Understanding flash flooding in the Himalayan Region: a case study

<strong class="journal-contentHeaderColor">Abstract.</strong> In recent years, flash floods repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past, and especially in the 1990s, floods were characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes (Pfister et al., 2004). The intention of this study is to link the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions to identify the cause for this apparent shift. Therefore, we hypothesise that an increase in extreme precipitation events has subsequently led to an increase in the occurrence of flash flood events in central western Europe and all that being caused by a change in the occurrence of flash flood favouring atmospheric conditions. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h^&minus;1 from radar data (RADOLAN, DWD). Moreover, we identified proxy parameters representative for flash flood favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity in the lower troposphere (q &ge; 0.004 kg kg^&minus;1), sufficient latent instability (CAPE &ge; 100 J kg^&minus;1) and weak deep-layer wind shear (DLS &le; 10 m s^&minus;1) proved to be characteristic for long-lasting intense rainfall that can potentially trigger flash floods. These atmospheric parameters, as well as the flash flood and precipitation events were then analysed using linear models. Thereby we found significant increases in atmospheric moisture contents and increases in atmospheric instability. Parameters representing the motion and organisation of convective systems occurred slightly more often or remained unchanged in the time period from 1981&ndash;2020. Moreover, a trend in the occurrence of flash floods was confirmed. The number of precipitation events, their maximum 5-minute intensities as well as their hourly sums were however characterized by large inter-annual variations and no trends could be identified between 2002&ndash;2020. This study therefore shows that the link from atmospheric conditions via precipitation to flash floods cannot be traced down in an isolated way. The complexity of interactions is likely higher and future analyses should include other potentially relevant factors such as intra-annual precipitation patterns or catchment specific parameters.

In recent years, flash floods repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past, and especially in the 1990s, floods were characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes (Pfister et al., 2004). The intention of this study is to link the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions to identify the cause for this apparent shift. Therefore, we hypothesise that an increase in extreme precipitation events has subsequently led to an increase in the occurrence of flash flood events in central western Europe and all that being caused by a change in the occurrence of flash flood favouring atmospheric conditions. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h−1 from radar data (RADOLAN, DWD). Moreover, we identified proxy parameters representative for flash flood favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity in the lower troposphere (q ≥ 0.004 kg kg−1), sufficient latent instability (CAPE ≥ 100 J kg−1) and weak deep-layer wind shear (DLS ≤ 10 m s−1) proved to be characteristic for long-lasting intense rainfall that can potentially trigger flash floods. These atmospheric parameters, as well as the flash flood and precipitation events were then analysed using linear models. Thereby we found significant increases in atmospheric moisture contents and increases in atmospheric instability. Parameters representing the motion and organisation of convective systems occurred slightly more often or remained unchanged in the time period from 1981–2020. Moreover, a trend in the occurrence of flash floods was confirmed. The number of precipitation events, their maximum 5-minute intensities as well as their hourly sums were however characterized by large inter-annual variations and no trends could be identified between 2002–2020. This study therefore shows that the link from atmospheric conditions via precipitation to flash floods cannot be traced down in an isolated way. The complexity of interactions is likely higher and future analyses should include other potentially relevant factors such as intra-annual precipitation patterns or catchment specific parameters.

In recent years, flash floods repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past, and especially in the 1990s, floods were characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes (Pfister et al., 2004). The intention of this study is to link the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions to identify the cause for this apparent shift. Therefore, we hypothesise that an increase in extreme precipitation events has subsequently led to an increase in the occurrence of flash flood events in central western Europe and all that being caused by a change in the occurrence of flash flood favouring atmospheric conditions. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h−1 from radar data (RADOLAN, DWD). Moreover, we identified proxy parameters representative for flash flood favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity in the lower troposphere (q ≥ 0.004 kg kg−1), sufficient latent instability (CAPE ≥ 100 J kg−1) and weak deep-layer wind shear (DLS ≤ 10 m s−1) proved to be characteristic for long-lasting intense rainfall that can potentially trigger flash floods. These atmospheric parameters, as well as the flash flood and precipitation events were then analysed using linear models. Thereby we found significant increases in atmospheric moisture contents and increases in atmospheric instability. Parameters representing the motion and organisation of convective systems occurred slightly more often or remained unchanged in the time period from 1981–2020. Moreover, a trend in the occurrence of flash floods was confirmed. The number of precipitation events, their maximum 5-minute intensities as well as their hourly sums were however characterized by large inter-annual variations and no trends could be identified between 2002–2020. This study therefore shows that the link from atmospheric conditions via precipitation to flash floods cannot be traced down in an isolated way. The complexity of interactions is likely higher and future analyses should include other potentially relevant factors such as intra-annual precipitation patterns or catchment specific parameters.

