Science of the Total Environment 556 (2016) 98–115

Contents lists available at ScienceDirect

Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

Review A review of atmospheric and land surface processes with emphasis on flood generation in the Southern Himalayan rivers

A.P. Dimri a,⁎, R.J. Thayyen b, K. Kibler c,A.Stantond, S.K. Jain b,D.Tullosd, V.P. Singh e a School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India b National Institute of Hydrology, Roorkee, Uttarakhand, India c University of Central Florida, Orlando, FL, USA d Water Resources Engineering, Oregon State University, Corvallis, OR, USA e Department of Biological and Agricultural Engineering, and Zachry Department of Civil Engineering, Texas A & M University, College Station, TX, USA

HIGHLIGHTS

• Floods in the southern rim of the Indian Himalayas are a major cause of loss of life, property, crops, infrastructure, etc. • In the recent decade extreme precipitation events have led to numerous flash floods in and around the Himalayan region. Sporadic case-based studies have tried to explain the mechanisms causing the floods. • However, in some of the cases, the causative mechanisms have been elusive. • The present study provides an overview of mechanisms that lead to floods in and around the southern rim of the Indian Himalayas.

article info abstract

Article history: Floods in the southern rim of the Indian Himalayas are a major cause of loss of life, property, crops, infrastructure, Received 9 January 2016 etc. They have long term socio-economic impacts on the habitat living along/across the Himalayas. In the recent Received in revised form 29 February 2016 decade extreme precipitation events have led to numerous flash floods in and around the Himalayan region. Spo- Accepted 29 February 2016 radic case-based studies have tried to explain the mechanisms causing the floods. However, in some of the cases, Available online xxxx the causative mechanisms have been elusive. Various types of flood events have been debated at different spatial fl Editor: D. Barcelo and temporal scales. The present study provides an overview of mechanisms that lead to oods in and around the southern rim of the Indian Himalayas. Atmospheric processes, landuse interaction, and glacier-related outbreaks Keywords: are considered in the overview. Himalayas © 2016 Elsevier B.V. All rights reserved. Flood Glacial lake outburst Cloudburst Lake dam outburst Precipitation

Contents

1. Introduction...... 99 2. Predominant atmospheric processes in Himalayan floodgeneration...... 100 2.1. OverviewofprecipitationmechanismsintheHimalayas...... 100 2.2. Orographicforcing...... 101 2.3. Convection – Orographydynamics...... 101 2.4. Large scale atmospheric flowconditions...... 103 3. Predominant land surface processes in Himalayan floodgeneration...... 104 3.1. Floodsofatmosphericorigin...... 104 3.1.1. Floods from large scale monsoon flowandtopographycontrols...... 104 3.1.2. Cloudburst floods...... 107

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.scitotenv.2016.02.206 0048-9697/© 2016 Elsevier B.V. All rights reserved. A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 99

3.2. Floodsofgeologicorigin:LandslideDamOutburstFlood(LDOF)...... 108 3.2.1. Riveravulsion...... 109 3.3. Floodsofcryosphericorigin...... 109 3.3.1. GlacialLakeOutburstFloods(GLOF)...... 109 3.3.2. Glacial surge dam burst floods...... 110 3.4. Floodsofhybridorigin...... 110 3.4.1. Earthbursts...... 110 4. Challenges and pathways to monitoring, predicting, and preparing for Himalayan floods...... 111 5. Conclusions...... 113 Acknowledgments...... 113 References...... 113

