What Serious Are the Flood Threats in LA?

When it comes to a big flood, Los Angeles County might be more vulnerable than many people think. According to a new study, the area’s 100-year flood risk is up to 30 times higher than federal assessments show. It’s also disproportionately dangerous for Black Angelenos, according to the researchers.

The study, published Monday in the journal Nature, looked at flood risks across the Los Angeles metro area. It found that current flood impact assessments underplay the importance of pluvial flooding hazards, which occur when a river channel becomes so overloaded that it spills over or breaches its sides and floods urban areas. This type of flooding is largely attributed to climate change, as rising temperatures lead to warmer weather patterns and heavier precipitation.

Those climate changes are already having an impact on the flood how big are Los Angeles flooding risks risk in LA, the researchers found. Over the past 50 years, LA has seen a significant increase in severe flood events. These are the types of floods that affect more than 10% of the city’s homes. In a typical year, these events cost the average homeowner about $21,000. The number of properties affected by this type of flooding is expected to increase over the next 30 years as LA continues to experience a warming environment and heavier storms.

Flood risk is based on a variety of factors, including watershed size, topography, and how much of the landscape is covered by streets, buildings, or other structures. The study used these factors to determine the likelihood of a 1-in-100-year flood event hitting a given property. The result showed that, if a major storm causing a 1-in-100-year flood hit today, it would affect a total of 276,846 LA county properties. The study also estimated that the cost of this type of flooding to an individual home would be about $10,444.

To understand why these numbers are so high, it’s important to note that a large majority of flood events in LA are caused by rainfall. These storms tend to be less frequent than the more intense hurricanes that cause major coastal flooding, but they have just as much of an effect on local communities.

What the study found is that our flood channels are often undersized and our streets lack the grasslands and other natural features that can absorb excess water and slow down runoff. As the population in Southern California grows, we’ve turned more of our natural wetlands into concrete canyons.

Sanders said these narrow, fast-moving, heavily paved areas are where the biggest floods occur because they have low topography and little vegetation that can absorb the water. “Decade after decade, these streams are getting less and less able to hold the big floods and when they get overtopped, they spill over into neighborhoods,” he said.

The good news is that there are ways to mitigate these risks. These include green infrastructure, which includes restoring wetlands and other natural land to their original functions, as well as resilience measures, which are community-wide strategies that help people recover more quickly from floods. Read more about these strategies in this article on our Risk Factor blog.