A 100-year flood does not mean a flood that occurs once every 100 years, but one with a 1 per cent chance of occurring in any given year.
Kerala’s 2018 flood was compared with the great flood of 1924, creating the impression that such events would not recur soon.
Repeated floods in Kerala since 2018, including flooding along the Pamba and Manimala rivers in 2026, show why that understanding is misleading.
Flood-risk mapping and 100-year flood estimation are now essential for Kerala’s infrastructure planning, evacuation preparedness and protection of low-lying communities.
After the great floods of 2018, many in Kerala were relieved by the belief that they would not have to witness another major flood in the near future. However, since 2018, floods have become a recurring phenomenon in the state. In 2026, the banks of the Pamba and Manimala rivers were submerged in a manner similar to the 2018 deluge, causing loss of lives and severe damage.
When the major floods struck in 2018, there were no comparable recent events to draw from. Experts and the media therefore compared it with the great flood of 1924, known in the Malayalam calendar as 1099 and locally remembered as the Thollayirathi Thonnootti Ompathile Vellappokkam, or the ‘99 Flood’.
The floods of 1924 and 2018 came to be described as ‘floods of the century’. In technical terms, the 2018 flood was also described as a ‘100-year flood’. It was a phrase that offered a sense of comfort, implying an event that happens only once every hundred years.
However, Kerala has faced repeated floods since 2018. Does this mean the technical term ‘100-year flood’ is wrong? The answer is no.
The misunderstanding arose because there had been no immediately preceding flood of similar magnitude, and the 94-year gap between 1924 and 2018 appeared to fit neatly with the ‘100 years’ in the phrase. This led many to misconstrue its actual meaning.
In reality, a ‘100-year flood’ does not mean a flood that happens once every century. It is a technical phrase used in flood mapping to denote a 1 per cent probability of a flood occurring in any given year. Although the phrase ‘flood of the century’ has long been used in public language, scientists describe such an event more accurately as a ‘1 per cent annual exceedance probability flood’.
Similarly, if the probability of flooding is 2 per cent, it is called a 50-year flood. If the probability is 5 per cent, it is called a 20-year flood. A 20-year flood means the flood risk in that area is five times higher than that of the commonly referenced 100-year flood. These are collectively understood through flood exceedance probability.
To put it simply, suppose the probability of getting heads when tossing a coin is 50 per cent. This does not mean heads cannot appear twice in a row. There is still a 25 per cent chance of that happening. Similarly, a 100-year flood does not mean one must wait 100 years for it to occur. There is a 1 per cent chance of such a flood in any given year. The probability remains 1 per cent every year. The severe flood of 2018, therefore, did not make flooding in the following years impossible.
A 100-year flood is determined by calculating the peak flow rate that causes flood elevations in rivers. This is done using flood-modelling software to analyse statistical data derived from historical river discharge rates or maximum rainfall amounts. Areas delineated in this way are called 100-year floodplains. Saying a region is a ‘100-year floodplain’ means it is an area where water from a flood with a 1 per cent annual probability is expected to reach.
In Kerala’s context, studies evaluated the August 2018 flood as roughly equivalent to a 100-year flood event. Before that, the previous flood of similar magnitude occurred in 1924. There was a 94-year gap between the two events. Broadly speaking, the 2018 flood was an event with a 1 per cent annual probability.
But flooding struck Kerala again in 2019 and 2026, with several non-trivial floods in between. Moreover, many of these floods affected the same areas that had been submerged earlier. These events helped correct the misconception that a 100-year flood would not repeat soon.
Kerala has a distinctive topography. Steeply sloped mountain ranges, 44 rivers and high-intensity rainfall over short durations in the Western Ghats make the state particularly vulnerable to flooding.
When calculating 100-year flood levels in Kerala, scientists cannot rely only on river discharge rates or simple rainfall volumes. They must also account for factors such as dams on rivers, encroachment of traditional rainwater retention systems such as paddy fields and wetlands, and construction along riverbanks. The reduced natural width of rivers and shallow beds caused by silt accumulation further increase flood risk.
Studies that include 100-year flood estimation and flood-risk mapping have become indispensable for Kerala’s future. Such research is crucial to ensure that infrastructure such as roads and buildings in flood-prone areas is raised above the designated flood line. It is also essential for helping the State Disaster Management Authority plan evacuations in advance.
Given the rapidly changing climate and the risks faced by people living in low-lying coastal and riverine regions, these are not merely academic exercises for Kerala. They are an urgent necessity for protecting lives.
Ganesh Radhakrishnan is a freelance journalist. Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth