Flash floods are one of the most devastating natural hazards with over 5000 deaths annually and considerable social, economic, and environmental impacts. Flash floods are defined as sudden floods caused by extreme rainfall events. Flash floods usually occur in a very short span of time after an extreme precipitation event, usually less than six hoursand characterized by sudden rise and recession of high flow over localized areas, which makes them extremely unpredictable and disastrous. Flash floods cause the highest mortality rate (ratio of death to the people affected) compared to riverine and coastal flooding.
India is highly susceptible to floods as more than 40 million hectares of geographical area is flood prone. Several river basins and urban areas (Mumbai, 2005; Leh, 2010; Uttarakhand, 2013; Jammu and Kashmir, 2014; Chennai, 2015; Kerala, 2018; Bihar, 2019; Hyderabad, 2020; Assam, Meghalaya, Arunachal Pradesh, 2022; Assam, 2024; Manipur,2025, three times so far) experienced flash floods in the past decade (EM-DAT), which severely impacted infrastructure and agriculture in India. Presently Manipur is experiencing impact of flash floods in various valley areas. The low-lying river basins in India are highly susceptible to flash floods. The occurrences of high intensity rainfall in a short duration causing flash floods are on the rise even in rainfall scarce regions, such as semi-arid region of western India. The rising frequency of flash floods and substantial economic losses underscore the need to reinforce the flash flood preparedness across the nation.
The combination of meteorological, geomorphological, and anthropogenic factors attribute to the occurrence of flash floods. The intense rainfall associated with tropical cyclones and the summer monsoon is a primary driver of flash floods. Saturated soil or impervious surface exacerbates flash flooding by causing instant runoff. Conversely, in arid regions such as desert landscapes the dry soil may lead to quick surface runoff generation. Especially in Manipur, frequent flash floods are due to deforestation in the hills of Manipur for wide range poppy cultivation. Cloudbursts events, which are defined as precipitation events exceeding 100 mm/hour intensity, cause flash floods in hilly terrains. In addition, dam breach or sudden release from dams can result in flash floods. Along with the extreme rainfall, the geomorphology of the region also plays an important role in deciding the inundation extent and severity of flash floods. Morphometric parameters like basin relief, drainage characteristics, and basin shape promote higher flood peaks and runoff. In addition, basin shape can favour flash flood occurrence as circular basins cause higher runoff in less time compared to elongated basins. Extreme rainfall at steep slopes and higher topographic relief can increase flash flood severity. For instance, regions like Assam, Arunachal Pradesh and Manipur are more flash flood prone due to their location at the foothills of Himalayas, which receives orographic precipitation.
The frequency of flash floods in India is projected to increase under the warming climate. Rising global mean temperature is linked to increased moisture holding capacity of the atmosphere, which can result in intense and more frequent rainfall events. In India, the summer monsoon season has become more erratic with prolonged dry periods followed by intense rainfall events. Despite a long-term decline in mean precipitation, an increase in the intensity and frequency of extreme precipitation events, can trigger flash floods. Precipitation variability, driven by climate change, poses significant challenges for managing and mitigating flash flood risks. Unlike other hydro-meteorological hazards, flash floods are not location specific. Every location where extreme rainfall occurs can be prone to flash floods, therefore, examining flash floods and their hotspots under the warming climate is essential. Moreover, identifying and mapping flash flood prone areas will help in flash flood mitigation measures and aid in regulating development in susceptible regions.
Implementation of effective flash flood early warning systems is crucial for timely preparedness and evacuation The lack of high-resolution climate simulations makes it challenging to quantify the changes in flash flood risks. Insufficient number of rainfall records and observing systems at hourly scale hampers effective flood detection and forecasting. Furthermore, there is a need for improved integration of satellite and ground-based observations to enhance flood monitoring capabilities. The effectiveness of flood risk management is hindered by a lack of awareness of potential flash flood risk zones. As the vulnerability and the population exposed to floods are increasing, examining the flash flood risk across the regions is crucial. As most flash floods typically originate from small catchments in complex terrains a sub-basin scale flash flood risk assessment is needed in India incorporating the hydro-meteorological aspects as well as the geomorphological aspects We find that flash floods in the Himalaya region are driven by the geomorphological factors while flash floods in central India and western Ghats are mainly controlled by the flashiness index. In addition, geomorphological and hydrological factors control flash floods in the Indian sub-continent regardless of the size of the basins. Most flash floods in the sub-continent occur due to the combination of wet antecedent soil moisture and extreme precipitation. Our findings highlight that the changing rainfall characteristics can alter flash flood hotspots in the future under warming climate, underscoring the need for adaptation.
South-east peninsular India experiences extreme precipitation events due to the cyclonic events and depressions that take place in the southern Bay of Bengal. Roxy et al. reported that the change in local precipitation across central India is primarily influenced by the atmospheric dynamics rather than local thermodynamics. The increase in extreme precipitation events across central India is attributed to the surge in moisture supply caused by the variability in low-level monsoon westerlies over the Arabian Sea. On the other hand, the Himalayan region receives moisture from different sources that can result in extreme precipitation and cloudburst events. The cloudburst events that occur in monsoon season across lower Himalayan region (particularly in western parts) cause localized flash floods, debris flows and landslides. In addition, cloudbursts that are driven by intense convective activity and influenced by mid-latitude and polar jet streams are observed in pre-monsoon season.
Among the river basins, most of the observed flash floods occurred in the Brahmaputra River basin followed by Ganga and Krishna River basins. We find an increase in the total number of flash flood events reported in India after 1995). The widespread flash flood events that happened across India (majorly affecting Assam province and East Coast) in summer and winter monsoon of 2008 caused severe casualty. The yearly frequency of extreme precipitation events in India has doubled during the pre-monsoon season. Moreover, significant increases of 56%, 40%, and 12.5% in extreme precipitation are reported during the monsoon, post-monsoon, and winter seasons, respectively. The increase in the intensity and frequency of extreme precipitation in the Indian sub-continent can be attributed to the warming climate. Climate change drives frequent and intense extreme precipitation events through increased atmospheric moisture, ocean warming, and altered monsoon dynamics. Heavy rainfall and rainfall on saturated soil can lead to flash floods and riverine floods in the Indian sub-continental river basins
Flash flood susceptibility based on geomorphology distinguishes the hilly terrains and Himalayan region as the most flash flood prone. The geomorphological parameters including relief ratio, relative relief, drainage texture, stream number, length of overland flow and ruggedness number highlight the highly flash flood prone sub-basins. Víg et al. reported that drainage texture and relief ratio directly impact flash flood occurrence while elongation ratio have an inverse relationship with flash flood intensity in the low mountain regions. Similarly, Ahmed et al. reported that length of overland flow as one of the significant predictors for flash flood susceptibility. While flashiness index provides dynamic information of the water behaviour in response to extreme rainfall events, geomorphological characteristics of river basins help us understand the role of physical landscape in channelling and storing water. Morphometric features like slopes, drainage networks, and soil types create local differences in flood susceptibility. For instance, morphometric features like steep slopes and narrow valleys can concentrate streamflow, increasing flood intensity in certain areas. Similarly, complex drainage patterns, such as dense networks or poorly defined channels, influence where and how flooding occurs.
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