According to research published on October 31 in the journal Nature Sustainability, up to 874,000 people in the city may be at risk of exposure to flooding, with disadvantaged communities the worse affected.
One of the deadliest floods of all time is thought to have occurred in China in 1931, where 3.7 to 4 million people died from the Yangtze–Huai River floods. In LA, a 1938 flood caused by two Pacific storms that were swept across the Los Angeles Basin killed between 113 and 115 people.
Brett F. Sanders, co-author of the new paper and a civil and environmental engineering professor at the University of California, Irving, told Newsweek: “Los Angeles is the second-largest city in the U.S. It has the third-largest economy in the world across all megacities, and it has a history of extreme flood events.
“The population has soared over the last century, yet until now, we have not known how many people are exposed, how much property would be exposed, and precisely who would be exposed.”
Flooding is caused by a variety of factors, including the overflow of water from bodies of water like the ocean, rainwater running off saturated ground, or when the flow rate of a river exceeds the capacity of its channel.
According to Sanders, their new study has modeled future flood risks and found that $108 billion in property could be exposed to flooding greater than 30 centimeters (1 foot) within the 100-year flood zone.
These are risk levels similar to some of the most destructive hurricanes in U.S. history, including Katrina in 2005, Sandy in 2012 and Irma and Harvey in 2017.
The 100-year flood zone is described by the authors in the paper as the magnitude of flood that has only a 1 percent chance of occurring or being exceeded in a given year: there is a 63.4 percent chance of one or more 100-year floods occurring in any 100-year period.
The risks of flooding in LA specifically are increasing due to land-use change, according to Sanders.
“More people are in harm’s way, and expansive use of impervious land surfaces (concrete, buildings) leads to rapid runoff of rainfall into flood channels that are no longer big enough to contain the flow,” Sanders said.
“This is also due to how we have historically measured exposure. Historically, federally-defined floodplains were not required to delineate areas exposed to street flooding from intense rainfall.
“Rather, mapping focused only on areas that could flood from channels that were too small to contain flood peaks, or coastal areas that could be inundated by a storm surge or waves.
“However, across major cities blanked by concrete and buildings, rainfall runoff presents significant flood risks. Here, using an innovative modeling approach, we accounted for all three possible modes of flooding (pluvial, fluvial and coastal) for the first time.”
While the effects of climate change may well play into this, it was not included in the researchers’ models of the flood.
“Other studies indicate that climate change (more intense rainfall) is also an important contributor globally and in California, but we did not attempt to quantify that aspect of the changing risk,” Sanders said.
These increasing floods and their damage to infrastructure and housing will most affect communities that are already struggling.
“Recent flood disasters across the U.S. have also shown that more affluent communities have the ability to recover from floods, while low-resource communities (which are often communities of color) may be slow to recover, or may never recover,” Sanders said.
“These risks are disproportionately higher for non-Hispanic Black and disadvantaged populations.”
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