1. Introduction tropics in summer and from extratropics in winter. Within the Himalayas, climate and hydrology vary at the regional to sub-regional Five major flood events in the Himalayan region from Pakistan to scale. The climate of western Himalayas is mainly driven by winter- Assam in the past 10 years have occurred, highlighting the increasing time weather (Dimri et al., 2015; Yadav et al., 2013), the climate of cen- prevalence of floods, inadequacy of infrastructure to combat floods, in- tral Himalayas is driven by summer and winter monsoon (Shrestha complete understanding of flood forecasting mechanisms, and the lack et al., 1999), and the climate of eastern Himalayas is dominated by sum- of preparedness in the region. Managing the Himalayan floods is a chal- mer monsoon (Jhajharia and Singh, 2011). The lifting of monsoon mois- lenging task, since the floods result from a combination of atmospheric ture along the steep southern slopes of the Himalayas is a major driver and land surface processes. The 2010 July–August flood in Pakistan is of floods in the region (Fig.1b). The principal non-monsoon flood re- considered to be the worst ever flood in the region caused by the meso- gime of the Himalayas lies in the north-west of the monsoon regime, scale convective systems (Houze et al., 2011; Webster et al., 2011; Lau where major floods are also caused by glaciers and landslide damming. and Kim, 2012). This was followed by the Ladakh flood in August The high altitude glacier regimes are also the potential zones of non- 2010, which was caused by multiple cloudbursts in a span of three monsoon floods, such as Glacial Lake Outburst Floods (GLOF). Hence, days generated by the passing low pressure system (Rasmussen and floods in the region have multiple geneses, offering a formidable chal- Houze, 2012; Kumar et al., 2014; Thayyen et al., 2013). In 2012, the lenge to flood management. Brahmaputra flood was also generated by torrential rains over steep Our understanding of atmospheric and land surface processes that mountain slopes. In June 2013, the Kedarnath floods were caused by dictate flood characteristics in the Himalayan region have many gaps. high intensity torrential rains coupled with Chorabari lake outburst This can be attributed to the complex geography and topographic inter- (Dobhal et al., 2013). The Kashmir flood in September 2014 was caused actions with large scale atmospheric flows and data gap/sharing issues. by an unusual convergence of monsoon and westerly winds, whereas Conditions and physical processes that produce floods in the Himalayan the 2015 Zanskar flood was caused by a Landslide Lake Outburst region can be highly complex, comprising multiple geneses that often (LLOF) (http://www.nrsc.gov.in/Phutkal.html). Apart from these involve coupled and dynamic atmospheric and land surface processes major flood events, many local floods were recorded across the (Barros et al., 2004; Dimri and Niyogi, 2012; Bookhagen and Burbank, Himalayas in the recent past as listed in Table 1.Theserecentflood 2006). Feedbacks between atmospheric, topographical, and geomor- events highlight the need for a better framework for flood hazard miti- phological attributes may result in highly variable patterns and out- gation and disaster risk reduction in the region. comes of water and sediment distribution in high altitude areas, Geographically and geomorphologically, the Himalayas have a including foothill and plain regions. Incomplete understanding of unique positioning, located along the Karakorum in the west to Assam these processes often hinder effective flood management practices, es- in the east (Fig. 1a), blocking the monsoon and trade winds from the pecially flood forecasting.

Table 1 Flood events reported during the recent past in the southern Himalayas.

Number corresponding to Fig. 1 Flood event Date Dominant forcings

Sutlej and Paree Chu Jun 2005 Landslide lake outburst 5 Nepal, Kosi River Aug 2008 Breach in the Kosi embankment 2 Myanmar May 2008 Tropical cyclone - Nargis?? 1 Pakistan, Indus River Late Jul and early Aug 2010 European blocking and tropical-extratropical interaction Pakistan, Indus River 2012 Ladakh Aug 2010 Cloudburst Zanskar, Ladakh 2015 Landslide Dam Outbursts Pokhara, Nepal May 2012 Landslide + glacier melt + ??? Uttarakhand (Kedarnath) Jun 2013 Westerly-ISM monsoon + lake outburst Kashmir Sep 2014 Torrential Rain??? Assam, India Brahmaputra River 2012 Torrential rain Assi Ganga, India 2005 Cloudburst Rudraprayag, India 2006 Cloudburst Ukhimath, India 2008 Cloudburst Chenab, India 1996/1997 Monsoon flood 3 Bangladesh, Brahmaputra, Ganges, Meghna Rivers 1988, 1998, 2004, 2010 Monsoon flood 4 Bangladesh, Surma, Kushiyara Rivers 1998, 2004, 2007, 2010 Flash flood 100 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Fig. 1. (a) Recent flood events in Himalayan rivers. Numbers correspond to events in Table 1. (b) Glacio-hydrological regime of the Himalayas, trans-Himalayan region.

The objectives of this paper therefore are to: 1) review the dominant Slingo, 2011; Saha et al., 2012). The Tibetan High plays a significant atmospheric and land surface processes that contribute to flood gener- role in defining the strength of ISM (Yasunari et al., 1991; Wu and ation in the Himalayan region, drawing upon the reported flood events; Qian, 2003). This high pressure region blocks the northward propaga- and 2) discuss challenges and pathways to overcome adversities related tion of the monsoonal flow. Many studies have defined varying dura- to predicting, monitoring, and preparing for disastrous flood events. tions of dominant monsoon spells or intraseasonal modes at 10– 20 days and 30–60 days time scales for ISM (Hartmann and 2. Predominant atmospheric processes in Himalayan flood Michelsen, 1989; Goswami and Ajaya Mohan, 2001; Hoyos and generation Webster, 2007). Monsoons usually advance into the eastern Himalayan region by the first week of June, steadily progresses westward and cover 2.1. Overview of precipitation mechanisms in the Himalayas most of the Himalayas by July 15. The influx of moisture to the eastern Himalayas is sourced from the Bay of Bengal, while the moisture into The Indian Summer Monsoon (ISM) provides a major part of annual the western Himalayas is appended by the Arabian Sea as well. During precipitation for the Indian sub-continent during summer (June, July, the intermittent lull in the monsoon activity over the core monsoon re- August, and September: JJAS). ISM is an intriguing synoptic-scale phe- gion known as monsoon break period, the monsoon activities along the nomenon controlled by varying drivers (Annamalai et al., 2007; foothill of the Himalayas is intensified (Maharana and Dimri, 2015). In KrishnaKumar et al., 1999; KrishnaKumar et al., 2006; Turner and this case also, the Himalayan topography plays a dominant role in A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 101 defining the advancement of quasi-intertropical convergence zone regional precipitation. The winter (DJF) precipitation investigated in (ITCZ) associated with ISM. many experimental studies illustrates that in general precipitation During the post-monsoon months of October and November (ON), peaks at and around ~4000 m and then decays even if topography in- the northeast monsoons (NEM) dominate over the Indian subcontinent creases (Fig. 2b) (Dimri and Niyogi, 2012). This suggests that in general and provide precipitation over the southeastern coast of the Indian pen- precipitation is limited up to ~4000 m elevation in the Himalayan re- insula. During NEM, the summer to winter circulation changes gradu- gion. Associated physical processes limiting the precipitation are mainly ally, reversing wind patterns from southwest to northeast (Kripalani due to the shedding of mass of saturated water vapor on the upslope and Kumar, 2004; Kumar et al., 2007; Yadav et al., 2007). However, side. these winds have little influence over most of the Himalayas. In winter For analyzing the role of topography and/or orographic forcings, the (December, January, February: DJF), the Indian winter monsoons latitudinal and longitudinal cross-sectional distributions of mean sea- (IWM) dominate and bring through western disturbances embedded sonal winter (DJF) precipitation simulated under two sets of modeling within large-scale subtropical westerlies and precipitate in the form of experiments as control (CONT) and subgrid (SUB) and the correspond- snow over mountainous regions (Dimri et al., 2015). The interannual ing observed (CRU and APHRODITE) precipitation are depicted in and intraseasonal variability associated with IWM has been a subject Fig. 2b. To project the interactive role of topography, these distributions of various studies (Laat and Lelieveld, 2002; Dimri, 2013a, 2013b; are also presented with the cross sectional topographies from model Yadav et al., 2013). During DJF the spatial distribution of precipitation simulation (CONT and SUB) and observations (GTOPO30 10 min). Inter- over central and western Indian Himalayas is strongly influenced by active and topographic feedback or dependence of precipitation on the the interplay of western disturbances with the orographic barrier of topography is evident in the spatial variability of precipitation amount the Himalayas (Dimri, 2004; Dimri and Niyogi, 2012). across the Himalayas. In the cross sectional distribution of topography, Extreme rainfall associated with flashfloods occurring over the model simulations have elevations similar to the observations. The cor- Himalayas is an outcome of various complex, non-linear interactive pro- responding precipitation distribution maxima and minima along the to- cesses. At times, the interactive orographic forcing invigorates flash pographic elevation in the model field and observations are seen. floods in tandem with atmospheric flows. Adhikari et al. (2010) have Beyond ~4000 m elevation, reduced precipitation is seen in both the ex- provided a Global Flood Inventory (GFI) which suggests that the major- periments and the observations. The temperature decrease along the ity of flood events (64%) are associated with short duration heavy rains, mountain upslope with elevation increase and the resistance to the up- followed by events due to torrential rains (11%) during the monsoon slope flow results in cloud and precipitation formation. This rise of air period. These latter heavy rain events are particularly