Scientists urged to end ‘bathtub modeling’ of flood risk

Recent decades have seen a rapid increase in flood damage and disruption. In a commentary article published in the journal The future of the EarthResearchers from the University of California, Irvine and the University of Bristol in the United Kingdom (the latter also a director of the British flood risk intelligence company Fathom) are calling on scientists to model these risks with more precision and to warn against overly dramatized communication of future risks. the news media.

In the paper, the researchers urge the climate science community to move away from an outdated approach to flood risk mapping known as “bathtub modeling,” which relies on the assumption that flooding spread over areas like a level swimming pool. This technique is often used as a simple way to visualize the impact of flooding in coastal areas but, according to the authors, it can lead to an overly simplistic and less realistic picture of flood risk than more advanced methods. The alternative to bathtub modeling, they say, is dynamic modeling that solves physics-based equations.

“Bathtub models can both overestimate and underestimate flooding,” said co-author Brett Sanders, UC Irvine Chancellor’s Professor of Civil and Environmental Engineering. “One of the leading causes of error is that bathtub designs do not accurately account for the systems in place to protect people and property, including storm drains, levees and pumping.”

He and his collaborators: Oliver Wing, scientific director of Fathom and honorary researcher at the University of Bristol; and Paul Bates, professor of hydrology at the University of Bristol and president of Fathom, note that bathtub modeling is limited in its ability to take into account at least six key factors:

• Flood mitigation of the effects of event dynamics and friction on flood propagation
• Tidal amplification associated with ocean tidal resonance in coastal bays
• Flood defenses such as levees and flood walls that can overtop during an extreme event while limiting the degree of flooding inland.
• Bulge of the water table
• Rise of groundwater under the combined influence of sea level rise and changes in hydrological budgets
• Pumping groundwater into land below sea level to mitigate flooding from rising groundwater

Based on a review of the flood risk literature, the research team summarizes the reduced accuracy of bathtub models using the Critical Success Index, which assesses the accuracy of the extent floods between 0 and 1, where 1 represents a perfect match based on field measurements.

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

“CSIs below 0.5 indicate that these models are worse than random classification,” Wing said. “In other words, a chimpanzee is better than a bathtub model at demarcating areas at risk of flooding.”

According to the researchers, studies that rely on bathtub modeling are frequently found in high-impact, short-format journal publications and attract considerable media interest. Although 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 underwater – is too often exaggerated, they say .

“Accurate maps of flood risk areas are critically important to 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 reliable information.”

Reliable flood risk models are necessary to effectively engage affected communities in adaptation processes and to implement effective and equitable mitigation and adaptation strategies, according to Sanders. Inaccurate models could lead to poor adaptation.

“Flood projections need to make sense to people, not only to better understand what is at risk, but also to decide what investments and policies will be implemented to manage them,” Sanders said. “In fact, numerous research papers have shown that people in at-risk areas are unlikely to trust projections of future flooding if they do not reflect their lived experiences. Research studies that simplify to extreme flooding and do not represent real-world data pose a threat to transformative action.”