Some floods are so severe that they rarely occur more than once a century, but by the end of the 21st century, rising sea levels could threaten coastal communities with extreme flooding occurring every year. A new study finds that even under a mild scenario in which carbon dioxide emissions peak by 2040, most coastal communities will experience 100-year floods every year by the end of the century. As early as 2050, regions around the world will experience 100-year floods on average every 9 to 15 years.
The 100-year flood is an extreme water level that has a 1% chance of being exceeded in any given year and is based on historical data. Despite the name, a 100-year flood may hit the same area for many years in a row, or not occur at all for a century. However, a new study finds that these historical trends no longer provide an accurate outlook for future flooding.
"We expect that in a warming climate, the threshold will be exceeded once every century on average until they are no longer considered a 100-year event," said Hamed Moftakhari, a civil engineer and professor at the University of Alabama overseeing the project. The research was recently published in AGU's journal Earth's Future, which conducts interdisciplinary research into the past, present and future of our planet and its inhabitants.
In May 2016, the Potomac River flooded the streets in Old Town Alexandria, Virginia. As sea levels approach coastal infrastructure, these floods will occur more frequently. Image source: NOAA
Rising sea levels: a looming threat
In coastal areas, extreme flooding can be caused by storms, tides and waves pushing water inland, but this study focused on one factor that causes flooding over longer periods of time - rising sea levels. As sea levels rise toward the coast, coastal infrastructure will be closer to the water, making storms, tides and waves more likely to impact communities.
The researchers used data from more than 300 tide gauges around the world to conduct trend analysis and estimate future extreme sea levels under two carbon emission scenarios outlined by the International Panel on Climate Change: if CO2 emissions continue to rise through the end of the century, and if CO2 emissions peak by 2040 and then decline. In both cases, they found that rising sea levels would lead to an increase in 100-year flood events in most of the sites they studied.
Proactive land planning, urban development and coastal protection measures can help communities reduce flooding and avoid disasters, which starts with realistic predictions of future coastal conditions, Mofatahari said.
Challenges in predicting flood patterns
Engineers who design structures like seawalls, sea walls and breakwaters to protect communities from extreme flooding rely on a concept called stationarity to predict future water levels.
"In terms of stationarity, we assume that the patterns we observed in the past will remain unchanged in the future, but there are a lot of factors that are modulating these patterns under climate change," Mofakari said. "We can no longer assume that coastal flooding is stable."
Previous studies have relied on stationary estimates of extreme sea levels to predict 100-year floods, but this study used nonstationary methods and found that extreme sea level changes were not uniform across many tide gauge locations.
As climate changes, rising ocean temperatures and melting glaciers cause sea levels to rise, increasing the frequency and severity of coastal flooding. Therefore, engineers need to accurately estimate future flood risk rather than assuming that our changing future will reflect historical coastal patterns.
"The reason it's so challenging is that most tools, design guides, practice manuals, etc. are based on the assumption of stationarity," Moftakhari said. "They need to be updated to enable us to keep up with the speed of change."
Local solutions to diverse challenges
Another study showed that more than 600 million people live in low-lying coastal areas. This number is expected to rise. Well-designed sea defense structures play an important role in the ability of coastal communities to withstand catastrophic floods.
While mean sea levels are rising, the results are not the same everywhere. Sea levels in high latitudes may fall as thick ice caps melt and the land beneath rises. Additionally, sea levels in areas such as the Gulf of Mexico are rising faster than the global average because the land is gradually sinking. Moftakhari said coastal communities require unique solutions based on local information to meet their needs.
"We know mean sea levels are rising, and the question is: How are we going to respond?" Moftakhari said. "We've already seen many parts of the coast permanently inundated and lost land, with many coastal cities and islands experiencing flooding more frequently than in the past - it's time to learn how to deal with non-stationarity."
But he is determined to remain optimistic, reminding us that disasters are the result of human decisions, not just hazards. “Don’t forget, it all depends on the water levels we are expected to experience without mitigation measures, and technological advances will increase community resilience.”