Building resilience into urban water management

Manhattan, New York, USA © shutterstock/ GCShutter

Climate change is making New York City rethink its water strategy. Rohit Aggarwala tells a story that is being written all over the globe.

It’s already a year since Mayor Eric Adams named me as New York City’s Chief Climate Officer and Commissioner of the Department of Environmental Protection (DEP), which runs the city’s water and sewer system. This dual appointment acknowledges just how inseparable future climate and water challenges will be for New York City (NYC). Like many others, we depend on regular rainfall and regular tidal patterns to keep our city liveable. The infrastructure we have today was built in anticipation of continued past levels of precipitation, which provide us with enough water to drink without overwhelming our sewers; even our water-sharing agreements with our neighbours rely on sea levels remaining steady far downstream, beyond our city. Our changing climate has the potential to disrupt many of the basic precedents that informed the creation of our water system, and the same holds true for cities all over the world. Above all, it requires water utilities to realise that – like it or not – their mission has expanded to include the new task of managing climate impacts.

New York’s water patrimony

New York is blessed with a rich natural heritage. Since the city was founded in 1624, we have generally enjoyed a relatively mild, forgiving climate. Aside from winter snow, the city traditionally does not have a history of extreme rainfall, or frequent hurricanes or other natural disasters. Our rainfall has historically been highly reliable and spread out across the year.

NYC also has some of the best-tasting and most affordable tap water in the world. We get our delicious water from three reservoirs north and west of the city that hold water chiefly from the Catskill Mountains and Delaware River. The mountain ranges capture water and allow us a supply of water fed entirely by gravity. More than 90% of NYC’s water supply doesn’t need to be filtered, although we do run it through the largest ultraviolet disinfecting treatment facility in the world.

As the city began to grow during the 1800s, far-sighted officials began building reservoirs north of the city, along with long aqueducts to bring that water into Manhattan. Two enormous water tunnels, roughly 250 metres below ground, were built to connect the reservoirs to the water mains throughout the city. This growth in the city’s water system lasted into the 1960s, when the most recent reservoir was completed.

Environmental cost of growth

While NYC was smart about getting clean water into the city, we weren’t as forward-thinking about how to deal with our sewage. NYC released sewage into its own rivers up until the 1980s and released sludge into the ocean into the 1990s. The city’s long history of ample water supply had led to laziness and waste, including resistance to water metering. The city’s fiscal difficulties in the 1970s, when city government effectively declared bankruptcy, disrupted investment and maintenance in critical infrastructure. But as the city came out of fiscal crisis, it began serious reforms that have made DEP the modern water utility it is today.

A major part of that reform was an institutional one that ensured NYC’s water investments would be at least partially insulated from political pressure, which has enabled roughly $14 billion in investments to be made over the past decade alone.

Environmental transformation

An ongoing, 50-year project to build a third water tunnel will provide full redundancy of supply to virtually all parts of the city when it is completed in 2032. Massive investments into our 14 wastewater treatment plants have returned New York Harbor to health. Today, we regularly see whales, dolphins, sharks and seahorses in the harbour. This is a testament to the fact that we have dramatically reduced overflows from our sewer system.

With nearly universal water metering and extensive conservation efforts in place, New Yorkers have reduced per capita water consumption by roughly one third since the 1980s. Our reservoirs now have the capacity to store more than 500 days of water use. And despite being the most expensive city in the USA, New Yorkers’ water bills are only average compared with other cities in the country, at an average of one cent per gallon.

The challenge ahead

With this background, my role as DEP Commissioner should be quite easy; keep everything humming along and enjoy the benefits of good work from decades and centuries past. We should be able to focus on normal management: constantly improving customer service, managing repair cycles and capital maintenance, maintaining a motivated and expert workforce, and implementing new technology as it emerges.