associated with sheds the condensed moisture along the upslope (Yasunari, 1976; localized causes, such as cloudbursts, lake bursts, landslides, and oro- Singh et al., 1995). This precipitation mechanism under the orographic graphic forcings, etc. These events are distinctly different from the influence leads to complex interactions. In the case of model, higher large scale monsoonal flow. The explicit differences in floods associated precipitation at high elevation points and less precipitation at low valley with large and localized flows are discussed in what follows. points than the corresponding observations are seen. In addition, model-simulated precipitation peaks are seen as in the corresponding 2.2. Orographic forcing observations with certain lead/lag in precipitation maxima at a few places. These differences in the values of SUB and CONT model fields Orographic interaction with large scale atmospheric flow and local- are due to land surface and topography interactions as seen in the SUB ized events determines the amplitude of rainfall over the Himalayan re- experiment which uses finer topography than that in the CONT experi- gion. At times this higher amplitude rainfall is one of the leading causes ment (Dimri and Niyogi, 2012).The longitudinal cross-sectional topog- of flood initiation. Examples of orographic control of the mountains raphy is more variable than the latitudinal topography, particularly aiding heavy rainfall are available for various mountain ranges across over the western Himalayas. Such aspects warrant further studies to un- the world. Over the Andes basin, the interaction of Andean mountain derstand the associated precipitation mechanisms and the interactive front increases the moisture flux to rise along it (Bookhagen and influence thereof. Strecker, 2008; Romatschke and Houze, 2011). Such pronounced inter- action of Andean mountain and the South American low level jet, asso- 2.3. Convection – Orography dynamics ciated with the South American monsoon, enhances precipitation (Boers et al., 2014). Based on a composite analysis of geopotential Cloudbursts in and around the foothills of the Himalayas are typical height and wind fields, Boers et al. (2014) showed northward propagat- convectively initiated orographically locked phenomena that subse- ing frontal systems and the associated anomalies to be the driver of ex- quently lead to flashfloods (Thayyen et al., 2013; Rasmussen and treme rainfall. In the United States, Colorado suffered from severe Houze, 2012; Kumar et al., 2014; Shrestha et al., 2015). Many local flooding in September 2013 due to the orographic interaction of Colo- flood events recorded across the Himalayas in the recent past were rado Front Range with North American monsoon (Kim et al., 2014). mostly generated by the cloudburst events. Localized events over Later, an unusual near-stationary low-pressure system led to a strong Asiganga (Gupta et al., 2013), Rudraprayag, Ukhimath (Shrestha et al., plume tropical moisture influx from the Pacific Ocean near Mexico to- 2015; Chevuturi et al., 2015), and Leh (Thayyen et al., 2013) are some wards the Colorado Front Range. of the examples from the recent decade. In the case of the Leh (Ladakh) In the case of the Indian Himalayas, steep topography and land use flood (4–6 August 2010), a cloudburst was the dominant atmospheric heterogeneity plays a crucial interactive role in defining the weather phenomenon, leading to flash flooding and subsequent debris flow and associated precipitation mechanisms. The latitudinal and longitudi- (Thayyen et al., 2013; Rasmussen and Houze, 2011; Kumar et al., nal cross sectional distribution of the topography along and across the 2014; Hobley et al., 2012). On 5 August 2010, a cloudburst had produced Himalayas highlights how the two-step topography determines the a lethal flash flood in the Leh town situated in the high arid Indus valley orographic forcing (Fig. 2a). Topographic variations along latitudinal in northwest India. The rare penetration of monsoon into an otherwise and longitudinal cross sections induce different precipitation forming non-monsoon area makes it difficult to predict and even more complex mechanisms leading to the associated precipitation amplitude. The cor- to understand it. During this particular time of ISM, the 500 hPa winds responding latitudinal and longitudinal cross-sectional topography over the Leh region are normally westerly. However, preceding across the western Himalayas at 30°N, 35°N, 40°N, 70°E, 75°E, and 03 days of the Leh flood, the 500 hPa geopotential height had shown 80°E is shown in Fig. 2a(1–3) and Fig. 2a(4–6), respectively, represents an easterly jet, which remained persistent for those days. This quasi- strong topographic gradients across the east – west and north – south stationary jet had led the mesoscale convective systems to move to- directions. Such a distribution of topography also influences the wards and over the Leh region. Simultaneously, there was a mid- 102 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Fig. 2. (a) Map of western Himalayan latitudinal cross sectional topography (∗1e2 m) corresponding to the CONT experiment, SUB experiment, and GTOP30 10 min topography, at (1) 30°N, (2) 35°N, and (3) 40°N, respectively. Similarly longitudinal cross-sectional topography at (4) 70°E, (5) 75°E, and (6) 80°E, respectively.