Impact of climate change on extreme rainfall events and flood risk in India

The occurrence of exceptionally heavy rainfall events and associated flash floods in many areas during recent years motivate us to study long-term changes in extreme rainfall over India. The analysis of the frequency of rainy days, rain days and heavy rainfall days as well as one-day extreme rainfall and return period has been carried out in this study to observe the impact of climate change on extreme rainfall events and flood risk in India. The frequency of heavy rainfall events are decreasing in major parts of central and north India while they are increasing in peninsular, east and north east India. The study tries to bring out some of the interesting findings which are very useful for hydrological planning and disaster managements. Extreme rainfall and flood risk are increasing significantly in the country except some parts of central India.     

Some characteristics of very heavy rainfall over Orissa during summer monsoon season

Orissa is one of the most flood prone states of India. The floods in Orissa mostly occur during monsoon season due to very heavy rainfall caused by synoptic scale monsoon disturbances. Hence a study is undertaken to find out the characteristic features of very heavy rainfall (24 hours rainfall ≥125 mm) over Orissa during summer monsoon season (June–September) by analysing 20 years (1980–1999) daily rainfall data of different stations in Orissa. The principal objective of this study is to find out the role of synoptic scale monsoon disturbances in spatial and temporal variability of very heavy rainfall over Orissa. Most of the very heavy rainfall events occur in July and August. The region, extending from central part of coastal Orissa in the southeast towards Sambalpur district in the northwest, experiences higher frequency and higher intensity of very heavy rainfall with less interannual variability. It is due to the fact that most of the causative synoptic disturbances like low pressure systems (LPS) develop over northwest (NW) Bay of Bengal with minimum interannual variation and the monsoon trough extends in west-northwesterly direction from the centre of the system. The very heavy rainfall occurs more frequently with less interannual variability on the western side of Eastern Ghat during all the months and the season except September. It occurs more frequently with less interannual variability on the eastern side of Eastern Ghat during September. The NW Bay followed by Gangetic West Bengal/Orissa is the most favourable region of LPS to cause very heavy rainfall over different parts of Orissa except eastern side of Eastern Ghat. The NW Bay and west central (WC) Bay are equally favourable regions of LPS to cause very heavy rainfall over eastern side of Eastern Ghat. The frequency of very heavy rainfall does not show any significant trend in recent years over Orissa except some places in north-east Orissa which exhibit significant rising trend in all the monsoon months and the season as a whole.     

Hydrometeorological Aspects of Floods in India

The Indian sub-continent being located in the heart of the summermonsoon belt, receives in most parts more than 75% of its annual rainfall during the fourmonsoon months of June to September. As the bulk of summer monsoon rainfall occurs withina period of four months, naturally majority of floods occur in Indian rivers during thisseason only. The ground conditions also help in generating high percentage of run-offbecause of the antecedent wet conditions caused by rainy spells occurring within the monsoonperiod itself. Besides mentioning different weather systems, which cause heavy rainfall and consequentfloods, a detailed discussion of 15 years' floods in different river systems has alsobeen given in the article. This study has shown that the flood problem in India is mostly confinedto the states located in the Indo-Gangetic plains, northeast India and occasionally in therivers of Central India.     

Relationship between atmospheric conditions at Dhaka, Bangladesh, and rainfall at Cherrapunjee, India

To improve flood forecasting, the understanding of the atmospheric conditions associated with severe rainfall is crucial. We analysed the atmospheric conditions at Dhaka, Bangladesh, using upper-air soundings. We then compared these conditions with daily rainfall variations at Cherrapunjee, India, which is a main source of floodwater to Bangladesh, and a representative sample of exceptionally heavy rainfall events. The analysis focussed on June and July 2004. June and July are the heaviest rainfall months of the year at Cherrapunjee. July 2004 had the fourth-heaviest monthly rainfall of the past 31 years, and severe floods occurred in Bangladesh. Active rainfall periods at Cherrapunjee corresponded to “breaks” in the Indian monsoon. The monsoon trough was located over the Himalayan foothills, and strong westerly winds dominated up to 7 km at Dhaka. Near-surface wind below 1 km had southerly components, and the wind profile had an Ekman spiral structure. The results suggest that rainfall at Cherrapunjee strongly depends on the near-surface wind speed and wind direction at Dhaka. Lifting of the near-surface southerly airflow by the Meghalaya Plateau is considered to be the main contributor to severe rainfall at Cherrapunjee. High convective available potential energy (CAPE) also contributes to intense rainfall.     