A short‐duration heavy rainfall events over India have been studied using hourly rainfall data of 126 stations for the period 1969–2015 for the Indian summer monsoon season. The events have been classified into two categories. The first category pertains to cloud burst (CB). The CB events have been further classified into two types, viz. “CBa” and “CBb.” CBa events are associated with heavy rainfall in the steep slope mountainous regions of Himalayas identified based on flash floods and damages to properties and human losses, irrespective of the rainfall amount. CBb events are associated with rainfall &gt;10 cm/hr as per the definition by India Meteorological Department (IMD). The statistics of CBb events has been provided in the paper. A total of 28 CBb events have been recorded over Indian region in the study period. These are found to occur over different parts of India. A new and second category of short‐term heavy rainfall event has been defined as “mini‐cloud burst” (MCB). It is an event in which rainfall in two consecutive rain‐hours is 5 cm or more. Statistical characteristics of these events have been investigated. MCB occurs in June over Western Ghats, over central India and foot hills of Himalayas in July and August. The frequency is low in the month of September. These events generally found to occur in the early morning hours at foot hills of Himalaya and along the west coast of India. In the interior of the land mass these are observed in the afternoon hours while in southern peninsula during night hours. Trend analyses indicate significant increase in these events at many places, except over northeast India. The statistics of CBb and MCB events provided in the paper will trigger numerical modelling studies for understanding the dynamics and invigoration of convection.

ABSTRACTThe present study aims to investigate how large‐scale atmospheric circulation may influence flash floods in Portugal. For this purpose, weather types (WTs) in a northeastern North Atlantic sector are used and flash floods between 1950 and 2003 were identified in six hydrographic basins: three in the north of Portugal and another three in the south of Portugal. An adapted methodology for flash flood detection on daily discharges is applied. A total of 131 flash floods were detected: 35 in the northern basins and 96 in the southern basins. WTs are identified using the National Oceanic and Atmospheric Administration 20th Century Reanalysis V2. The sea level pressure composites for flash flood days by WT reveal the large‐scale atmospheric patterns underlying their occurrence. The results show that the cyclonic (C) type was strongly associated with flash flood days in the northern basins. The C type pattern features deep low pressure systems north of Iberia, much stronger than average for this WT. For the southern basins, the C type still shows the highest relevance for flood occurrence, but to a lesser extent, since the easterly wind (E) and dual anticyclonic (AA) types also acquire some importance. The flash flood‐inducing AA‐type events are characterized by atypical largely zonal troughs extending towards Portugal. The flash flood‐inducing E type events hint at the occurrence of anomalously strong cut‐off low pressure systems over southern Portugal. Although the significance of these systems to precipitation in Portugal was addressed in previous studies, a systematized assessment of their role on flash flood occurrences is herein provided for the first time. The presented flash flood detection methodology can be applied to other regions where hourly flow discharge data are not available, thus being an alternative to the more conventional detection algorithms.

Quantifying regional-scale flash flood susceptibility using watershed geomorphometry-based approach along the high range, west of Chiang Mai Basin, Northern Thailand

This paper was initially motivated by a flash flood index (FFI) study performed by Bhaskar et al. and others that explained flash floods using a runoff perspective by extracting a rising curve gradient, the flood magnitude ratio, and flood response time variables from runoff hydrograph. However, the runoff data are not sufficient to estimate a robust flood magnitude ratio in Korea. In addition, the existing model is not able to consider high rainfall intensity, which is an important flash flood characteristic in Korea. Flood magnitude ratio variables, thus, were replaced by rainfall intensity variables. Then, the results were compared with the existing FFI. The spatial distribution of the flood event in the Han River basin that occurred in July 2006 showed that a modified FFI, which reflects rainfall characteristics, outperformed in representing damaged (flash flood occurrence) areas in the past. Furthermore, the FFI for three different time windows (early, middle, and late periods) for the Han River basin during the localised intensive rainfall in July 2006 were analysed. Analysis of spatiotemporal distribution showed that sudden intensity was pronounced in the Seoul area during the first 10 days of July. During the second 10 days of July, a probable high‐intensity flash flood appeared in the Gangwon‐do area. Meanwhile, an onsite survey of the Gangwon‐do area showed that the actual occurrences of a high‐intensity flash flood were largely in agreement with the predictions made by the model.

