one hand, since 2010 urbanization and development in high-risk coastal zones has been occurring at rapid rates in many parts of the world. These trends are putting an increasing number of people in harm’s way. Human activities, such as ground water extraction, are exacerbating risks as land subsidence is increasing the extent and intensity of flooding. On the other hand, the increase in the estimated flood exposure headcount is methodological; the use of more accurate and higher resolution data, as well as the inclusion of pluvial flooding mean that this study captures flood risks more comprehensively than past studies.

In addition, the Global Assessment Report (UNDRR 2017) offers estimates of risks from five major natural hazards (including flood risks), though it focuses on monetary quantifications of disaster risks. Besides such global assessments, the recent availability of high-resolution flood and population maps has enabled countless local and national risk assessments, for instance in Tanzania, the United Kingdom, the United States, and Vietnam (Chakraborty et al. 2014; Erman et al. 2019; Fielding 2012; Braese et al. 2020).

# **Flood risks and poverty.**

Many analyses have explored the link between natural hazards – flood risks in particular – and poverty; a detailed review of this literature is offered by Hallegatte et al (2020). Studies at the country level illustrate the complex factors at play. For instance, Rentschler et al. (2020) show that people in coastal provinces in Vietnam are by far the most exposed to flood risks. In particular, the low-lying and densely populated Red River and Mekong Deltas display a concentration of people, economic activities, and assets. Coastal provinces also consistently have the lowest poverty rates in the countries. For instance, in Ho Chi Minh City, the country’s economic powerhouse, extreme poverty is nearly eradicated – and yet more people than anywhere else in the country live in flood hazard zones. In other words, high-income provinces and districts tend to be more exposed to flood risks. However, a different study finds that in Ho Chi Minh City, the poor are disproportionally exposed to flooding (Narloch and Bangalore 2018). How do these results fit together?

Indeed, at the local level the relationship between poverty and flood exposure can be inversed. Flood hazards tend to be highly localized, with some neighborhoods being at risk from frequent inundation, while other nearby neighborhoods may benefit from higher elevation or better drainage systems. Hallegatte et al. (2017) review the literature and conclude that land and housing markets often push poorer people to settle in these riskier areas, especially where land is scarce. Indeed, a meta-analysis of the literature suggests that a 1 percentage point increase in the yearly probability of flooding is associated with a 0.6 percent decrease in housing prices (Daniel, Florax, and Rietveld 2009). In Ho Chi Minh City, for instance, qualitative surveys suggest flood-prone areas can be much cheaper than non-flood-prone areas for the same quality of accommodation (World Bank and Australian AID 2014). Using a household survey in Dar es Salaam, Tanzania, Erman et al. (2019) find that flood-prone dwellings are valued at 30 percent less on average than safe ones.

Reduced housing prices, then, make it possible for poor people to access housing opportunities that could be out of reach in the absence of risk (Husby et al. 2015). In developing countries with informal markets, land scarcity can be particularly acute and land markets function poorly (Durand-Lasserve, Selod, and Durand-Lasserve 2013). In these places, it may not be the prices that push poor people into risky places but simply the availability of land with appropriate access to jobs and services. Informal settlements are often located in hazard-prone locations such as on hillsides, close to riverbanks, or near open drains and sewers—Pune (India), Dhaka, Caracas, Rio de Janeiro, and Mumbai have many such settlements (Lall and Deichmann 2012; Lall, Lundberg, and Shalizi 2008).

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