Cloudburst Management Comes to New York City 

This past month, reports flooded in of the deluge of rain hitting California. Californians were faced with dire situations, and at least 22 lives were lost. In all, over 30 trillion gallons of water fell in the state, according to a student scientist. But in a state with an unyielding drought, the majority of that water flowed right back out to sea. 

Back in 2011, the city of Copenhagen in Denmark experienced a similar – although short-lived – deluge, when 150mm – almost 6 inches – of rain fell in a two-hour period, the most rain in a 24-hour period in over 55 years. Coupled with a study from 2021 noting that “a large increase in intense, slow-moving storms” would happen in the future in Denmark and across Europe, the city began to develop a plan to manage the water from these types of short-burst intense rainfalls that are predicted to be more regular occurrences not just in Copenhagen, but across the globe, as a result of the climate crisis. 

The result was the Cloudburst Masterplan, a multi-municipality plan that will help to control the damaging effects of intense rain in Copenhagen. 

“In total there are more than 250 larger scale projects across the city,” said Martin Zoffmann, communication manager of Ramboll Water, the company that is helping plan and implement the master plan, in an e-mail. “The projects are interconnected in branches and [it] provides protection and improved livability throughout the city via it’s multifunctional and blue-green design.”

As an example: St. Annae Square, in central Copenhagen. This area, once a flood-prone field, has been redesigned so that the rainwater now heads out to the harbor. This has not only helped to prevent the water from pooling in the main square where people go, but also from damaging adjacent buildings. The area was redesigned into a bowl shape, and new storm pipes and gutters were built so that excess water flows out to the harbor. 

Ramboll is using what’s called ‘blue-green’ infrastructure in its designs. Blue-green infrastructure can be defined as “hydrological functions with urban nature, landscaping and urban planning” – in other words, planning with both water and land being considered together. 

And other flood-prone cities are taking notice. Another example? The Copenhagen Masterplan caught the eye of the NYC Department of Environmental Protection (DEP). Hurricane Sandy, though a once-in-a-lifetime event, showed that New York City was sorely lacking in water management and particularly in management in times of intense downpour. When the DEP got word of the Masterplan, they visited Denmark on five occasions to explore the project. Back in 2015, the two cities signed a cooperation agreement to improve climate resilience in both cities by sharing knowledge and technical information. And when Hurricane Ida hit the New York area with 10 inches of rain in a three-hour period in 2021 – the very definition of a cloudburst – it made the creation of some kind of heavy rain mitigation plan that much more urgent. 

During a visit in 2022, Alan Cohn, the managing director of Integrated Water Management at the NYC DEP, said, “What inspired me most this visit was Copenhagen’s climate adaption efforts integrated with urban renewal work. This includes pooling different types of expertise and resources to revitalize open space in communities, while incorporating cloudburst management in the process.”

What did this eventually lead to? In January of this year, NYC’s Mayor Eric Adams announced a $400-million expansion of the Cloudburst project that will target four flood-prone neighborhoods across the city: Corona and Kissena Park, Queens, Parkchester, Bronx, and East New York, with more to come in the years ahead. The infrastructure projects will incorporate grey and green infrastructure, which uses natural lands in combination with pipelines, reservoirs and treatment plants, to absorb, store, and transfer excess storm water. 

“Expanding our cloudburst programs is key in helping us protect New Yorkers from extreme rainfall, and making our city greener,” said Mayor’s Office of Climate and Environmental Justice Executive Director Kizzy Charles-Guzmán in a statement. “Green infrastructure projects are much-needed in environmental justice communities, and the proposed ideas will expand and improve access for pedestrians and cyclists.”

New York already has two Cloudburst pilot projects underway in Southeast Queens. These projects complement sewer and green infrastructure projects. The NYC Green Infrastructure 2020 Annual Report included a rendering of this Cloudburst project in a large NYCHA courtyard:

An article posted on Grist last October goes into detail about this plan, and how the New York City Housing Authority has been leading the charge in cloudburst management projects. 

So, this unique partnership between Copenhagen and New York City has resulted in shared knowledge and the implementation of climate resilience projects in both cities. Ramboll, the Copenhagen company, is also working with Miami, Singapore, and other cities to help with flood mitigation. And while the management of large bursts of water remains the focus, it is all a part of water management – and perhaps one day, California, with its hugely complex water management systems, can figure out a way to retain some of the water being lost back out to sea, while at the same time reducing the devastating impact of flooding a result of huge volumes of liquid.