Scientists urged to pull the plug on ‘bathtub modeling’ of flood risk

Recent decades have seen a rapid surge in damages and disruptions caused by flooding. In a commentary article published in the journal Earth’s Future, researchers at the University of California, Irvine and the University of Bristol in the United Kingdom—the latter also executives of U.K. flood risk intelligence firm Fathom—call on scientists to more accurately model these risks and caution against overly dramatized reporting of future risks in the news media.

In the paper, the researchers urge the climate science community to turn away from an outdated approach to mapping flood hazards known as “bathtub modeling,” which is an assumption that floods spread out over areas as a level pool. The technique is often used as a straightforward way to visualize flood impact in coastal areas but, according to the authors, can lead to an oversimplified and less realistic picture of flood risk than more advanced methods. The alternative to bathtub modeling, they say, is dynamical modeling that solves physics-based equations.

“Bathtub models can both overpredict and underpredict flooding,” said co-author Brett Sanders, UC Irvine Chancellor’s Professor of civil & environmental engineering. “One of the biggest causes of error is that bathtub models fail to accurately account for the systems in place to protect people and assets, including storm drains, levees and pumping.”

He and his collaborators—Oliver Wing, chief scientific officer at Fathom and an honorary research fellow at the University of Bristol; and Paul Bates, a University of Bristol professor of hydrology and Fathom chairman—note that bathtub modeling is limited in its ability to account for at least six key factors:

• Flood attenuation from the effects of event dynamics and friction on flood spreading
• Tidal amplification associated with the resonance of ocean tides within coastal embayments
• Flood defenses such as levees and floodwalls that may overtop during an extreme event but still restrain the degree of inland flooding
• Shoaling of the groundwater table
• Surfacing groundwater from the combined influence of rising sea levels and changing hydrologic budgets
• Pumping of groundwater within lands below sea level to mitigate inundation by rising groundwater

Based on a review of literature pertaining to flood risk, the research team summarizes the reduced accuracy of bathtub models using the critical success index, which scores flood extent accuracy between 0 and 1, where 1 represents a perfect match based on field measurements.

The CSI for bathtub models analyzed in the literature is consistently under 0.5, well below the threshold of 0.65 that experts suggest models need in order to have local relevance and therefore produce useful results when applied in impact analyses.

“CSIs under 0.5 indicate that these models are worse than a random classification,” Wing said. “In other words, a chimpanzee has more skill than a bathtub model in delineating flood hazard areas.”

Studies that rely on bathtub modeling are frequently found in short-format, high-impact journal publications and attract considerable interest from the news media, according to the researchers. While the biases and uncertainties of bathtub modeling are often acknowledged in these technical papers, the message communicated to the public and policymakers—sometimes with compelling visualizations of cities under water—is too often an exaggeration, they say.

“Accurate maps of areas at risk of flooding are of paramount importance for everyone from home and business owners to insurers, banks and governments,” Bates said. “We all have a role to play in reducing flood losses, but it all starts with trustworthy information.”

Reliable models of flood risk are needed to effectively engage impacted communities in adaptation processes and to implement effective and equitable mitigation and adaptation strategies, according to Sanders. Inaccurate models could lead to maladaptation.

“Projections of flooding need to make sense to people, not only for building understanding of what’s at risk but also for deciding upon the investments and policies that will be made to manage it,” Sanders said. “In fact, numerous research papers have shown that residents within at-risk areas are unlikely to trust projections of future flooding if they don’t reflect their lived experiences. Research studies that oversimplify flooding and don’t represent real-world data pose a threat to transformative action.”