(b)1980–2001 averaged winter (DJF) precipitation (mm d−1) (on left hand axis) and topography (m) (on right hand axis) along a latitudinal cross-section at (1) 30°N, (2) 40°N, and (3) 45°N and a longitudinal cross section at (4) 70°E, (5) 75°E, and (6) 80°E. CONT-Pr and CONT-Topo correspond to control precipitation and topography, respectively. Similarly SUB-Pr and SUB-Topo correspond to subgrid experiment. CRU-Pr corresponds to CRU precipitation. APH-Pr corresponds to APHRODITE precipitation (for details please refer Dimri and Niyogi, 2012). A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 103 tropospheric transient vortex providing a strong barotropic shear seen. The modeled daily accumulated precipitation over this area (Krishanmurti et al., 1981) associated with ISM situated over northwest along with its complex topography at a 2.8 km resolution is shown in India. This vertical shear of the vortex was instrumental in bringing ad- Fig. 3c. The model simulated cumulative maximum daily precipitation ditional moisture over the Leh region (Rasmussen and Houze, 2012). for this cloudburst event was up to over 200 mm/day (Fig. 3d). It can Kumar et al. (2014) hypothesized that mesoscale convective systems be seen that the zone of maximum precipitation shifted somewhat associated with cloudburst events were steered towards the Leh region from the Ukhimath and it was consistent with the corresponding by the 500 hPa wind, and low level moist air from the Arabian Sea and TRMM observations. In the model environment, precipitation started Bay of Bengal rose up to the Himalayan barrier. The corresponding rain- around 1700 UTC (+0530 LT) and reached its peak by the midnight of fall intensity reconstructed from the flood hydrographs showed a very 13 September 2012. The modeling of this particular event, specifically high intensity rainfall up to 320 ± 35% mm constrained over a the probable location and time of the associated maximum, has helped 1.6 km2 area for 8.8 min and 209 ± 35% mm rainfall over 0.842 km2 in better understanding the process associated with cloud burst events for 11.9 min (Thayyen et al., 2013). that are otherwise elusive due to their occurrence in remote regions of Another such event occurred on 13 September 2012 over the the Himalayas. Ukhimath, Uttarakhand, in the central Himalayas. The Ukhimath region In the case of tropical cyclone-led flooding on 2 May 2008 in is situated on the protruding foothills of the central Himalayas valley Myanmar, cyclone Nargis made a landfall, causing a worst natural floors having an opening towards southwest on both sides. Shrestha flood disaster. The geomorphological situation of the Myanmar with et al. (2015) showed the development of an easterly low-level wind the Khasi and Jaintia hills, an extension of the Himalayas, is important along the Gangetic Plain caused by a low pressure system over the cen- to mention here. Cyclone Nargis was of category 4 on the Saffir- tral Gangetic plain. This situation led to the convergence of moisture Simpson Hurricane Scale (Fritz et al., 2009). Sustained winds of the over the north-western part of India, hence an increase of potential in- order of 210 km/h, with gusts up to 260 km/h, persisted before the land- stability of the air mass along the valley recesses, which was capped fall on 2 May 2008. Due to increased convection owing to the influence by an inversion located above the ridgeline and strengthening of the of ITCZ and cyclonic circulation, a low-pressure system formed over the north-westerly flow above the ridges. This supported the lifting of po- Bay of Bengal. Due to the warmer oceanic condition, lower vertical shear tentially unstable air over the protruding ridge of the foothills of the and pole-ward outflow, a deep depression developed (Raju et al., 2012). Himalayas and triggered a shallow convection that, on passing through On account of the positioning of anticyclone in its west this storm adjacent topographic folds, initiated a deep convection. steered northwest, intensifying as a cyclonic storm. Based on the flood With specific elaboration on the cloudburst dynamics, a brief pream- direction, inundation distances reached up to 50 km inland from the ble with a case study of 13 September 2012 cloudburst over Ukimath in nearest coastline. The atmospheric mechanism leading to flash flooding the central Himalayas is provided. This case was studied with a Consor- in this case was different from the case of cloudburst discussed above. In tium of Small-scale Modeling (COSMO) (Doms and Schaettler, 2002; this case, tropical cyclone formation and interaction led to subsequent Steppler et al., 2003; Baldauf et al., 2011) at spatial scale of 2.8 km in flooding, whereas in the above cloudburst case convective driven mech- the convection permitting mode (COSMO-DE). Comparison of the spa- anism locked with the orography led to flooding. tial pattern of precipitation field by the model with the corresponding observation of the Tropical Rainfall Measuring Mission (TRMM) 3B42 2.4. Large scale atmospheric flow conditions (V7) showed that the model underestimated the extent of daily accu- mulated precipitation compared to the corresponding observations. Four of the most devastating floods (Pakistan 2010, Uttarakhand However, a consistent spatial pattern for the precipitation maxima 2013, Kashmir 2014 and Brahmaputra 2012) in the Himalayan region along the lower and upper foothills of the Himalayas (Fig. 3a and b) is in the recent past resulted from torrential rains associated with large

