With increasing hazard extremes due to climate change, people settled in places that flood rarely are at risk
Photo: iStock In floodplains sensitive to rare and extreme floods, the population density is higher compared to places that place frequent, low-magnitude floods. With hazard extremes increasing due to climate change, this distribution puts the first population group at risk, according to a new report.
Laterally unconfined floodplains experience flood extents that grow most rapidly during rare, extreme magnitude events. Populations show a tendency to settle on these rarely flooded zones worldwide, the report published in Nature Communications showed.
Although only making up 5 per cent of the populated river reaches, laterally unconfined floodplains are home to 412 million people, thus accounting for 21 per cent of the total population exposure globally.
The highest population density on laterally unconfined floodplains is found in Asia and North Africa. These regions also have the highest density of exposure per river reach on these floodplains, with 4,519, 2,536, 1,973 and 902 people exposed per kilometre of reach length in South, East, South-East Asia and North Africa, respectively.
In comparison in North America and Europe, this comes to only 183 and 247 people exposed per kilometre of reach length.
With potential increases in the frequency of extreme river flows in the projected future for these regions, an exponential growth in population impacted by flooding could be experienced in locations that typically would not have flooded within living memory, the study warned.
For example in China, 82 per cent of the reaches with laterally unconfined floodplains have the highest density of exposure found in the rare, extreme magnitude flood zones — “69 million people living in the 100-year flood zone or greater”.
These regions have experienced rapid population growth, urbanisation and industrialisation in the last two decades, including the growth of some of the world’s largest urban agglomerates on large river floodplains and deltas, such as Shanghai, Cairo, Dhaka and Bangkok.
Urban population is expected to continue to grow in the future with 90 per cent of the projected increase taking place in Asia and Africa. This continued growth will put additional strain on the existing floodplain settlements, placing even more people at risk from flooding in the future.
When it comes to frequently flooded zones of floodplains, more people live in these areas as they also cover a greater area. This means that people have either found ways to cope with flooding from frequent, low-magnitude events, or have taken on risk through choice or necessity, said the study led by Laura Devitt, University of Bristol, United Kingdom.
Some of the countries that have larger populations on frequently flooded floodplains also have high standards of protection, the study found.
In Europe, 22 countries (58 per cent) have a mean standard of protection of at least a 50-year return period, while 10 countries (26 per cent) have very high standards of protection of at least a 100-year return period (Austria, Belgium, Germany, United Kingdom, Croatia, Hungary, Ireland, Netherlands, Poland, Portugal, Romania and Slovakia).
Around 39 million people are settled on partially confined floodplains in these countries, accounting for 48 per cent of the total exposure on this floodplain type in Europe.
Flood extents on partially confined floodplains grow most rapidly during frequent, low-magnitude events. On this floodplain type, nearly all countries are found to have the highest density of exposure in the rarely flooded zones of the floodplain. However, the study found regional differences in whether more people are living in the frequently or rarely flooded zones.
In Europe, countries exhibit a mostly even distribution of population density throughout all flood zones, with population totals being greatest on areas of the floodplain that would naturally flood frequently, given that they make up the largest areas. For Europe and North America, this is likely due to significant investments in structural defences to protect against flooding from frequent events.
However, structural protection tends to encourage development in flood-prone zones, a socio-hydrological process known as the “levee effect”. This effect can lead to floodplain settlements becoming vulnerable to low-probability but potentially high-consequence flood events, which is “problematic under climate change induced non-stationarity of flood extremes”.
This effect was for example seen during the catastrophic 2005 flooding of New Orleans brought by Hurricane Katrina and the devastating impacts of the European floods of 2021.
Asian countries have a similar settlement pattern on partially confined floodplains. Given the particularly high density of population on this floodplain type in this region — 14 per cent of total land area experiences flooding but 35 per cent of the total population are settled there. It is likely that pressures on land have resulted in people settling in areas of the floodplain at risk from frequent flooding.
South America is the only continent where more people live in rarely flooded areas of partially confined floodplains relative to the frequently flooded areas. These countries also have the highest population densities in these low-hazard zones.
Standards of protection are generally low, however, the relatively low population density on floodplains in general has allowed for preferential settlement in less hazardous locations.
The study found that around 24 per cent of total land area in South America has potential for flooding, but only 13 per cent of the total population lives there.
The researchers looked at the three dominant floodplain categories — confined, partially confined and laterally unconfined.
Confined floodplains are typically found alongside steep streams in mountainous bedrock regions. In these confined floodplains, flood extents typically grow most rapidly during rare, extreme magnitude events due to the steep slopes next to the river channel.
Partially confined floodplains are the most common type. They are found on transitional streams like valley bottoms, and in these floodplains, flood extents typically grow most rapidly during frequent, low-magnitude events.
Laterally unconfined floodplains (like deltas) are very wide, flat and unbound, which allows for exponential growth in the inundation area.
The extent of flooding is typically discharge-limited, making them sensitive to changes in extreme discharge, and therefore flood extents grow most rapidly during rare, extreme magnitude events.
The study can help in understanding the global flood risk and the relationship between rivers, their floodplains and human settlements. Clear regional differences can be seen in settlement patterns that are related to whether floodplains are most sensitive to flooding from frequent and low magnitude, or to rare and extreme magnitude events.
Adaptation will be a critical factor in determining the severity of impacts from flood hazards in the coming decades. However, the flood-prone states in India have not identified or demarcated affected areas, in spite of recommendations by multiple committees, a report by the Comptroller and Auditor General of India (CAG) in 2017 found.
In 1975, the Central Water Commission (CWC) circulated a model bill for flood plain zoning among states for enactment. Flood plain zoning demarcates zones or areas likely to be affected by floods of different magnitudes or frequencies the damage can be mitigated.
The CAG report observed that only three states — Manipur, Rajasthan and Uttarakhand — had enacted Flood Plain Zoning Acts.
The report said that if these acts were implemented, the destruction in the 2013 floods of Uttarakhand as well as the Chennai floods of 2015 would be lower.
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