Environment: Higher and disproportionate flood risk in Los Angeles predicted

Up to 874,000 people may be at risk of exposure to flooding in Los Angeles, California, according to a modelling study published in Nature Sustainability. This risk — which is higher than federal estimates — may not be equally distributed across the city, but disproportionately affects traditionally disadvantaged communities.

Although the majority of flood damage in the United States is caused by hurricanes along its gulf and eastern coasts, extreme rainfall can cause catastrophic flooding in the western United States, as evidenced by the California Great Flood of 1861–1862. Despite this, current Federal Emergency Management Agency maps for Los Angeles County only place 0.3% of the population (just over 23,000 people) as living in an area within the 100-year flood zone (the magnitude of flood that only has a 1% chance of occurring or being exceeded in a given year).

Using a new, high-resolution flood inundation model, Brett Sanders and colleagues predict that — at the upper end of their model estimates — up to 874,000 people and up to US $108 billion in property may be at risk of exposure to flooding levels of over 30 cm within the 100-year flood zone — risk levels similar to the most damaging hurricanes in US history. The authors reveal racial and socioeconomic disparities in who is threatened by this flood risk. Non-Hispanic Black, Hispanic and other already disadvantaged groups are predicted to make up nearly three-quarters of the population at risk of being exposed to fluvial floods, and over half those at risk from floods originating in dam reservoirs or lakes and compound flood events. Coastal flood risk, however, is found to predominantly affect affluent non-Hispanic white communities.

Based on this new capacity to identify precisely who is exposed to different modes of flooding (rainfall, fluvial, coastal) across major cities, the authors suggest that future efforts to address flooding can now explicitly consider the distribution of risks across populations, from community to regional scales, and measure benefits in new ways such as shifts in equity and/or geographic focus.