Los Angeles Flood Risk

The Los Angeles metropolitan region is built out over a coastal plain surrounded by steep mountains to the North and East and the Pacific Ocean to the West and South. This results in three distinct types of flood hazards: rainfall hazards that create street flooding due to dense development with hardened surfaces across the coastal plain, streamflow hazards from runoff, mud and debris flowing into and overtopping drainage channels, and coastal hazards from high tides and waves.

Severe flooding is linked to so-called Atmospheric River events that produce rainfall day after day for many weeks, saturating the ground surface and filling flood control reservoirs before a day-long episode of intense rainfall sends flood water, mud and debris across the coastal plain.

A widely used scenario to prepare for the possibility of flooding is termed the 100-year flood. It represents a flooding intensity that is likely to occur or be exceeded, on average, once every 100 years. The 100-year flood is sometimes termed the 1% annual chance flood, because in any given year, the probability that flooding equal or greater than the 100-year scenario occurs is 1%.

Using an innovative new urban  flood risk modeling system , a  UCI-led team mapped the 100-year flood  zone across the Los Angeles coastal plain to reveal the populations affected.  This work strives to understand both the scale of flood risks, and precisely who is affected – since this bears directly on whether communities recover or spiral downwards. The map below is representative of the 100-year flood depth considering rainfall hazards, streamflow hazards and coastal hazards.

UCI-led modeling reveals that roughly 1.7 million people and $265 billion in property are exposed to flooding greater than 3 cm, and roughly 425 thousand people and $56 billion in property are exposed to flooding greater than 30 cm. Shallow flooding  (3-10 cm) is mainly related to rainfall hazards and deep flooding (> 100 cm) is mainly related to streamflow hazards.

Severe flooding represented by a 100-year event is, by definition, very unlikely occur in any given year, but it becomes more likely over longer and longer periods. For example, the probability of flooding increases to 10, 26 and 40% after 10, 30 and 50 years, respectively. This means that communities and infrastructure within the 100-year flood zone will eventually experience flooding, and by understanding the areas at risk, steps can be taken to save lives, reduce losses and enhance the ability of impacted communities to recover. The most effective steps include land use decisions that enable natural processes to diffuse the impact of floods and keep highly vulnerable assets, infrastructure and populations out of harms way; building codes and standards so structures are capable of enduring flooding with minimal need for repairs; and public communication of risks so decision-makers at household, community and regional levels are aware of flood risks and have options to manage it. In the U.S., the National Flood Insurance Program (NFIP) is designed to assist owners of property within the 100-year flood zone in rebuilding after a damaging flood.

Addressing Social Vulnerability

Across the U.S., the rise in flood damages and impacts have not been evenly distributed across social and demographic groups. Poor and non-white populations have been disproportionately affected. Additionally, these same communities have been supported less with flood risk reduction and disaster recovery by governments, compared to more affluent communities. This has led to prolonged and incomplete recovery among socially marginalized, low-wealth, and vulnerable communities.

The U.S. is presently lacking a nationwide approach to provide flood assistance to communities considering the severity of the risk that they face and their capacity to manage it. The U.S. system relies upon local communities to advocate for assistance including funds for mitigation, preparedness and recovery through State and Federal programs, and recent reports have shown that less affluent communities—where support is most needed—are not adequately resourced to effectively advocate.

By modeling of flood risks at household scales as we have done for Los Angeles, we offer an objective top-down approach for identifying the communities most in need of assistance – one that considers several factors including the number of people affected, the severity of the hazard, and the capacity of the community to manage flood impacts. For example, in the graphic below, we show the number of people exposed to at least 3 cm of flooding by the 100-year flood hazard, the average depth of flooding across the exposed population, and the average neighborhood disadvantage index (NDI, a measure of disadvantage with respect to flood recovery) at the scale of local government. We deliberately avoid combining these factors into to single risk index to enable dialogue, coordination and cooperation at the regional level and with state and federal agencies, and to encourage flexibility and innovation with respect to flood risk management. Hence, the aim here is to combine bottom-up planning and coordination with a top-down approach for making state and federal resources available equitably,  an approach that could be replicated nationwide .

The NDI represents a non-racial indicator of social vulnerability to flooding, one that governments can use to prioritize resources for flood management without bias towards any particular racial or ethnic group. Nevertheless, examination of flood exposure along racial and ethnic lines is important for understanding whether infrastructure is serving communities equitably and fairly. As mentioned above, previous research in other parts of the U.S. such as New Orleans indicates that non-white populations have been less well protected. To test for racial and ethnic inequities in flood exposure in Los Angeles, we used Census data and parcel land use data to estimate population fractions for Hispanic, non-Hispanic Black, non-Hispanic Asian and non-Hispanic White residents of Los Angeles at the scale of land parcels, and we intersected these estimates with the parcel-based 100-year flood depth to assess precisely how flood risk impacts each racial/ethnic group. The next graphic presents a side-by-side comparison of the spatial distribution of flood depth (right panel) and the spatial distribution of population for non-Hispanic Black, Hispanic, non-Hispanic Asian, and non-Hispanic White populations (left panel).

The graphical comparisons above show that the overall flood hazard does not align precisely with any of the four racial/ethnic groups considered. However, with our parcel-level dataset that includes socio-economic variables including property value, personal income, exposure, race, ethnicity, disadvantage (NDI) and the U.S. Social Vulnerability Index (SoVI) developed at the University of South Carolina and adopted by FEMA, we can quantitatively test for inequities in exposure using an Inequality Index known as a Gini Coefficient. This Inequality Index measures whether a particular variable (i.e., property value, income, NDI, SoVI, population fraction by race/ethnicity) signals an over- or under-exposure to flood risks. Over-exposure is indicated by a value greater than zero, and under-exposure is indicated by a value less than zero. A value of zero means that exposure to the hazard falls across the population in a way that is even, or equitable.

The table below shows that strong inequalities in Los Angeles include the exposure of non-Hispanic Black populations to streamflow hazards (0.51), the under-exposure of non-Hispanic White populations to streamflow hazards (-0.33), and the exposure of non-Hispanic White populations to coastal hazards (0.95). Weak inequalities include the exposure of Hispanic populations to streamflow hazards (0.12), and the exposure of more disadvantaged communities (NDI) to streamflow hazards (0.21).

Guiding Future Infrastructure Investments

Moving forward, what is arguably most important now is not the inequities facing communities, but identifying and implementing the projects that will address these inequities while reducing risks for everyone and making urban communities more livable and prosperous. The proposed modeling framework is designed to meet this need. The system can be updated to account for proposed infrastructure such as  levee projects , channel widening, green infrastructure and even land use changes and building codes which impact who is exposed and where. In turn, a new set of impacts and inequalities will be revealed at community and regional scales. Some proposals may reveal localized benefits, others regional benefits, and others will surely combine benefits to one area with costs or heightened risks to another. Nevertheless, with this  innovative modeling , Los Angeles and other major cities of the U.S. will have an unprecedented opportunity to coordinate flood responses across local, regional, state and federal levels precisely as the U.S. turns to make a generational commitment to restoring infrastructure, reducing risks and inequities, and enabling greater social justice.

This storymap is based on  research originally published in Nature Sustainability  by Brett Sanders, Jochen Schubert, Daniel Kahl, Katharine Mach, David Brady, Amir AghaKouchak, Fonna Forman, Richard Matthew, Nicola Ulibarri & Steven Davis.