Scientists urge an end to ‘bathtub modeling’ of flood risk

The damage and disruption caused by floods has increased rapidly in recent decades. In an explanatory article published in the journal Earth’s Future, researchers from the University of California, Irvine and the University of Bristol in the United Kingdom, the latter of whom is also an executive at British flood risk information company Fathom, asked scientists to assess these risks. We are calling for more accurate modeling. And beware of overly dramatized coverage of future risks by the news media.

In their paper, the researchers urge the climate science community to move away from an outdated approach to mapping flood risk known as “bathtub modeling,” which assumes floodwaters spread across an area as a pool of water. are. Although this technique is often used as a simple way to visualize the effects of flooding in coastal areas, the authors say it oversimplifies and understates flood risk compared to more sophisticated techniques. It may no longer be possible. An alternative to bathtub modeling, they say, is dynamic modeling, which solves physics-based equations.

“Bathtub models can overpredict or underpredict flooding,” said co-author Brett Sanders, a professor of civil and environmental engineering at the University of California, Irvine. “One of the biggest sources of error is that bathtub models don’t accurately account for systems in place to protect people and property, such as storm drains, levees, and pumps.”

He and his collaborator Oliver Wing, Fathom’s chief scientific officer and honorary research fellow at the University of Bristol. Paul Bates, Professor of Hydrology at the University of Bristol and Chairman of Fathom, notes that bathtub modeling is limited in its ability to account for at least six key factors.

Flood attenuation due to the effects of event dynamics and friction on flood spread; Tidal amplification associated with resonance of ocean tides within coastal bays; Potential for overtopping during extreme events, but still reducing the extent of inland flooding Flood defenses such as levees and seawalls that suppress groundwater levels in shallow rivers Surface groundwater levels due to the combined effects of sea level rise and changes in the hydrological budget Pumping of groundwater within land below sea level for mitigation Flooding due to rising groundwater

Based on a review of the flood risk literature, the research team uses a critical success index to summarize the reduced accuracy of the bathtub model. This index scores the accuracy of flood coverage between 0 and 1. 1 represents perfect match based on field measurements.

The CSI of bathtub models analyzed in the literature is consistently less than 0.5, which experts suggest is necessary for the model to be locally relevant and produce useful results when applied to impact analysis. well below the threshold of 0.65.

“A CSI of less than 0.5 indicates that these models are inferior to random classification,” Wing said. “In other words, chimpanzees are better at delineating flood-prone areas than bathtub models.”

Studies relying on bathtub modeling are frequently published in short-form, high-impact journal publications and receive significant interest from the press, the researchers said. Although the biases and uncertainties of bathtub modeling are often acknowledged in these technical papers, the message conveyed to the public and policy makers is sometimes conveyed through compelling visualizations of the city beneath the surface. , they say, is often greatly exaggerated.

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

Saunders said reliable models of flood risk are needed to effectively engage affected communities in the adaptation process and implement effective and equitable mitigation and adaptation strategies. Inaccurate models can lead to maladaptation.

“Flood forecasting is meaningful for people, not only to understand what is at risk, but also to make investment and policy decisions to manage floods,” Sanders said. There is a need.” “In fact, numerous research papers have shown that residents of at-risk areas are less likely to trust predictions of future floods if they do not reflect their actual experience. Research that oversimplifies flooding and does not reflect real-world data can pose a threat to transformative action.