The present study aims to analyse the human perceptions and adaptations to recurrent devastating flash floods in Khyber Agency, FATA. The study is mainly based on both primary data sources including field observations, structured questionnaire survey and focus group discussions supported by secondary data sources. Three hundred adult residents in five sample villages along Chora River and its tributaries were interviewed by using a structured questionnaire which covered approximately 20% of the total households. Respondents’ perceptions were investigated about the occurrence of flash floods, flood damages, nature of affinity towards flood prone areas and adaptation strategies. Cartographic and statistical techniques were applied to process and analyze the data. As a result, the data were analytically discussed and presented in the form of tables, maps and plates. The analysis reveals that people of the area perceived about the recurrence of future flash floods during rainy season. Most of the people perceived the flash floods as it hit the study area with devastating impacts and the extent of flood damages. In spite of flood risk, the level of affinity towards living in the flood prone area was found high among the community as they have blood ties with the land, kith and kinship. In the study area, people had adopted various flood mitigation strategies to reduce the devastating impacts of flash floods. However, the resident had greater motivations and concern on socio-economic factors rather than adjustments to frequent flash flood.

Projected frequency of low to high-intensity rainfall events over India using bias-corrected CORDEX models

A study has been conducted on Extreme Weather Events (EWEs) induced mortalities in Jammu and Kashmir, India during 2010-2022. In the present study, we used the frequency of heavy rain, heavy snow, lightning/thunderstorm, Hailstorm and squall during the period 2010 to 2022 of 10 stations of J&amp;K from India Meteorological Department. The mortalities occurred due to these extreme weather events for each district were collected from the Meteorological Centre Srinagar. The mean monthly precipitation and number of rainy days for each month was calculated for each station based on 40 years data (1982 to 2022). During the past 12 years, (2010-2022) a total of 2863 EWEs occurred over J&amp;K in which 552 deaths occurred till 31st December 2022. Among the various EWEs, lightning (1942) and heavy rainfall (409) events were more frequent. When we compare the mortality per event, the heavy snow was more destructive compared to any other EWEs. The mortality per event due to heavy snow was highest (4.33) as compared to other extreme events, although the number of events of heavy snow is less (42) as compared to heavy rain (409), flash floods (168) and lightning (1942). District wise results of EWEs results revealed the highest deaths due to heavy snow were observed over Kupwara, Bandipora, Baramulla and Ganderbal. Similarly for flash floods, the highest deaths were observed over Kishtwar, Anantnag, Ganderbaland Doda. The Pearson correlation results revealed highest correlation of total deaths for heavy rain (0.77) and heavy snow (0.69) (significant at p value p&lt;0.01) followed by flash floods (0.492) (significant at p value p&lt;0.05). Negative correlation result was observed between heavy snow and windstorm (0.584) (significant at p value p&lt;0.05). The present study has shown that, for the union territory as a whole, the heavy rain and heavy snow have been two major disasters causing mortality, though flashfloods, thunderstorms and windstorms are gaining importance. The trend analysis results also revealed that there is a significant increase in mortality over the years particularly due to flash floods (R2 value 0.434) and windstorm (R2 value 0.371).

&amp;lt;p&amp;gt;Since the impacts of climate change on the environment have been constantly rising over the last decades, scientists have paid much attention to understanding the effects of this phenomenon. Climate change leads to different kinds of extremes, such as heavy rainfall events, characterized by short duration and high intensity, and drought, which can cause the problem of water scarcity over a certain area. These types of extreme events cause several damages for the affected areas since they can result in loss of human lives and economic damages. In particular, heavy rainfall events, which are often associated with convective precipitation because of their characteristics, may result in flash floods, especially when they hit small catchments with low times of concentration, thus causing economic damages and, more relevantly, human lives losses.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The increasing occurrence of heavy rainfall events in many areas of Europe, also in Italy, over the last few years, has contributed to raising the importance of understanding which factors could be recognized as drivers of these events. In this perspective, it is possible to identify in atmospheric circulation one of the causes of severe rainfall events occurrence since some air fluxes, generated from certain schemes of atmospheric circulation, could lead to the accumulation of moisture within a certain volume of the atmosphere, hence to the occurrence of rainfall.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Since even the Sicily (Italy) has been experimenting heavy rainfall events and consequent flash floods and urban floods in the last years, this work aims to find out a relationship between some weather circulation patterns, developed by the UK Met Office, and the rainfall Annual MAXima (AMAX) for the Sicily, recorded by the rain gauge network of Autorit&amp;amp;#224; di Bacino - Regione Siciliana. The possible connection between AMAX and WPs has been investigated in order to define some specific schemes of atmospheric circulation that are responsible for leading to the occurrence of AMAX in Sicily. In order to do this, a database containing the AMAX of all the available gauges for the Sicily has been used. A distinction between AMAX occurred in summer and winter season and their related WPs has been performed as well, with the goal to understand the possible influence of WPs on the summer and winter AMAX. Furthermore, in order to distinguish convective from stratiform AMAX, some analyses on reanalysis data, namely the CAPE and the Vertical Integral of Divergence of Moisture Flux (VIDMF), have been done.&amp;lt;/p&amp;gt;

