impacts the wider city and its economy.

Incorporating climate risk Cities will have to react swiftly and plan emergency response protocols for such events. Within days of the 2021 floods, Mayor of London Sadiq Khan did just that, convening a group of senior leaders from the UK Environment Agency, Thames Water, the London Resilience Partnership, London Fire Brigade and London’s boroughs to work on a coordinated strategy for flooding. Mayor Khan has invested over GBP 20 million (US$26.2 million) in programmes to increase London’s climate resilience.

New York’s Climate Design Guidelines apply scientific climate projections to design interventions for buildings and infrastructure. The city can use sea-level rise projections from the guidelines to adjust a design according to the asset’s expected useful life. Any stakeholder can apply the guidelines to ensure that building and infrastructure designs are based on climate projections.

These examples of emergency response and resilience planning are the kind of actions mayors will need to take to safeguard critical healthcare infrastructure and services. More examples of how to create a climate-resilient system of infrastructure can be found in  C40’s Focused Adaptation  report.

• Incorporate climate risk into urban planning to tackle the impact on hospitals and healthcare infrastructure. e.g., limit the construction of hospitals and other critical infrastructure in high-risk areas.

When Hurricane Harvey hit land in 2017, some 300,00 people in Houston, Texas were cut off from communication and plunged into darkness. High winds and rapid flood waters swept away and damaged power lines. The 800-year flood event also caused blackouts in the city of Austin, leaving more than 25,000 homes without power.

Cities in the US are no strangers to energy disruptions from stormwater flood events. More recently, in 2021, Hurricane Ida left 1.2 million US homes without electricity across eight states, including the city of New Orleans. Some 56% of service stations across the city were also left without petrol, leaving thousands of people stranded for days and sometimes weeks.

More than 300 energy generation facilities in C40 cities are at risk of flooding by 2050.

Flooded power facilities Energy supplies will be impacted around the world, particularly in the US and China. Cities account for more than 75% of global primary energy use, making them particularly vulnerable to energy disruption. Most power generation facilities consider flood protection when they are built, but flood models show that the world is set to experience more intense, larger floods, more frequently. Flood and storm surges impact energy in several ways. Rising water can submerge energy infrastructure if they are not flood protected. Transformers and transmission wires can also be damaged or destroyed by high winds and waves.

Energy disruption in cities In March 2022, floods cut off power for tens of thousands of people across New South Wales and Queensland, Australia, affecting  54,000 homes  in the city of Brisbane alone. The knock-on impacts have also been felt across vital city services and sectors. Drought can also cause disruption to energy generation, as water is needed for hydroelectric generation, which accounts for 6.4% of the world’s energy. Cities in China, Brazil and Canada could be the worst hit, as these countries are the greatest producers of hydroelectric power.

Much energy generation takes place outside a city’s borders, but plants will still feel the impacts if drought strikes. The Zambian capital, Lusaka, gets 94% of its electricity from hydropower. In 2017, low levels of rainfall on Lake Kariba and an increase in demand for power left the city facing a power deficit of 985 megawatts, resulting in load shedding and blackouts. The Itaipu hydroelectric power plant on the Parana River between Paraguay and Brazil has faced similar problems.

In August 2021, critical reservoirs providing hydroelectric energy across Brazil had fallen to just 10‒15% of required levels. The country experienced its worst drought in more than a century. Sixty percent of Brazil’s power comes from hydroelectricity, meaning many of its key cities experience electricity disruption when drought risks are high. In drought conditions, the grid shifts to more expensive, carbon-intensive, fossil-fuel electricity, impacting the economy and adding to greenhouse gas emissions. Energy is often sourced from outside city borders, but disruptions from floods and drought can impact the infrastructure that transports energy and electricity to cities.

Transport, water and waste treatment, telecoms, food distribution, healthcare and businesses are critical sectors that experience disruption when energy supply is not available. Cities will, therefore, have to identify and address the climate risk to interdependent infrastructure. C40’s past work on  understanding interdependencies  delves deeper into how cities are planning resilience in the face of interdependent risk.

Safeguarding vital services Mayors need to act now to protect the vital services that serve billions of urban dwellers and keep cities running. Energy and healthcare are just a few of the critical sectors vulnerable to flooding and drought.

To tackle disruption to electricity supplies and protect hospitals from the impacts of climate change, cities need to integrate climate risk into city and infrastructure planning to avoid malinvestment. Data is crucial in this regard. Cities will need accurate local projections and data to know who is at risk, what the risk is, and how to prevent the impact. Such data will inform policy decisions and adaptation projects, but cannot be left to cities alone.

Research institutions, universities and private stakeholders, such as utility companies and insurers, also need to be involved. They will need to share data and forecasts and incorporate these into plans to build new energy and health infrastructure. They will have to incorporate future flooding and drought scenarios, so they don’t build in high-risk areas. Existing energy infrastructure, meanwhile, should be assessed and retrofitted to accommodate higher levels of flooding and redundancy should be built into drought scenarios.

Cities will have to plan robust emergency response protocols, so that when floods and droughts impact electricity supply or other critical sectors, the city has a back-up plan. Rio de Janeiro has set up emergency shelters in public buildings across the city where citizens can find refuge from floods and landslides following extreme rain. Many shelters have back-up energy generators, should there be power disruption to homes and buildings, so people can charge their phones and reconnect online.

Finally, the hydrology of a city is interconnected. Water governance is not restricted to a city's borders. Mayors and city decision makers will need to work with national governments and often governments at the regional level. For instance many cities take their water sources from outside of the city and sometimes from different countries. Therefore coordination is important to protect against the impacts of flooding and drought from these shared water resources and to keep people and cities water safe.

• Implement critical system efficiency (for example, power, transport and water) to build resilience.
• Flood-proof critical infrastructure (such as energy systems) to protect against sea-level rise, coastal storms and inland flooding.

To understand the data used to create the interactive maps and key messages, please see the Technical Methodology and refer to the FAQ document which you can access in the buttons below to download.

The above narratives highlight the water risks to cities, however the Water Safe Cities technical assistance programme has also worked to develop the solutions that cities are implementing to safeguard people, infrastructure and services.