When we think about flooding, the image that comes to mind is likely a river bursting its banks or a home inundated with storm surge and high waves. However, these are not the only types of floods that can occur. Flooding can be caused by a variety of events, including heavy rains, urban runoff and construction projects. This is why it’s important to know your risk, which is usually available through your local government’s flood maps.

But many of these maps underestimate the amount of risk flooding risks LA in communities by focusing only on fluvial (river) or coastal (storm surge, waves and storm tides) hazards without accounting for rainfall runoff. These errors can have devastating consequences, especially for low-income and marginalized communities. And that’s why a team at the University of California, Irvine, has mapped the compound flood risks of Los Angeles County using a computer model that can evaluate each property for risk.

Their research shows that more than 20 percent of LA’s properties are at risk for flood damage over the next 30 years. That number will rise as climate change continues to alter weather patterns and bring stronger storms. The authors’ findings also highlight how racial disparities affect flood vulnerability in the region. Non-Hispanic Black residents, for example, are 79 percent more likely to be exposed to flooding greater than 100 centimeters, or three feet, than their white counterparts. In addition, the study finds that low-income residents are more at risk for floods than their higher-income neighbors.

The findings are based on a computational model called PRIMo, which uses topography, land use, watersheds and a database of flood damage to generate a map of each property’s potential for a “100-year” event. The model also considers how these factors interact, such as how the capacity of a flood control channel may be reduced by the buildup of sediment and vegetation.

Compared to traditional methods of mapping flood risk, the model is much more detailed and accurate. For example, other models often overestimate the impact of storm surge and wave on urban areas by ignoring the effects of street gutters and city drainage infrastructure. The model also accounts for the fact that clogged channels can reduce the amount of water they carry.

These findings can help cities and governments prioritize where to invest in flood mitigation and resilience measures. This approach, called “green infrastructure,” involves creating sustainable flood management systems that gather and remove excess water at its source rather than engineering expensive concrete or steel structures that are typically used in conventional building construction.

The researchers hope their work will inspire others to create similar studies in other parts of the country. They would like to see more communities engage in these efforts with an equity lens, ensuring that the benefits are shared across the entire community. For more information about the study, and how to get involved, visit this area’s Risk Factor pages.