Flash floods are considered as the most intense hazard therefore rapid identification is needed. Tsunami also causes flash floods as it is commonly generated around the Pacific Ocean. Flash floods are also caused by the severely blocked streams in heavy rainfall. Floods have ended up so many lives more than the other natural hazards and also devastated precious belongings and infrastructures. Cattles have also been affected by the floods event. Floods devastate the construction and infrastructure like roads, bridges and buildings that comes in the vicinity of effected area by flood. Breakdown and overflow of dams may produce the deadly flash floods to the populated area and environs. Many strategies and methods have been followed to determine the flash floods on early basis so that evacuation announcements may be propagated. Numerous researches have been studied and carried out to accomplish this task. Development of dams and reservoirs have been given more significance. Artificial Intelligence based competent decision-making algorithms like Bayesian classifier, PSO, ANN, NNARX, SVM and GA have been applied to achieve more accuracy in predictive analysis. Direct observations from the sensors and data from the meteorological department have also been used for the predictive analysis of flash floods. Many yardstick parameters have been proposed in past researches to identify the flash floods vigorously like environmental CO2 levels, precipitation velocity, wind speed, upstream level, height of the water, pressure, temperature and cloud to ground flashes. In this research papers a novel Artificial Intelligence based approach Modified Cuckoo Search (MCS) has been adopted to forecast the flash floods more rapidly and accurately. Obtained results in the MATLAB have proved that Modified cuckoo search with the combination of Artificial Neural Network worked better than the recent available methods. Results have also been validated by comparing the MLP-PSO.

&amp;lt;p&amp;gt;In the second half of the 20&amp;lt;sup&amp;gt;th&amp;lt;/sup&amp;gt; century, hydrological regimes in central Western Europe were largely characterised by large-scale winter floods. This type of event was predominantly triggered by westerly atmospheric fluxes, bringing moist and mild air masses from the Atlantic Ocean to the European continent. Since the late 1990&amp;amp;#8217;s, major flooding events seem to have shifted in time and magnitude. Flash flood events, while being a well-known phenomenon in Mediterranean catchments, are increasingly also reported at higher latitudes. Unlike the large-scale winter flood events, flash floods are of very narrow spatial extension and triggered by rather short, but highly intense rainfall events.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Here, we focus on the specific case of rivers in Luxembourg that have experienced several flash flood events in recent years, while only small to moderate winter flood events have been reported since the late 1990&amp;amp;#8217;s. National hydro-meteorological monitoring and flood forecasting systems have been designed for large-scale floods and are not suited for simulating local flash flood events. Therefore, there is a need to increase our understanding of the hydro-meteorological processes underlying flash flood occurrences in our area of interest.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;While increasing air temperature is known to allow a higher air moisture content that can lead to more intense rainfall events and possible flooding, we moreover hypothesize that the recent increase in flash flood occurrences in Luxembourg is reinforced by a change in atmospheric circulation patterns. To test this hypothesis, we analyse the prevailing atmospheric patterns on rainy days during summer and winter months over the period 1954 - 2019, with a particular focus on rainfall events that lead to moderate and extreme floods. In a next step, we intend to extend our findings for Luxembourg in a larger European context. This analysis should allow to better assess the current situation of hydrological extreme events in central Western Europe in order to take precaution measures and prepare for a diversifying hazard.&amp;lt;/p&amp;gt;

The geographical distribution of a flash flood's occurrence could be an important consideration in assessing its severity and social impacts. In this study, the spatial distribution of survey responses is analysed with respect to features such as the stream network (natural water path), the drainage network, buildings, and catchments. This provides information on both flash flood distribution causes in an urban setting and residents' perceptions of the hazard that would otherwise not be revealed. The study demonstrated the possibility of taking into account people's experience in detecting the location of potential flash flood hazards. It also illustrates the usefulness of GIS-supported spatial analysis of survey results in identifying factors that increase the occurrence of urban flash floods.

The Indian summer monsoon is a spectacular seasonal phenomenon on the globe. Large part of Indian summer monsoon rainfall is an outcome of various synoptic scale disturbances, many of which are extremely intense. Monthly and seasonal rainfall amounts are considered for describing general behaviour of summer monsoon. These amounts may give a misleading impression that monsoon is a robust and slowly evolving system. The mean rainfall, however, is the rainfall averaged over many sporadic weather events having spatial scales from 100 to 1,000 km (Stephenson et al. 1999). In past decade, there have been many instances of very heavy and extreme rainfall events over India (Rajeevan et al. 2008). Recently, in the month of June 2013, the state of Uttarakhand experienced heavy rainfall event owing to passage of well-formed synoptic systems. Incessant rainfall from 14 to 17 June 2013 accompanied with melting snow and Himalayan terrain resulted in flash floods in this region especially over Kedarnath valley and adjoining areas.