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Inequitable patterns of US flood risk in the Anthropocene

Institute for Water Resources
Published Jan. 31, 2022
Present and future US flood risk by county

Present and future US flood risk by county

Relative US flood risk for different demographic groups

Relative US flood risk for different demographic groups

Mr. William Lehman, of IWRs’ Hydrologic Engineering Center (HEC) in Davis, CA, wrote an article with Oliver Wing et al which is now published in the most recent issue of Nature Climate Change (January 31, 2022), that describes how expected annual damage (EAD) was computed as a distribution expressing our uncertainty in the EAD estimate for every structure in the United States.


Flood risk is managed globally and assumes that history is a good predictor of the future. Be it enforcing regulations within flood zones defined using historical water-level records, modelling the cost–benefit ratio of mitigatory actions based on historical flood probabilities, or not considering future risk when permitting new development, ubiquitous flood risk management tools fail to recognize that the nature of floods is changing.


Current flood risk mapping, relying on historical observations, fails to account for increasing threat under climate change. Incorporating recent developments in inundation modelling, here we show a 26.4% (24.1–29.1%) increase in US flood risk by 2050 due to climate change alone under RCP4.5. Our national depiction of comprehensive and high-resolution flood risk estimates in the United States indicates current average annual losses of US$32.1 billion (US$30.5–33.8 billion) in 2020’s climate, which are borne disproportionately by poorer communities with a proportionally larger White population. The future increase in risk will disproportionately impact Black communities, while remaining concentrated on the Atlantic and Gulf coasts. Furthermore, projected population change (SSP2) could cause flood risk increases that outweigh the impact of climate change fourfold. These results clearly indicate the need for adaptation to flood and emergent climate risks in the United States, with mitigation required to prevent the acceleration of these risks.


The article seeks to deepen the understanding of US flood risk in the following ways: (1) estimate the national average annual flood loss and its geographic spread, (2) robustly quantify the uncertainty in these estimates, (3) project changes in risk due to climate and demographic change, and (4) uncover the social justice implications of who bears present and future risk.


As such, the paper provides (1) a methodological framework for comprehensive, high-resolution and forward-facing flood risk estimation, with an innovative approach to characterizing large-scale uncertainties, and (2) empirical insights into US flood risk and its heterogeneity across time, space and demography.


To read the entire article in Nature Climate Change, go to citation: Wing, O.E.J., Lehman, W., Bates, P.D. et al. Inequitable patterns of US flood risk in the Anthropocene. Nat. Clim. Chang. (2022).


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