历史极端雨涝事件研究——1823年我国东部大范围雨涝

1823年(清道光三年)我国发生大范围、多流域的严重雨涝,这是在小冰期寒冷气候背景下的重大气象灾害和极端气候事件.文章依据历史文献记载复原了1823年的气候实况并绘图显示,指出该年我国华北夏季雨期长、多大雨,北京6～8月雨日53天、降水量663mm超过现代(1971～2000年)平均值5成;长江中下游全年多雨,梅雨期长...     

Flood events, fatalities and damages in India from 1978 to 2006

High temporal and spatial variability of rainfall qualifies India to be highly vulnerable to floods. Recurring floods of various magnitudes play havoc with the lives and property of the people, leading to unplanned development and unchecked environmental degradation, thwarting and retarding the overall development of the country. Therefore, the purpose of the present study is to analyze the types and trends in terms of flood events, frequency, number of people killed, injured, missing and economic damage both in space and time on the basis of a nationwide database published by India Meteorological Department, Pune, from 1978 to 2006. Analysis of these long-term data has revealed that 2,443 flood events claimed about 44,991 lives with the average of 1,551 lives each year. In terms of population size, these figures translate into a loss of 1.5 human lives per million of the population. A majority (56 %) of flood fatalities were caused during severe flood events. However, the frequency of these events was just 19 % in comparison with heavy rainfall events (65 %). In spatial context, flood-related fatalities are distributed all over the country with highest fatalities in Uttar Pradesh (17 %), Maharashtra (13 %), and Bihar and Gujarat (10 % each). Most fatalities occurred during the summer season monsoon months of August (30 %) followed by July (29 %) and September (20 %). The country suffered a cumulative flood-related economic loss of about 16 billion US$ between 1978 and 2006 and a maximum economic loss of 1.6 billion US$ in the year 2000 alone. The study further suggests that both flood events and fatalities have increased in India over a period of time.     

Hydraulic routing of extreme floods in a large ungauged river and the estimation of associated uncertainties: a case study of the Damodar River, India

Many developing countries are very vulnerable to flood risk since they are located in climatic zones characterised by extreme precipitation events, such as cyclones and heavy monsoon rainfall. Adequate flood mitigation requires a routing mechanism that can predict the dynamics of flood waves as they travel from source to flood-prone areas, and thus allow for early warning and adequate flood defences. A number of cutting edge hydrodynamic models have been developed in industrialised countries that can predict the advance of flood waves efficiently. These models are not readily applicable to flood prediction in developing countries in Asia, Africa and Latin America, however, due to lack of data, particularly terrain and hydrological data. This paper explores the adaptations and adjustments that are essential to employ hydrodynamic models like LISFLOOD-FP to route very high-magnitude floods by utilising freely available Shuttle Radar Topographic Mission digital elevation model, available topographical maps and sparse network of river gauging stations. A 110 km reach of the lower Damodar River in eastern India was taken as the study area since it suffers from chronic floods caused by water release from upstream dams during intense monsoon storm events. The uncertainty in model outputs, which is likely to increase with coarse data inputs, was quantified in a generalised likelihood uncertainty estimation framework to demonstrate the level of confidence that one can have on such flood routing approaches. Validation results with an extreme flood event of 2009 reveal an encouraging index of agreement of 0.77 with observed records, while most of the observed time series records of a 2007 major flood were found to be within 95 % upper and lower uncertainty bounds of the modelled outcomes.     

“8.12”陇南市特大暴雨与落区和泥石流点预报

2010年8月11至13日,陇南市连续发生强降雨,引发暴洪、泥石流和滑坡等地质灾害.本文讨论暴雨的天气实况、环流背景和物理特征,从云图和降雨特征分析降雨机理.研究发现,副高边缘低槽东移形成低涡,在涡旋过程中单点降落,是形成暴雨的根本原因;又因单点的强度太大,短时形成灾害.     

