Graphic designed by C40 that reads: "Is your city taking water action?". The graphic contains C40's iconic visual language with illustrations representing sustainable, wellbeing cities and water-related climate action.
© C40

More than half of the world’s population live in urban areas, which face water-related climate hazards such as water scarcity and flooding every day. They are threats to our communities and our future.

The impact will be disproportionately felt by those living in the Global South, who are ten times more likely to be affected by water-related disasters arising from the climate crisis.

C40 research shows that if cities take action today, millions of lives can be protected from the risks of severe flooding and drought by 2050, and billions of dollars in urban damages can be saved.

Mayors around the world are united in action to address water-related hazards, showing that real political leadership can save lives and make communities more resilient to floods and droughts. Keep reading to learn about city action that addresses water-related climate hazards.

Expanding Dhaka North’s green space to create a more livable city

Dhaka North is highly susceptible to flooding due to its low-lying geographical position, heavy monsoon rains and cyclones, deforestation, and the rapid expansion of urban areas. These hazards are becoming more unpredictable and intense due to climate breakdown.

Dhaka North is also one of the most densely populated cities on Earth. The city welcomes an estimated 2,000 new arrivals daily, 26% of whom are climate migrants fleeing from floods and droughts in rural areas to find a safer life. Urban density contributes to a need for more access to clean, green urban spaces for millions of residents.

Dhaka North is taking action to manage rapid urban growth and enhance its resilience to water hazards by boosting rainwater harvesting facilities and increasing urban green space by 70%. This move will improve climate resilience to flooding, increase permeable land, curb greenhouse emissions, and create more green areas for residents to enjoy.

Green space expansion on Oct. 6, 2022 in Dhaka North, Bangladesh. The city is increasing urban green space by 70%. The spaces will increase climate resilience to heat waves and flooding, reduce greenhouse emissions, and create areas that are both equitable and accessible. (Turjoy Chowdhury/AP Images for C40)
© Turjoy Chowdhury / AP Images for C40

Reducing flood risk and improving biodiversity through São Paulo’s rain gardens

São Paulo is at high risk for urban flooding due to the city’s landscape and increasing rainfall. To improve flood resilience, the city is converting concrete and asphalt areas into green, absorbent, multi-functional zones called rain gardens.

The gardens act as small retention basins for rainwater, and once full, excess water is diverted to the stormwater system. At present, the gardens are mainly located in the Central region as part of the city’s pilot project to test nature-based solutions for flooding.

The rain gardens provide green spaces that benefit local biodiversity and residents while helping reduce urban temperatures. São Paulo’s rain gardens also feature native plants, supporting diverse local fauna like frogs, insects, and birds.

Mitigation meets adaptation on Rotterdam’s rooftops

Facing climate hazards and high urban population density, Rotterdam is using its rooftops to address climate challenges and make the city more livable.

Risks such as flooding, air pollution, and a lack of green space are all addressed using a multifunctional approach that optimises space on top of buildings. Roofs are categorised by colours that represent unique adaptive functions: blue roofs retain water, green roofs add green space and biodiversity, and yellow roofs produce renewable energy.

Rotterdam, The Netherlands, June 2, 2019: section of the Dakakker (Rooftop Field) with a flower garden and a cafe on top of a 1960's office building
© franswillemblok / Getty Images

As of 2017, green roofs already reduced water treatment costs by US$75,000 annually, and 75,000 m2 of solar panels had been installed, preventing roughly 10,000 tons of CO2 emissions.

Phoenix and Tucson are united in action to secure water supplies

The Phoenix Metropolitan area has managed multi-year droughts before, but the city is taking action to be prepared for possible future water shortages. Working with the city of Tucson, the Phoenix-Tucson Water Partnership helps both cities increase water supplies during times of drought.

Tucson has access to large, clean groundwater stores that Phoenix lacks. Meanwhile, Phoenix has better access to ground that is water saturated compared with Tucson, but the city has more limited water storage options. 

The partnership means Phoenix can use Tucson’s pumping infrastructure to store more water while enabling Tucson to use less energy to pump water, as it can access Phoenix’s water stores in times of need. Phoenix stores part of its unused Colorado River entitlement at Tucson’s recharge facilities in exchange for accessing Tucson’s Colorado River water allocation during shortages.

