Temporal variation in flood occurrences with different affected areas
There were 1279 Level III floods, 1718 Level II floods, and 1852 Level I floods during 1985–2019. The median and 90th percentile of deaths in descending order were Level III > Level II > Level I. Level II floods have the highest average amount of deaths. This may have been due to severe casualties caused by some individual flood events, such as the floods in Bangladesh in 1991 and Myanmar in 2008, and the impact of individual disastrous events was also reflected in the large standard deviation of Level II flood mortality (Table 3). At a global scale, annual flood occurrence showed an increasing trend during the study period (with a Z value of 2.08 in the MK test). The monotonous increasing trend of global flood frequency was mainly caused by the increase in Level II and Level III flood frequencies, while Level I floods show an decreasing trend but not significant (Fig. 1 and Table 4). We also found that the average area affected by individual flood event increased yearly at a global scale (Table 4).
At the continental scale, Level II and Level III floods in Africa, South America, and Oceania had a tendency to increase at the 5% significance level (Table 5), and Level III floods exhibited the largest trends. Level II floods in Asia had a monotonous increasing trend at the 10% significance level. Europe was dominated by Level I and Level II floods, and only the Level I floods showed an increasing trend at the 10% significance level; North America was dominated by Level I and Level II floods, and only Level III floods increased yearly at the 5% significance level.
Spatial distribution of flood frequency and mortality with different affected areas
Using the affected area for each flood event given in the DFO database, we obtained the spatial distribution of global flood occurrence during the study period (Fig. 2). The regions with high flood frequency were mainly distributed in the low latitude areas of the Northern Hemisphere and coastal areas, such as the Yangtze River Basin and coastal areas in eastern Asia, the Ganges–Brahmaputra River Basin in South Asia, and the Mississippi River Basin in North America. In Africa, flood-prone areas were found near the Nile River, Niger River, and the eastern coast of Africa; floods in South America mainly occurred along the eastern seaboard and the Magdalena River in the northwest. Level I floods mostly occurred in Southeast Asia, the Ganges River Basin, the northern Mediterranean Sea, around the Black Sea, and along the coast of Latin America and island countries (Fig. 2b). Level II floods were more distributed in the northern Bay of Bengal, Southeast Asia, the northern Mediterranean of the Danube River Basin in Europe, eastern North America and Latin America (Fig. 2c). Level III floods mainly occurred in South Asia and the Yangtze River Basin and along the eastern coast of China and the Mississippi River Basin (Fig. 2d).
For flood-induced mortality, Level III floods in Africa and South America caused the highest proportion of cumulative deaths (48% and 83% of all levels on each continent) (Fig. 3b) and had the largest proportion of affected populations (77% and 62%, respectively) (Fig. 3c), while South America had the highest proportion of Level III floods (49%) (Fig. 3a). Asia had the largest proportion of cumulative mortality (79%) resulting from Level II floods but also had the largest proportion of cumulative affected population (71%) in Level III floods. Europe had the lowest number of Level III floods (18%) and the lowest proportion of cumulative related mortality (19%) but had the highest proportion of people affected (53%); of the floods in Europe, 44% were Level I floods. Level III floods in North America caused the largest proportion of cumulative deaths (54%), but Level II floods affected the largest proportion of cumulative population (78%) among three level floods. For Oceania, Level I floods occurred at the highest frequency (39%) and resulted in the greatest cumulative mortality (56%) and most affected populations (53%).
Frequency and mortality of floods with different affected areas in the top 30 countries
We selected the top 30 countries with the highest flood frequencies, flood-induced mortalities and affected populations, respectively, and analysed the proportion of floods by different categories of affected areas in these countries. In countries with less than 70 floods over the study period, the proportion of Level I floods was approximately 40–60%, and the proportion of Level III floods was approximately 10–30%. Countries with more than 70 floods had approximately 20–50% Level I floods, and the proportion of Level II floods was approximately 30–50% (Fig. 4a).
The top ten countries most frequently affected by floods were the USA, China, India, Indonesia, the Philippines, Australia, Vietnam, Russia, Brazil, and Bangladesh. The USA had the highest flood frequency (467 flood events), with no significant difference in the proportion of Level I (37%) and Level II floods (36%). However, the USA ranked 14th in the world in terms of flood-induced mortality; 49% of the deaths were caused by Level III floods (Fig. 4b), which accounted for 27% of all flood events in the USA. During 1985–2019, there were 396 floods in China; the proportions of Level I, Level II and Level III floods were 31%, 28% and 41%, respectively; the deaths caused by Level III floods accounted for 64% of the flood-induced mortalities, and the affected population accounted for 83%. In India, of the three categories of floods, Level III floods occurred the most frequently and resulted in the most deaths and affected populations.
In Fig. 4b, approximately three-quarters of the dots are distributed close to the hypotenuse on the right side of the triangle, indicating that flood-induced mortalities in these countries were mainly caused by Level II or Level III floods. Thailand, Bangladesh, and Myanmar, at the top corner of the triangle, were the top three countries with the highest flood-induced mortality in the world, with most of the deaths caused by Level II floods. In Venezuela, India, Pakistan, and Somalia, at the bottom right of the triangle, the majority of deaths have been caused by Level III floods; Level I floods caused over half of the deaths in only four countries (Japan (93%), Tanzania (90%), Dominican (86%) and Indonesia (62%)) of the 30 countries. For South Korea, the proportions of mortality caused by Level I and Level II floods were 49.3% and 48.9%, respectively. In Fig. 4c, most of the countries are located in the lower half of the right hypotenuse of the triangle, indicating that most flood-affected populations were caused by Level III floods. In addition, at the top of the triangle, the cumulative population affected by Level II floods in Nicaragua (20 flood events) and Myanmar (25 flood events) accounted for 99% and 85% of the total affected populations, respectively; Level I floods accounted for 85%, 57% and 56% of the affected populations in South Korea (33 flood events), Sri Lanka (58 flood events) and Indonesia (206 flood events), respectively. These differences in flood-affected populations support the need for different countries to implement appropriate flood coping strategies and prevention measures. In particular, Level III floods warrants special concern. These floods generally caused a greater proportion of deaths and affected people, although they are not the most frequent type of floods in some countries.
Event-based analysis of influencing factors and contributions to flood-induced mortality
For this study, we selected flood events during the period 2001–2019 according to overall data availability and used Geodetector to calculate the contribution rate of each factor to the spatial heterogeneity of the mortality caused by individual flood events.
The main influencing factors were affected population and per capita GDP. The contribution rates of the influencing factors significant at the 5% level, in descending order, were affected population (26.0%) > GDPper (10.7%) > flood duration (8.9%) > female (% of total population) (5.5%) > maximum hourly precipitation (4.9%) > affected area (4.4%) > curve number (2.1%) > mountainous area ratio (1.5%) (Fig. 5).
The eight significant influencing factors were further divided into several groups to examine the corresponding distribution of flood-induced mortality (Fig. 6). The number of deaths per event increased with flood duration, maximum hourly precipitation, affected area and population. As shown in Fig. 6, flood-induced mortality first increased with economic development (GDP per capita) and then decreased as more investments can be put on flood prevention measures. The mortality per flood event decreased with an increase in the proportion of females. When the curve number value was less than 79, flood-induced mortality increased as the curve number increased. The proportion of mountainous area is an integrated indicator that represents topographic factors. In areas with mountainous area ratio lower than 20%, the death per flood increased with the increase in mountainous area ratio. When the percentage of mountainous area was greater than 20%, the same rule was applied. The affected area with a mountainous area ratio between 3 and 20% had the highest mean mortality.