Fig. 3. (a) Daily accumulated precipitation on 13 Sept. 2012 from TRMM 3B42 data, (b) Daily accumulated precipitation on 13 September 2012 simulated using COSMO (D2 domain). The two bands of precipitation are outlined, along with the borders of Uttarkhand state, (c) Zoomed modeled accumulated precipitation with the local topography at the resolution of D2 domain (2.8 km). The topography contour interval is 500 m, from 2000 to 6000 m elevation, (d) Time-series of accumulated precipitation at location marked “o” and the surrounding grid cells, starting from 12 Sep. 2012. The precipitation accumulation for spatial plot is from 0000 UTC to 2330 UTC (Shretha et al. 2015). 104 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 scale atmospheric flows, where monsoons played a major role. The oc- downdraft occurs in a rainy region. When sequences of such convective currence of the 2010 Pakistan flood was due to the interaction of up and down drafts occur, the precipitating cloud system takes on the tropical-extratropical flows linked with the European blocking form of mesoscale convective system (Houze et al., 2011). During this (Webster et al., 2011; Houze et al., 2011; Lau and Kim, 2012). This process, some convective cells disintegrate, while new ones emerge. July–August 2010 flood was the worst over the Pakistan region. On 28 The older dying storms group together to form regions of stratiform July 2010 the mid-tropospheric 500 hPa height deviated from its mon- rains. On the upslope side of the Himalayas, mountains can sustain me- soonal normal conditions (Houze et al., 2011). This anomalous condi- soscale convective systems, as the airflow over the rising terrain triggers tion brought in dry air into the western Himalayan foothills from the new intense convective cells, thus sustaining mesoscale convective sys- Afghan plateau. In tandem with such a situation, a low pressure mon- tems for longer periods and the orographic upward air motion may sus- soon system over the Bay of Bengal brought in very humid deep layer tain and broaden the stratiform region of the mesoscale convective of moisture wind leading to a major rain storm. Hong et al. (2011) systems. Such conditions are suggested to be the mechanism of extreme have shown the role of large-scale circulation perspective in strong rainfall in the eastern Himalayas (Houze et al., 2007) and is suggested as tropical-extratropical interaction due to the coupling of monsoon the mechanism behind the 2010 Pakistan flood as well. Further studies surges and extratropical wave activity downstream of the European in this direction will improve our understanding of extreme rainfall blocking. The teleconnection of Russian Heat wave and Pakistan flood across the Himalayas. is important (Lau and Kim, 2012), generating anomalous southwesterly flows along the foothills of the Himalayas in association with northward 3. Predominant land surface processes in Himalayan flood propagation of the monsoonal intraseasonal oscillation brought in generation abundant moisture transport from the Bay of Bengal. Such exceptionally huge deluge can possibly be predicted using an Ensemble Prediction The Himalayan terrain and processes shaping the land surface System. strongly influence and are, in turn, influenced by flood generation and In another case, a large scale atmospheric flow in the central Hima- propagation. By monitoring atmospheric moisture, routing surface run- layan region caused one of the most devastating extreme rainfall and off, and controlling the supply of sediment/debris, the landscape of the flood events over upper Ganga basin of Uttarakhand during 16–17 Himalayas and its influence on allocyclic and autocyclic response is a June 2013. The monsoonal low pressure system, providing increased first-order control to fluvial processes, including flooding. Given the var- low level convergence and abundant moisture, interacted with the ied flood-generating mechanisms related to the Himalayan topography mid-level westerlies and generated a strong upper level divergence. and patterns of geologic uplift and weathering, we present three broad Such interaction led to a faster northward progression of the monsoon categories for the origin of Himalayan floods: (1) atmospheric origin, ITCZ to cover the entire Indian sub-continent and yielded heavy rainfall (2) floods of cryospheric origin, and (3) floods of geologic/geomorphic over the region (Joseph et al., 2014; Chevuturi et al., 2015). The large origin. Because flood events are often a product of interaction across scale wind and geopotential height variation at low, mid and upper tro- these categories, we also discuss hybrid floods generated by combined pospheric levels is shown in Fig. 4.Thefigure depicts curved south- effects (Fig. 5). westerly flow associated with the ISM at 850 hPa. This is a cross- equatorial flow termed as low level jet (LLJ) (Blanford, 1884; Joseph 3.1. Floods of atmospheric origin and Raman, 1966; Findlater, 1969) and has a direct relation with mon- soonal rainfall over peninsular India (Joseph and Sijikumar, 2004)The 3.1.1. Floods from large scale monsoon flow and topography controls LLJ is responsible for the moisture transport which is required for sus- A large swath of the Himalayas comes under the monsoon flood taining the storm (Figure not presented, refer Chevuturi et al., 2015). zone, extending from Kashmir in the west to the eastern frontiers of Moisture incursion over the Indian sub-continent during 15–16 June the Himalayas (Fig. 1). Annual floods driven by monsoon rainfall are a 2013 is from Arabian Sea and Bay of Bengal. On 17 June 2013 there is ad- nearly universal signature of the Himalayan rivers. River channels are ditional moisture flow from Arabian Sea, which is also reported shaped by annual peak flows that may be of the orders of magnitude (Ranalkar et al., 2015). The monsoon depression in the figure greater than the mean annual flows (Kale, 2003). One of the major fac- strengthens as it moves northwards in the successive time periods. In tors controlling rainfall intensity across the Himalayas is the mountain the upper-troposphere, a trough is seen in the subtropical westerly jet topography (Nandargi and Dhar, 2011; Bookhagen and Burbank, (STWJ), which extends till the north-western India. This is the western 2010). Moisture is pushed up by the mountain barrier and this upward disturbance (WD) travelling in the STWJ and its axis shows a slight east- trajectory is regulated by the mountain slopes. Hence, precipitation oc- ward movement in the different time steps. It is seen as the confluence currence along the major parts of the Himalayan arc is concentrated in of the two systems: monsoon trough of ISM and WD in the STWJ occurs distinct orographic bands (Bookhagen and Burbank, 2010). One of on 17 June 2013. Considering the two systems are present in the lower these bands is along the lower ranges of the Shiwaliks. Major extreme and upper parts of troposphere respectively, this merging could have one-day rainfall is mainly reported from an altitude b1000 m above extended right from lower to upper-troposphere. At 500 hPa, a trough sea level along the foothill zone (Nandargi and Dhar, 2011). The central with a deeper extent than the WD is observed. This trough is showing Himalayas are characterized by a two-step topography, where two dis- the merged low pressure zones of both monsoon trough and WD tinct relief zones of the mountains, represented by the lesser Himalayas trough. This trough is the cause of the transient cloud system, formed and the greater Himalayas, aiding the orographic lifting of the monsoon along the same axis centered over Uttarakhand. The corresponding ob- moisture and resultant higher rainfall 10's of kilometers offset to those servations (MERRA dataset) of large scale wind and geopotential height relief zones (Fig. 6). On the other hand, western Himalayas are mainly show similar results (figure not presented, refer Chevuturi et al., 2015). characterized by a one-step topography, in which only one major Singh et al. (2015) illustrated the active phase of Madden Julian Oscilla- mountain relief facilitates the orographic lift and the corresponding tion (MJO) that led to the faster northward propagation of the monsoon higher rainfall. Precipitation observed by the TRMM reveals higher an- ITCZ. In the case of the Jammu and Kashmir floods of 2014 and 2015, a nual rainfall along this dominant mountain relief (Bookhagen and southward extension of western disturbances (Dimri et al., 2015) and Burbank, 2006; Bookhagen, 2010; Bookhagen and Burbank, 2010). its merging with the monsoon core was the major cause for the torren- In the monsoon flood zone (Fig.1) inputs of summer monsoon pre- tial precipitation leading to one of most severe floods in the region. cipitation dominates the annual water budget, with snowmelt as a sec- Some general characteristics of orographic rainstorms have been ondary contribution (Bookhagen and Burbank, 2010). Thus, most of the elaborated by Houze et al. (2011). Rainstorms over the mountains result geomorphic draining by the eastern and central Himalayan rivers occurs when air is lifted over the terrain. In an unstable environment, a during the summer monsoon flood season, while more than 80% of the A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 105