Unfortunately, climate change has other plans for us. NYC’s climate is getting more hostile. As a coastal city, we face the risk of coastal inundation because of more frequent violent storms and rising sea levels. Ten years ago, Hurricane Sandy was our initial indication. In addition to the deaths of 40 New Yorkers, damage from the storm put unprecedented levels of organic matter into our normally very clean reservoirs and flooded many of our treatment facilities and pump stations, which are, in general, located on our shorelines and very close to sea level.

We have realised that we are highly vulnerable to changing rainfall patterns. The most intense rainstorm before 2021 had measured 4.5 centimetres per hour in 2004. Before that, the record had been four centimetres per hour in 1967. These records for hourly rainfall go back to 1943, but, in September 2021, Hurricane Ida brought rainfall that was twice as intense as these previous events, with 9.5 centimetres falling in one hour. The ensuing flooding killed 13 New Yorkers. Barely a month before Hurricane Ida, Hurricane Henri had set the record for the most intense rain event in the city’s history, at 4.9 centimetres in one hour.

While Ida was the most extreme, we’ve now seen two summers with regular rainfall at or approaching historical records. For the first time, we’ve begun to see how storm surges and repeated coastal flooding may be weakening specific designs in our sewer system. We recently had two sinkholes, only a year apart, in the same city block. This was caused by the pressure that these new intense rainstorms are creating and the weakness of one particular design in our sewer system. That sewer dated from 1916, so it served the city well for more than a century, but it had never been tested like it was recently. The reality is that extreme weather will create more situations where we find our infrastructure just isn’t designed for our new normal.

Climate change will also soon affect our water supply. Half of our water comes from the Delaware River, which flows through upstate New York and on to supply water to the city of Philadelphia and much of New Jersey. We are worried that rising sea levels will drive salt water up the river and reduce how much water we can divert without impacting other states in the region. Climate change is not only damaging our infrastructure, but also revealing interdependencies and patterns that we had not noted previously.

Time for action

All of this requires aggressive action. We are forced to continue the work started after Sandy to protect NYC from storm surges, with a network of barriers, both natural and mechanical. This extensive set of projects is far from complete and requires renewed urgency.

We must also develop an integrated approach to using green infrastructure to complement our sewers. Basically, we will need to build enough green infrastructure to equal the existing absorption capacity of our sewer system.

Our sewers are designed to handle between 4.5 and 5 centimetres of rain per hour. But Hurricane Ida gave us 9.5 centimetres. To prepare for an Ida-level storm, we need to build a green infrastructure network that can handle, on its own, as much water as the city’s entire sewer system built up over more than a century.

We know combating climate change will force us to work more closely with other city agencies and with the private sector, and invest huge sums of money rapidly and wisely. We have already started by building and planning for green-infrastructure manufactured lakes, called ‘blue belts’, that can hold stormwater until the system can process it. We’ve also started building ‘rain gardens’. In the past year, the city has finished construction on 2300 rain gardens, which can hold rainwater runoff that flows from roofs, driveways, patios, and lawns. We are also incorporating more porous pavement into projects.

We will also need to address the vulnerabilities of our wastewater treatment plants. We will be required to modernise and potentially consolidate existing plants to locations that are more secure and away from shorelines. Better predictive capabilities will also be required so we can see when patterns emerge and prepare for failures in our infrastructure. Climate change is pressuring our infrastructure in ways that we are not familiar with, and intuition alone cannot be our guide.

While we prepare for flooding, we must also figure out how we would handle a severe drought, which the north-eastern United States has not faced since the 1960s. This, in turn, will require us to alter the zero-sum approach towards shared water supplies that characterises so many of our traditional regional water disputes. NYC’s examples reflect the same priorities all water utilities are forced to embrace; building green infrastructure and natural barriers, cross-border collaboration, planning for extremes that fall outside our historical knowledge, and massive investments. What that says to me is that, around the world, the real question is no longer what we should do to tackle these problems, but how we go about doing it. This planning must be the core of the climate agenda.

