China is home to some of the planet’s fastest growing cities. In fact, the percentage of China’s total population that lives in cities has risen to over half. To balance this rapid growth of urbanisation with the increasing frequency of extreme weather events the government has invested more than $12 billion in a programme to develop over 30 ‘sponge cities’ which aim to use features such as pervious pavements, urban wetlands, rain gardens, collection wells and green roofs to absorb and recycle water during storms. These features are designed to gradually release water, preventing heavy rainfall from overwhelming urban draining systems or local waterways and reducing the severity of flooding.

 While this system has worked on some levels, recent extreme weather events have exposed its weaknesses. In 2021 heavy rainfall in the city of Zhengzhou – where the government has spent more than $80million implementing the sponge city programme – quickly overwhelmed the areas designed to absorb water. Experts have noted that although these features are effective at absorbing infrequent heavy rain and more frequent lighter rain, they cannot withstand the frequency and intensity of rainfall events which are being experienced.

Enghaveparken in Vesterbro, Copenhagen provides valuable urban green space and flood resilience. Built in the 1920s, the park required renovation and the city council used this opportunity to design an innovative flood park, in keeping with the original neo-classical design. It has a 22,600m³ reservoir to store water during heavy rain events, holding up to 6 million gallons during 100-year rain events. At the foot of a hill, existing topography is utilised by a dyke on the west-east slope slowing water during heavy rain and acting as a seating area for visitors during drier times.

Rainwater is collected from surrounding rooftops and a closed piping system transfers this to a retention basin under the park. After passing through soil filtering and UV treatment, the water is used for a children’s play fountain, absorbed by trees and plants and street cleaning. The park integrates basins with leisure facilities, such as multi-purpose sports pitches that also act as water storage facilities. Submergible plant species have been chosen for planned flooding areas. After being cleaned, the park is reopened to the public.

The renovation was a collaboration between the municipality, Områdefornyelsen Vesterbro (municipal district renewal initiative) and HOFOR – Greater Copenhagen Utility. Several foundations contributed to the project and residents were invited to be a part of the project too.

Constructed in 2005 to channel stormwater and reduce flood risks, the 8-hectare site can contain 165,000 m³ of water. Poor management of wastewater has led to 70% of it now being untreated wastewater, causing significant problems for residents of Grand Yoff. Heavy rains can cause the water retention basin to overflow, flooding the neighbourhood with unhygienic waste. 

A partially submersible urban park has been proposed, for recreation during dry months and water retention during the rainy season. The $13m project will reduce flood risks and associated economic losses, limit disruption of social and economic life, improve living standards for Grand Yoff residents, improve solid waste and wastewater management whilst also creating a valuable GI recreational space. Cities like Dakar can be caught in a cycle of repairing flood damage only to be faced with further damage after the next heavy rains, integrating flood management into urban planning can reduce poverty, protect municipal budgets, enhance service delivery and infrastructure development. This can be particularly valuable in cities that function as the economic powerhouse of their country.

Bordered by the East River on one edge, New York is vulnerable to changing tides and extreme weather events such as Hurricane Sandy in 2012 and Ida in 2021. To help address this vulnerability City Officials have implemented a range of flood-prevention measures including the creation of Hunter’s Point South Park- an 11-acre floodable park located in Queens.

The park includes hidden channels to help storm water drain away, paving areas constructed with porous concrete and marshland to help absorb the high tide which rolls over parts of the park twice a day. To avoid overloading the urban sewer system, the park instead takes a “catch and release” approach, purposefully collecting water in certain areas and using a planned run-off system to drain it at a steady pace. The park also includes wooden ‘rafts’, kayak launch sites and a playpark. This former industrial area successfully integrates resilient planting with hard landscaping and flood defences and has been heralded as “a new model of urban waterfront development”.

However, it’s important to note that such developments are not without controversy. A similar project which plans to rebuild Manhattan’s East River Park to make it a buffer from storm surges has sparked a fierce debate over the impacts of these projects on Green Gentrification.