Fig. 4. Wind circulation (m/s; vector) at 850 hPa, geopotential height (×102 m; contour) and wind speed (shaded) at 27 km horizontal model resolution for 16 June 2013 (1200UTC) for (a) 850 hPa (wind speed N10 m/s shaded), (b) 500 hPa (wind speed N10 m/s shaded) and (c) 200 hPa (wind speed N25 m/s shaded). (d–f) for 17 June 2013 (0000UTC) and (g–i) 17 June 2013 (1200UTC). Green plus sign depicts Kedarnath. annual rainfall across the central Himalayas and Ganges Plain occurs regions this elevation zone extends from 500 to 4000 m above sea level. during the monsoon season (Bookhagen, 2010)andb50% of annual Overall, the highest annual rainfall occurs in the eastern region. Flood rainfall occurs during the monsoon season in the western Himalayas. severity at the foothill zone is also possibly linked with the rainfall dis- Additionally, in contrast to the eastern and central Himalayas, a large tribution regulated by the mountain topography. This east to west de- proportion of runoff from the western Himalayan rivers is comprised crease in the rainfall amount is also reflected in the peak flood of snowmelt, which is beyond the scope of this study. Bookhagen and discharge. However, peak erosional processes, which trigger landslides, Burbank (2010) estimated snowmelt contributions of 66% and 57% in rockfalls, debris flows, and other mass movements, occur deep within the Indus and Sutlej basins, respectively, while reporting that water the orographic interior frequently triggered by rainfall (Wulf et al., budgets in the central and eastern Himalayas comprise b20% snowmelt. 2010). Reflecting the varied water phases across the western Himalayas, there In this discussion on floods, particularly related to the large-scale at- are often two associated discharge peaks that occur during the summer mospheric flow, a case study of the recent Uttarakhand flood of 2013 is monsoon, a less intense earlier peak attributed to combined snowmelt discussed. Fig. 7a–d shows different vertical distributions related vari- and rainfall and a more intense later peak primarily due to monsoon ables whose physical and dynamical processes associated with the rainfall (Pal et al., 2013). In addition to the elevation gradients in the flood storm are discussed here. On day 1, 16 June 2013, an increasing mean annual rainfall across the Himalayan orography, the percentage low level relative humidity is seen from 0600 UTC to 1800 UTC, which of rainfall that occurs during the monsoon season is regionally variable. in mid to upper-troposphere increases after 16 June 2013 1200UTC Bookhagen and Burbank (2010) reported gradients of monsoon precip- (Fig. 7a). An increase in the associated vertical wind is also seen itation, where monsoon rainfall decreases, moving west from the Bay of (Fig. 7a). The higher low level moisture is due to the moisture incursion Bengal. from the Arabian Sea (figure not represented) along with the low level In the central Himalayan region, annual rainfall amount is concen- jet in the monsoonal south-westerly flow. It is seen that the monsoonal trated below 500 m above sea level, whereas in the western and eastern convergence zone moved faster northwards in this particular year. This 106 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Fig. 5. Various types of flood generating mechanism in the Himalayan region. increased moisture reached the Uttarakhand region, in association with with increased reflectivity: 16 June 2013 1200UTC and 16 June 2013 the increased vertical wind that led the low level moisture to rise up- 1800UTC onwards. Though there was continuous rainfall reported slope mountains and rise up to mid and upper-tropospheric levels. throughout the time period, however, during these two distinct time The sudden rise of moisture in the water vapor form was forced to go periods due to increased hydrometeor formation first and second through various microphysical processes, thus forming different hydro- heavy precipitation episodes occurred one before (16 June 2013) and meteors, viz., cloudwater, cloud ice, rainwater, snow and graupel second during (17 June 2013). These two episodes brought in the cas- (Pruppacher and Klett, 2010). The signature of these hydrometeor con- cading effect for the Uttarakhand disaster. Further investigation with centrations and the corresponding mixing ratios and the hydrometeor the corresponding vorticity and divergence (Fig. 7c) showed a low dependent reflectivity signifies the formation of clouds over the region level positive vorticity on 16 June 2013 that led to the development of (Fig. 7b). During the storm, two time slices are very distinctly seen instability in the region due to the monsoon trough of ISM. The positive