A change in the mindset of water utilities will be required for us to fully prepare for the future. The water and sewerage sector are traditionally dominated by high-quality engineers who act deliberately and plan over long timeframes, but we now need to act more quickly. Everything from planning and engineering to our procurement and customer service processes need to be revamped to embrace speed and scale without sacrificing quality. Our climate is changing faster than we can currently respond. It will do us little good to get everything done decades after climate change has done its worst to our cities.

We need to think about science differently. Water has long been a field that is driven by science and engineering; but it is also often driven by intuition and experience. Changing conditions mean that intuition based on the past is no longer as solid a predictor of the future as it once was. So, we need to ensure that we are relying on current empirical data and analysis, and new technology, to predict how our systems will perform under situations for which we have no precedents.

We also need to create new group dynamics in watersheds. We must act regionally, not just locally, so that we don’t revert to the zero-sum political games of the past, but act as true partners who are all facing the same growing challenges and whose needs must be met equitably. In some cases, this will require new institutions. In others, it may simply require new mindsets. It may also require a new willingness to conduct joint projects or commit to new and unusual shared funding models. It will most assuredly require rebuilding trust where it has been lost.

Finally, we will need to think differently about funding.

In NYC, our user-paid utility model has served us very well for two generations. But there is an increasing realisation that basing payments only on drinking water consumption ignores the extent to which land use decisions drive stormwater management needs. We will have no choice but to wrestle with the question of who pays, and to develop equitable approaches that move towards a transition to a green economy.

It is a difficult challenge we have ahead of us. I am cognisant of the fact that, despite the challenges I list, NYC has it easy compared with cities where climate change is having more dramatic impacts on supply, and where geography and history have not provided so many advantages.

This is why it is so important that we collaborate across borders. NYC has benefited from collaborations with Thames Water in London, UK, Waternet in Amsterdam, The Netherlands, and the City of Copenhagen, Denmark, all of which have much to teach us in New York. NYC is particularly grateful to Copenhagen for hosting last year’s World Water Congress and Exhibition, where I signed a memorandum of understanding that further deepened our work together on green infrastructure. NYC is also fortunate to have worked with the Asian Development Bank, and colleagues in Fiji, Samoa, Brazil and Pakistan, to share our expertise globally. These are the kinds of exchanges that IWA exists to foster, and exactly the ones we must build.


As NYC’s Chief Climate Officer, I am often asked to list my most important climate initiatives. There are two: reducing carbon emissions and speeding up our procurement process (the latter often leads to raised eyebrows).

About 75% of our emissions come from buildings, so implementing NYC’s laws requiring buildings to become more carbon efficient will be key to this.

This is our only way to meet our Paris targets. Speeding up my agency’s procurement process does not sound like the kind of topic frequently discussed in climate circles, but it should be. At the most fundamental level, procurement reform is climate action, because if we cannot figure out how to select, design, procure and implement capital projects faster, there is no way we can make NYC resilient to climate change in time for it to matter.

Over the past several decades, climate action has changed rapidly from a challenge of policy to a challenge of implementation. Not too long ago, we were still talking about whether to even undertake climate action. Today, we are no longer asking whether to take action, but examining how fast we can do it and whether it will be too late. We are asking CEOs, project managers, lead engineers and facility chiefs: when and how?

Water systems inspire history. From the aqueducts of ancient Rome and the canals of ancient China to the water supplies of 20th century Los Angeles and the treatment systems of 21st century Singapore; these are great engineering feats that inspire marvel. But those will not be the water histories of the next generation. The historians who write about us, will not be focused on our engineering work. They will primarily ask the question of whether we were good enough as operators to act quickly enough today to keep the water flowing and protect our cities from the impacts of climate change. Our children will be asking the same question and living with the consequences of the answer.

Dr Rohit T Aggarwala is Chief Climate Officer of New York City and Commissioner of NYC Department of Environmental Protection. He gave a keynote presentation at the IWA World Water Congress & Exhibition 2022.