UC Irvine and University of Bristol researchers recommend analysis of complex factors
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In the paper, the researchers urge the climate science community to turn away from an outdated approach to mapping flood hazards known as “bathtub modeling,” which is an assumption that floods spread out over areas as a level pool.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.
Those are: 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; and 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.”
This work was supported by the U.S. National Science
Foundation, the National Oceanic and Atmospheric Administration, and the
Natural Environment Research Council in the U.K.
About UC Irvine’s Brilliant Future campaign: Publicly launched on Oct. 4, 2019, the Brilliant Future campaign aims to raise awareness and support for UC Irvine. By engaging 75,000 alumni and garnering $2 billion in philanthropic investment, UC Irvine seeks to reach new heights of excellence in student success, health and wellness, research and more. The Henry Samueli School of Engineering plays a vital role in the success of the campaign. Learn more by visiting https://brilliantfuture.uci.edu/the-henry-samueli-school-of-engineering.
