Smaller storms over time can cost more than extreme, infrequent events

Researchers find rising long-term impact of climate change on American cities

21 February 2017

Joint Release

The effects of climate change are being felt now in American cities. With the steady rise in sea level over the past few decades, events such as an especially strong tide can result in flooding in coastal regions, a new study finds.
Credit: FEMA.

WASHINGTON, DC — Global climate change is being felt in many coastal communities of the United States, not always in the form of big weather disasters but as a drip, drip, drip of nuisance flooding.

According to researchers at the University of California, Irvine, rising sea levels will cause these smaller events to become increasingly frequent in the future, and the cumulative effect will become comparable to extreme events such as Hurricane Katrina or Superstorm Sandy.

“Catastrophic storms get a lot of media attention and are studied, but we wanted to know more about the non-extreme events,” said Amir AghaKouchak, associate professor of civil and environmental engineering at the University of California Irvine (UCI) and co-author of a new study on cumulative hazards in Earth’s Future, a journal of the American Geophysical Union.

“These diffuse floods happen multiple times a month or year,” AghaKouchak said. “They don’t kill anyone, they don’t damage buildings, but over time they have extremely high-cost outcomes, and it happens without us realizing it.”

In Washington, D.C., for instance, the number of hours of nuisance flooding per year has grown from 19 between 1930 and 1970 to 94 over the last two decades. Projections suggest that there could be as many as 700 hours of nuisance flooding per year by 2050. The capital’s monuments, marinas, parks, public transportation infrastructure, roads and businesses could be affected. The UCI researchers found similar potential impacts in four other American cities: Miami, New York, Seattle, and San Francisco.

Climate change is driving the growth of cumulative hazards, the team noted. A full moon on a clear night triggering higher tides is enough to now cause flooding, because the ocean levels are so high.

“The frequency is going higher because of sea level rise,” he said. “We call it clear-sky flooding. There’s no rain, but if you have a higher than usual tide, the ocean level is already high, so you get flooding in these coastal areas.”

This graphic shows how higher tides are now enough to cause flooding, because sea levels are so high.
Credit: Jennie Brewton/UCI.

While not catastrophic at the time, these episodes degrade infrastructure and can damage roads and building foundations. More immediately, nuisance flooding forces municipalities to spend resources to pump water out of streets. Communities suffer school closures, traffic interruptions and reverberating waves of cost and inconvenience. Degraded sewer infrastructure result in heightened public health risks.

Hamed Moftakhari, a UCI postdoctoral scholar and lead author of the new study, said people in often-hit regions are growing somewhat inured to the problem. “In a recent social science survey, people weren’t really interested in knowing the depth of the water, they just wanted to know how long they would be flooded,” he said. “Their main concern was finding out when they could get back to their schools and businesses.”

But public officials can’t afford to take cumulative hazards in stride, said Richard Matthew, professor of planning, policy & design at UCI and co-author of the new study. Policy makers faced with limited capital funds often defer action or make incremental improvements, the researchers note, when major investments may be critical to fortify their communities. The team created a Cumulative Hazards Index that could accurately pinpoint which locations would experience the greatest long-term risk.

“This index gives them a tool that could potentially help them decide to move beyond the convenient but potentially very costly strategies of deferral and incrementalism, and promote more transformative policies where these make sense,” Matthew said.

Brett Sanders, UCI professor of civil and environmental engineering and co-author of the study added, “the index is particularly useful for predicting future hot spots for nuisance flooding across the U.S., where adaptation measures are needed the most.”


The American Geophysical Union is dedicated to advancing the Earth and space sciences for the benefit of humanity through its scholarly publications, conferences, and outreach programs. AGU is a not-for-profit, professional, scientific organization representing 60,000 members in 137 countries. Join the conversation on FacebookTwitterYouTube, and our other social media channels.

Founded in 1965, the University of California, Irvine is the youngest member of the prestigious Association of American Universities. The campus has produced three Nobel laureates and is known for its academic achievement, premier research, innovation and anteater mascot. Led by Chancellor Howard Gillman, UCI has more than 30,000 students and offers 192 degree programs. It’s located in one of the world’s safest and most economically vibrant communities and is Orange County’s second-largest employer, contributing $5 billion annually to the local economy.

Notes for Journalists
This research article is open access. A PDF copy of the article can be downloaded at the following link:

Journalists and PIOs may also order a copy of the final paper by emailing a request to Lauren Lipuma at [email protected]. Please provide your name, the name of your publication, and your phone number.

Neither the paper nor this press release is under embargo.


“Cumulative Hazard: The Case of Nuisance Flooding”

Hamed R. Moftakhari, Amir AghaKouchak: Department of Civil and Environmental Engineering and Blum Center for Poverty Alleviation, University of California, Irvine, Irvine, California, U.S.A.;

Brett F. Sanders: Department of Civil and Environmental Engineering, Blum Center for Poverty Alleviation and Department of Planning, Policy and Design, University of California, Irvine, Irvine, California, U.S.A.;

Richard A. Matthew: Blum Center for Poverty Alleviation and Department of Planning, Policy and Design, University of California, Irvine, Irvine, California, U.S.A.

Contact information for the authors:
Amir AghaKouchak: +1 (949) 824-9350, [email protected]

Hamed Moftakhari: [email protected]

Brett F. Sanders: +1 (949) 824-4327, [email protected]

Richard A Matthew: +1 (949) 824-4852, [email protected]

AGU Contact:

Lauren Lipuma
+1 (202) 777-7396
[email protected]

University of California Irvine Contact:
Brian Bell
+1 (949) 824-8249
[email protected]