Flooding hit Raleigh, North Carolina this week, killing at least one person.

More Frequent Smaller Floods Lead to Higher Costs Over Time

Major flooding that hit Raleigh, North Carolina, and surrounding areas this week is the type of catastrophic event that typically makes the news. But a recent study suggests that smaller floods of just a few inches can be more damaging and costly over time.

Flooding hit Raleigh, North Carolina this week, killing at least one person. Visual: Raleigh News & Observer / Contributor

The threat of these events varies widely by region — depending on the proximity of buildings to waterways, land subsidence, and weather patterns. The researchers therefore created a cumulative hazard index as a tool that coastal communities can use to compare the probable cost of minor floods to those of extreme events. Published in the journal Earth’s Future in February, the analysis looked at 11 U.S. coastal cities and found that in New York, Washington, D.C., Miami, San Francisco, and Seattle, the long-term inundation risk to property (which the team used as a proxy for cost) from minor floods equals or exceeds that from major floods.

Minor floods, typically lasting hours rather than days, can make streets impassable for cars, cause sewer systems to dump raw material into local waterways, contaminate drinking water, expose buildings to damaging saltwater, and temporarily shut down businesses, railways, bridges and tourist attractions.

Due in part to rising sea levels brought about by climate change, the frequency of these minor events, also known as nuisance floods, has increased significantly along U.S. coasts in the past half-century, according to a 2014 report by the National Oceanic and Atmospheric Administration.

Hamed Moftakhari, a civil and environmental engineer at the University of California, Irvine, and the lead author of the Earth’s Future study, says his team’s cumulative hazard index could inspire communities to look more closely at contemporary data that typically are overlooked.

“We encourage those communities affected and threatened by these minor events to keep a record of the costs that they pay for these events,” he says, such as the cost of pumping saltwater out of streets. “It is like throwing a penny [with every step] while you are walking. But if you keep track of the pennies, it would add up to something large to be considered in your future sustainable management.”

Such calculations could help policymakers decide when to move beyond Band-Aids to transformative measures such as sea walls, berms, levees and more. With nearly 40 percent of Americans living in counties with shorelines as of 2010, these minor floods likely will affect more of us every year.

Donald Trump took action last month to roll back measures aimed at mitigating climate change, but both Washington, D.C., and Florida, between which the president splits his time, are feeling its effects. In D.C., for example, nuisance flooding increased from an average of 6.8 days annually between 1956 and 1960 to 28.6 days annually between 2006 and 2010, according to a 2014 analysis.

A sign on display at the March for Science last Saturday in the nation’s capital warned the president about the dangers of ignoring the growing threat of flooding. It read: “Donald! You’ll learn soon enough. Mar-a-Lago is only 10 feet above sea level.”