Since 2014, Phoenix has stored 160,000 acre-feet of Colorado River water and expanded its storage facility. The partnership provides operational flexibility and cost savings for both cities.

Protecting Tokyo’s “green dams” to secure drinking water supplies

Tokyo has a long history of managing its water resources effectively. The city depends on forests in the Tama region, which covers approximately 10% of Tokyo’s area, for its drinking water supply. Rainfall in the forest is absorbed by the soil and slowly flows into rivers. This process stabilises the water level in the river, preventing extreme droughts. Because of this, the forest is known as a “green dam.”

A city view of the business district with skyscrapers, there is a green forest of the park in front of it.
© Kohei Hara / Getty Images

Protecting forested areas in the Tama region is a key water strategy for Tokyo, and both city leaders and community volunteers take an active role in their management through the Tama River Water Resources Forest Team. The city also purchased private forest land that could impact local water supplies, enabling the volunteer conservation programme to more effectively regenerate unkempt forest areas, ensure a stable flow in the Tama River, and conserve the Ogouchi Reservoir. 

Promoting forest preservation activities such as tree thinning and pruning is ongoing, helping the city protect urban nature and its drinking water supply and quality.

Restoring Durban’s rivers to improve flow and protect ecosystems

Durban‘s eThekwini Metropolitan Municipality is addressing climate risks in the Great Durban Area through river restoration and community-led management programmes.

18 major river systems intersect the city; most are heavily polluted due to solid waste and invasive plant species that thrive in Durban’s climate. The city also has large areas of impermeable surfaces, increasing the flood risk of the river systems and posing a severe threat to residents’ lives and city infrastructure. The city has implemented river management projects in response, focusing on river rehabilitation to restore flow and ecosystem function.

One example is the Sihlanzimvelo Stream Cleaning Programme initiated in 2011. The programme engages local communities by creating cooperatives to manage stretches of river and riverside corridors; each cooperative is responsible for the 5-kilometre stretch adjacent to their community. 

Durban (eThekwini) river restoration project site showing a riverside, trees, green space along a path and locals enjoying outdoor space.
© Durban eThekwini Metropolitan Municipality

The city also led the Palmiet River Rehabilitation Project, which focused on constructing riverside wetlands to filter runoff. As demonstrated during recent flooding events in Durban, the projects have significantly enhanced the city’s resilience.

Reducing residential water consumption in New York City

For more than 15 years, New York City has been working to improve environmental protection and resource conservation. As part of this effort, guidelines and incentives have been established to encourage water reuse projects.

One example is Solaire – the USA’s first residential high-rise building designed to be environmentally friendly and promote sustainable urban development. It was built in 2003 with an on-site water reuse system that collects rainwater and blackwater (from toilets) for reuse. The system is connected to New York City’s water system.

Solaire’s water reuse system has reduced water consumption by 50% and wastewater discharge by 56% compared to the average residential high-rise building in New York City. This has been achieved by integrating low-flow water conservation fixtures and reusing rainwater and wastewater for flushing toilets in the building’s 293 apartments.

Nanjing’s “sponge city” initiative tackles urban flooding and promotes sustainability

Nanjing is using “sponge city” methods to reduce urban flood risk by developing new areas and transforming existing neighbourhoods. 

The city has a pilot site – the Jiangdao Science and Technology Innovation Centre – which showcases sponge features such as green roofs and permeable pavements. The water storage facilities can recycle 4,500 tons of rainwater per year, saving 30-50% of water. Nanjing aims to upgrade more than 40% of its built-up areas to sponge city standards by 2025.

Focusing on the areas Jiangxinzhou and Dingjiazhuang, the city has completed 748 sponge projects, with 21% of the city’s built-up areas meeting sponge city requirements by 2020.

Mayor Chen Zhichang emphasised the importance of improving urban planning, construction, and governance to make the city more livable while strengthening ecological functions and preserving cultural traditions.

Water Safe Cities: A blueprint for climate action

C40 is working with cities worldwide to promote innovative and sustainable solutions, increase awareness and education, and build a culture of water resilience.

On 22 March 2023, C40 Cities, Grundfos and The Grundfos Foundation will host an event at the UN Water Conference alongside key partners International Water Association, W12+ Programs and the Global Water Operators’ Partnerships Alliance to discuss challenges and actionable steps cities can take to become more resilient to water-related climate risks.

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