Occurrence of heavy rainfall around the confluence line in monsoon disturbances and its importance in causing floods

It is well known that heavy rainfall occurs in the southwestern sector of the monsoon depressions due to strong convergence in that sector. By examining the rainfall distribution associated with the monsoon disturbances (lows and depressions) in one of the central Indian river basins, ‘Godavari’, the author found that when the disturbance-centre is away from the basin, heavy rainfall may also occur in the basin area close to the confluence line and cause severe floods in the river. The confluence line is the zone of convergence between the northeasterlies to the west of the disturbance centre and the monsoon westerlies. This study further reveals the importance of the position and movement of the confluence line with respect to the basin, on which the intensity and the raising period of the resulting flood depend.     

Space-based monitoring of severe flooding of a southern state in India during south-west monsoon season of 2018

Kerala, a southern state of India, experienced a severe flooding due to multi-day extreme rain events during July and August months of 2018. This disaster was one of the worst floods to hit the state and resulted in heavy losses of lives and property. Natural Disaster Management Authority of India reported that 483 people lost their lives and more than 50 lakhs population were affected severely. This short communication focuses on examining this flood event using satellite remote sensing. It is reported that Kerala received an excess of about 56% rainfall during July and August from multi-day extreme rainfall episodes. Few regions of Kerala received the rainfall in the range of 270–300 mm on August 14 and 15. Hourly rainfall events in the excess of 25 mm have also been reported during heavy rainy days. The present study reports that multi-day heavy rainy events during July and August brought an accumulated rainfall of about 1600 mm, which resulted in extreme flooding over Kerala.

     

Surge simulations for 1999 Orissa super cyclone using a finite element model

The Orissa super cyclone which crossed the Orissa coastal region near Paradip on October 29, 1999 proved to be disastrous. The strong winds, torrential rains with heavy rainfall and high storm surge associated with the cyclone caused havoc that resulted in the death of thousands of people, cattle and extensive damage to agricultural land, paddy crop, transmission lines, power supply, roads and buildings. In the present study, a fine resolution finite element model is used to simulate surges due to this super cyclone. The model results are compared with observed surges available from post-storm survey reports. Comparison of results show that they are in good agreement with the observed surges, and the finite element model can be used for real time surge forecasts.     

Landslides associated with the colluvial soils overlying the Natal Group in the greater Durban region of Natal, South Africa

In the last 25 years, many of the landslides that have occurred in the greater Durban region have been associated with the colluvial soils overlying the Natal Group, most of which occurred during the very heavy rains of September 1987. Subsequently, a very heavy rainfall event in February 1999 also gave rise to landslides. In fact, prior to 1987 these colluvial soils were considered relatively stable. A critical precipitation coefficient has been developed which included the cumulative precipitation up to a landslide event. In addition, an attempt has been made to establish a threshold value for triggering of landslides for the colluvial soils from a study of pluviometric data. The results indicate that when a rainfall event exceeds 12% of the mean annual rainfall, small-scale landslides are likely to occur. When a rainfall event is greater than 16% of the mean annual rainfall, a moderate number of landslides take place. Major landslides are associated with rainfall events with intensities in excess of 20% of the mean annual precipitation. An example of a landslide which occurred on the Natal Group due to construction operations is provided, as well as an account of those which took place during September 1987. In the latter case, most of the slides took the form of mudflows and were responsible for some of the worst damage which has occurred in the Durban region. The colluvial soils involved were relatively thin and therefore became quickly saturated by the heavy rainfall. In some places the situation was further aggravated by liquefaction of the soils. Received: 15 June 1999 · Accepted: 30 August 1999     

Application of weather prediction models for hazard mitigation planning: a case study of heavy off-season rains in Senegal

