Where flood hazard increases most under climate change may not be where its societal consequences are greatest. Here, Will Dow, Anya Hawkins and Kirsty Styles from JBA Risk Management’s Climate Change team take Mobile, Alabama, as a case study to explore what changes when flood hazard is considered alongside exposure and social vulnerability.
Climate adaptation often involves decisions about where to focus limited resources. Flood modelling can show how physical hazard may change, but the consequences of flooding also depend on who and what is exposed. Where social vulnerability is relevant to a decision, considering these different dimensions of risk together can produce a different picture from looking at changing hazard alone.
From flood hazard to flood impacts
Flood hazard modelling provides information about where flooding may occur, how deep or extensive it could be, and how frequently it might happen. This is fundamental to understanding how flood hazard may change as the climate warms, but the consequences also depend on the people, buildings and infrastructure exposed, and their vulnerability to flooding.
Research has explored how these consequences can be distributed unevenly across society. Studies following Hurricane Harvey in Houston, in rural New South Wales, and in New Orleans have examined relationships between flood exposure, impacts and social vulnerability. The findings vary between locations and studies, reflecting differences in the physical and social context, but show how information about social vulnerability can add to an assessment based on flood hazard and exposure.
Bringing these sources of information together can therefore provide a broader view of potential flood impacts. For organisations considering adaptation, resilience or climate risk, it can help explore where changing flood hazard coincides with communities that may be less able to prepare for, respond to or recover from flooding.
Understanding future flood impacts in Mobile, Alabama
To explore this question, we used JBA's climate change flood maps to investigate Mobile, Alabama, a region projected to experience increasing flood hazard under future climate change. Our analysis focused on river flooding and examined how projected changes in hazard may vary under different levels of global warming.
Just considering the flood hazard, the climate change flood maps show an increase across the region. For instance, under 3°C of global warming, river flood depths are projected to increase by approximately 25%, while the number of exposed buildings across the city is projected to increase by around 68%.
These statistics tell us something about how the flood hazard and exposure may evolve, but they leave another risk-management question: are the communities experiencing the largest increases in flood hazard also those that may be most vulnerable to its consequences?
To explore this, we combined projected changes in river flood hazard from our climate change flood maps with building exposure data and the CDC Social Vulnerability Index. We used these data to identify future climate risk “hotspots”, defined here as locations where increasing flood hazard coincides with greater exposure and social vulnerability.
This is an illustrative way of combining these dimensions of risk, rather than a universal measure. The locations identified as hotspots depend on the datasets and measures used, and on how they are combined.
What we found
Across Mobile, we found no clear relationship between projected increases in flood depth and levels of social vulnerability. The communities experiencing the largest increases in flood hazard were not consistently the most socially vulnerable.
This is consistent with a mixed picture in the wider literature. While some studies have found that socially disadvantaged communities are disproportionately exposed to flood risk, others have identified weaker or more complex relationships that vary by location, flood type and scale of analysis. Our results from Mobile provide another example of this variability and show why changes in hazard alone may not identify the communities where wider societal impacts could be the greatest.
Nevertheless, hazard remains an important part of the picture. We found that 75% of identified hotspots were also among the top 10% of locations experiencing the largest increases in flood depth.
Incorporating social vulnerability and exposure did, however, change which communities emerged as hotspots. One community emerged as a future hotspot despite a relatively moderate increase in flood depth: its high social vulnerability and substantial building exposure increased its overall hotspot ranking. Another community experienced one of the largest projected increases in flood depth but did not emerge as a hotspot because its social vulnerability was comparatively low.
The hotspots also changed with global warming. Some communities moved from outside the highest-risk group under lower warming levels to within it under 3°C of warming. The analysis therefore illustrates how the locations highlighted by a combined assessment of hazard, exposure and vulnerability can change under different climate scenarios.
Overall, the Mobile results show how considering exposure and social vulnerability alongside changes in flood hazard can alter the picture of where future flood impacts may be concentrated.
Why this matters for decision-makers
The Mobile analysis shows that how future flood risk is assessed can affect which locations stand out. Looking at projected changes in hazard alone highlights some communities; adding exposure and social vulnerability changes the picture.
What matters will depend on the decision being made. Where social vulnerability or distributional impacts are relevant, climate change flood maps can be combined with other datasets to explore those questions rather than treating changes in physical hazard as a proxy for wider societal impact.
Looking ahead
This analysis focuses on river flooding in one city, so it does not tell us whether the same patterns will emerge elsewhere. Applying the approach to other locations, flood types and warming levels would help us understand how these relationships vary.
The Mobile case study demonstrates how climate change flood maps can provide a starting point for questions that extend beyond physical hazard. Understanding where flood hazard may change remains essential; for decisions concerned with societal impacts, combining that information with data about exposure and vulnerability can reveal patterns that hazard alone cannot.
If you’d like to explore how JBA’s climate change flood data could support your own risk, resilience or adaptation assessments, contact us at hello@jbarisk.com.