Nuisance floods, now routine in some coastal areas, receive far less attention than big weather disasters, such as hurricanes and tornadoes, but they could wreak as much costly destruction over time, according to a new study.

Here are 6 things you should know about this low-profile, unfolding threat, even if you live well inland:

1. What causes nuisance flooding? Sea levels have risen in the past few decades. That means high tides are, well, higher. When a full moon or new moon coincides with high tide, seawater routinely reaches greater heights, thanks to the combined gravitational pull of the moon and sun. Salt water relentlessly pushes up through drains to swamp roads, sidewalks and lawns.

2. So, what’s the big deal? Over time, the cumulative cost of battling more frequent nuisance floods may in fact exceed the cost of dealing with extreme weather events that happen less often.

“These diffuse floods happen multiple times a month or year,” said Amir AghaKouchak, associate professor of civil and environmental engineering at University of California, Irvine, and co-author of the study on cumulative hazards published in the American Geophysical Union journal Earth’s Future. “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.”

3. What’s a nuisance flood look like? These events, also called “clear-sky” or “sunny sky” floods, don’t trumpet their arrival with dark clouds, violent wind or heavy rain. Pools of water just… show up. And then show up again sometime later and maybe deeper. Communities afflicted by come-and-go floods use pumps to remove water from streets and sidewalks that become impassable. Traffic is rerouted. Neighborhood schools might close. Roads and building foundations may erode.

4. Where is it a problem? Coastal areas. The UCI researchers created a measure, the Cumulative Hazard Index, to pinpoint future hot spots for nuisance flooding across the U.S. The hazard index was positive in five major metropolitan areas – Miami, New York, Seattle, San Francisco and Washington, D.C. – meaning the cumulative exposures to minor floods in these cities equals or exceeds the threat from extreme weather events.

In Washington, D.C., for example, 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. 

5. What’s the response so far? Many of us don’t even recognize a nuisance flood when we splash into one. People in affected areas tend to focus on how long they will be inconvenienced rather than how deep the water is, according to Hamed Moftakhari, a UCI postdoctoral scholar and lead author of the UCI study.

Municipalities typically take lower-cost, incremental steps to drain the excess water rather than making plans to fortify communities for the future and deter the rising tide, said co-author Richard Matthew, UCI professor of planning, policy and design.

6. What’s next? The authors of the study believe policymakers need more tools such as the Cumulative Hazard Index to help figure out when low-cost incidents are likely to aggregate into high-cost catastrophes.

Harnessing “big data” to monitor CHI nationally and internationally, they concluded, could help to identify locations where high-frequency, low-impact problems like nuisance floods are expected to be most severe. It can also help policy makers move past deferring or taking minor steps to remediate problems and instead pursue more transformative solutions.