Heavy off-season rains in the tropics pose significant natural hazards largely because they are unexpected and the popular infrastructure is ill-prepared. One such event was observed from January 9 to 11, 2002 in Senegal (14.00° N, 14.00°␣W), West Africa. This tropical country is characterized by a long dry season from November to April or May. During this period, although the rain-bearing monsoonal flow does not reach Senegal, the region can occasionally experience off-season rains. We conducted a numerical simulation of the January 9–11, 2002 heavy off-season rain using the Fifth-Generation NCAR/Pennsylvania State University Mesoscale Model (MM5) and the Weather Research and Forecasting (WRF) model. The objective was to delineate the meteorological set-up that led to the heavy rains and flooding. A secondary objective was to test the model’s performance in Senegal using relatively simpler (default) model configurations and local/regional observations. The model simulations for both MM5 and WRF agree satisfactorily with the observations, particularly as regards the wind patterns, the intensification of the rainfall, and the associated drop in temperatures. This situation provided the environment for heavy rainfall accompanied by a cold wave. The results suggest that off-the-shelf weather forecast models can be applied with relatively simple physical options and modest computational resources to simulate local impacts of severe weather episodes. In addition, these models could become part of regional hazard mitigation planning and infrastructure.     

Anomalous atmospheric events leading to Kyushu’s flash floods,July 11–14, 2012

During July 11–14, 2012, deadly floods and landslides triggered by a series of unprecedented heavy rains hit Kyushu, Japan, causing at least 32 deaths and around 400,000 evacuations. We focus on synoptic anomalies identified after inspecting rainfall patterns and documenting the conditions associated with this tragic event using data combined from the Global Rainfall Map in Near Real Time data, the NCEP/NCAR Reanalysis dataset, and the global forecast system. Rainfall maps indicated that there were many heavy rains in Kyushu in these days and this disaster was associated with the pattern of forecasts and standardized anomalies. A weather trough with positive height anomalies appeared, the center of which moved to the north of Japan over this period, which might cause wind anomalies and whereby lots of water vapor were transported to Kyushu area with up to 90 m s^?1, and high values of precipitable water formed with up to 60 mm. These results suggest that a larger-scale pattern is conducive for heavy rainfall and the anomalies put the pattern in context as to the potential for an extreme rainfall event, which can provide insights and methods for predicting extreme events’ or something similar.     

The influences of land use and sanitation infrastructure on flooding in Kumasi,Ghana

Each year during the rainy seasons, the Ghanaian media is dominated by banner headlines of flooded urban landscapes along with emergency response teams doling out relief items to the victims. However, when the rains subside and the floods are no longer newsworthy, the key stakeholders return to their normal duties only to await the next cycle of rains with bated breath. Based on the analyses of secondary and fieldwork data, the paper found no significant change in recent rainfall patterns that could account for the city’s flood problems. Rather, the impervious surfaces created through the removal of wetlands, riparian lands and urban vegetation, along with deposits of sediments and domestic wastes into streams and drains, have combined to slow down the velocity of stormwater flows and allowing it to disperse into the surrounding built up areas in lower terrains. The paper recommends the construction of modern city-wide sewerage systems to cope up with the current and predicted rainfall patterns, complemented by non-structural approaches including the protection of wetlands and urban vegetation to reduce the amount of storm water flows. Planners should anticipate and project future land use demands to cater emerging activities such as LPG stations which currently take sites in open space or flood ways and manage and direct physical development into low risk locations. Other measures would include the consideration of free domestic waste collection systems, mandatory flood insurance schemes on properties, with premiums assessed based on site risks, the development of early warning systems to minimize the effects of floods.     

Development of a GIS-based alert system to mitigate flash flood impacts in Asyut governorate,Egypt

Natural Hazards - Egypt is one Arab country that is vulnerable to flash floods caused by heavy and intensive rainfall. Different locations in Egypt are vulnerable to the hazards of flash floods,...     

