Flooding is one of the most devastating effects of a changing climate. In the US, it has cost tens of billions in damages and has forced millions of people to evacuate or move to higher ground. The risks of flooding can be even more pronounced in urban areas. A new study from UC Irvine has found that the most vulnerable residents of Los Angeles face significant and inequitable flood risks.
The study is based on detailed flood modeling of the city that shows in nearly house-by-house detail how a “100 year” flood event would impact homes flooding risks Los Angeles and businesses. It finds that affluent neighborhoods along the coast are more at risk of storm surge and waves, while poorer communities in the inland areas are more vulnerable to floods caused by intense rainfall that can overwhelm street channels and sewage systems. These different flood hazards require different mitigation strategies.
Despite the well-publicized risks of coastal flooding, most of the risk in LA is due to rainfall and flash flooding. This is because the city’s built environment has encroached on traditional river wetlands, and most of its drainage systems are outdated and inefficient. The study finds that the city is 10-40 fold more at risk of flooding from these kinds of events than suggested by current FEMA maps.
A reevaluation of the city’s flood zones is needed. Sanders and his colleagues found that 105,574 properties are at a high or severe risk of flooding. The city is working to lower these risks by improving drainage and retrofitting buildings, but the work will be costly and take time. Moreover, many of these at-risk properties are in low-income and minority communities.
Flooding events can be triggered by hurricanes, tropical storms, melting snow or heavy rainfall. A growing number are also being caused by “atmospheric rivers” driven by climate change that bring large amounts of precipitation to densely populated regions. The city’s infrastructure is especially poorly suited to these events, and the city’s poorest residents are at a higher risk of loss.
To assess the risks to urban areas from these events, the researchers used a computer model that subjected the city to 24 hours of rain at levels that have a 1 percent chance of occurring in any given year. They found that the highest risk areas were primarily in the area south of Downtown Los Angeles, including Norwalk and Gardena. The areas are characterized by low topography that can cause rainwater to build up and overflow drains and streets. The team says that creating a kilometer-wide network of slow-moving, vegetation-rich natural rivers through this area could mitigate some of these risks. But such a project would be expensive and probably politically unfeasible. The study authors are planning further research to explore how different communities can reduce these flood risks using green and grey infrastructure approaches that incorporate an equity lens. They hope to repeat this kind of detailed modeling in other major cities, and explore solutions that take into account both flood risk and climate resilience.