The use and performance of mesoscale models over the Indian region for two high-impact events

High-impact mesoscale weather events, occurring in different parts of India in all seasons, lead to major weather- and climate-related disasters. Several research groups and operational weather forecasting centres in India have adopted mesoscale models for research and operational usage. This paper reviews the work done by different groups with respect to two specific events, (1) unprecedented locally heavy rainfall near Mumbai (Santa Cruz) on 26 and 27 July 2005 and (2) the Orissa super-cyclone of 29 and 30 October 1999 from its incipient stage on 24 and 25 October 1999. Considerable variability in the prediction of the intensity and location of mesoscale heavy rainfall, as well as in the intensity and path of the super-cyclone, are found. In order to reduce uncertainty in dynamical prediction, it is necessary that the model dynamics, physics, resolution, boundary conditions and availability of data on land–ocean surface processes are tuned separately to the specific event types, such as heavy monsoon rainfall, tropical cyclone genesis and movement and severe local thunderstorms, as the processes controlling such types of events may require suitable treatments for their proper simulations through appropriate dynamics, physics and resolution.     

滑坡预警中的降水时空变异性——以云南头寨沟为例

目前已有大量不同尺度的临界降水标准对区域性滑坡进行预警,但仍存在预警空间外延性差、时间精度低、临界降水阈值不科学等问题。针对当前构建的临界降水阈值实现滑坡预警模型,以云南昭通盘河流域头寨沟为例,在研究区两个不同海拔高度位置分别布设气象站,进行了为期一年的降水观测,并比较了研究区两个不同位置的降水特征。结果表明:研究区降水梯度高达190 mm/hm,两个不同海拔位置降水变异性显著;降水持时是造成山上(2#站)与山下(1#站)次降水量差异的主要影响因素;发生小雨时2#站与1#站的降水差异性很小,但1#站发生中雨及以上的降水事件时,2#站的降水等级比1#站高1~2个等级,且有59.26%的概率会发生大雨或者暴雨;1#站为暴雨时,2#站100.00%概率为暴雨甚至大暴雨;2#站与1#站夜间降水均大于白昼,且强降水事件多发生于夜间。     

Reconstruction of the summer rainfall regime for the last 500 years in China

The anomalies of summer rainfall (June–August) for the last 500 years in China have been estimated according to the relationship between the instrumentally observed rainfall and the degrees of floods and droughts for the last 25 years. It is shown that calculated anomalies of average rainfall for 10, 20 and 30 year periods have sufficient accuracies. The long-term climatic variations for the last 500 years are discussed based on the estimated anomalies of summer rainfall of 25 stations over the E part of China for the last 500 years. For example, there were prolonged drought in the N of China at about the end of Ming dynasty, but there were extensive floods in China for the early times of Qing dynasty. The anomalies of rainfall for the last 500 years were averaged along latitude zone and their decade running mean were formed. It was found that the long-term variations are very clear. The 80 year cycle is predominating. Besides, an about 200 year cycle is also shown in the charts of 50-year mean rainfall anomalies. It is indicated that both above mentioned cycles relate closely to the solar activity.     

Investigating the impacts of typhoon-induced floods on the agriculture in the central region of Vietnam by using hydrological models and satellite data

Flooding associated with landing tropical cyclones (TCs) is one of the major natural hazards in the coastal region of Vietnam. Annually, approximately 5 or 6 TCs make landfall in Vietnam, bringing heavy rains and inducing flooding, particularly to the central coastal region because of its topography and geographic configuration. This study focuses on the modelling of typhoon-induced floods that have resulted in widespread damage to agriculture over the central Thua Thien Hue Province of Vietnam by coupling two well-known hydrological models, KINEROS2 and HEC-RAS (Daniel et al. in Open Hydrol J 5(1), 2011), and using GSMaP (Global Satellite Mapping of Precipitation) data as the satellite rainfall input. Landsat imagery and GIS are also used for mapping and analysing the inundated areas. The discharge and water level from the KINEROS2 and HEC-RAS models displayed acceptable results for the floods modelled from three selected typhoons; both the Nash–Sutcliffe simulation efficiency coefficient (NSE) and the coefficient of determination (R²) were greater than 0.6. The simulated inundation maps of these typhoon-induced floods were compared with those extracted from the Landsat imagery to assess consistency. The result revealed a similar spatial extension of the inundated agricultural areas. This information, together with the forecasted TC movements and associated rainfalls, will be helpful to plan methods for mitigating potential typhoon-induced flooding and damage, particularly damage to agricultural regions.