Fig. 6. Monsoon over Himalaya is driven by the orographic processes and mountain topography controls the precipitation distribution. Two step topography describes two distinct relief of the mountain aiding higher precipitation 10 of kilometre offset to the relief. Regions with one step topography have only one major relief and corresponding higher precipitation zones. These higher precipitation zone are the potent source areas of Himalayan flood. (Source: Bookhagen and Burbank, 2010). A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 107

Fig. 7. Time-pressure plot for (a) vertical wind (m/s; shaded) and relative humidity (%; contour); (b) reflectivity (dbz; shaded) and sum of mixing ratios of hydrometeors: cloudwater, cloudice, rainwater, snow and graupel (g/kg; contour); (c) vorticity (×10−4 s−1; shaded) and divergence (mm; contour) and (d) CAPE (J/kg; shaded) and CINE (J/kg; contour) (Chevuturi et al., 2015). vorticity on 17 June 2013 dominated in the mid-troposphere. This an area not exceeding 20–30 km2 (IMD, 2010). Though a few observed remained the zone of confluence of western disturbance and monsoon data document the hydrologic response to cloudbursts, the hydrologic trough. The low level convergence and upper tropospheric divergence signature of a cloudburst event may consist of extremely rapid led to increased vertical winds and disturbance in the subtropical west- hydrograph rise and fall within small catchments (Fig. 8d). However, a erly jet, providing the divergence aloft. The convective lifting in tandem cloudburst is often associated with sustained low pressure system and with orographic forcing invigorated the storm. As precipitation patterns associated rainfall over a wider region extending 2–3 days, as experi- were reported to be increasing from 16 June 2013–17 June 2013 (data enced in the Leh region in 2010, and the corresponding hydrograph re- not presented), the corresponding model convective available potential sponse at larger basin scales may be muted and lagged (Fig. 8c). Because energy between 16 June 2013 0600UTC and 1200UTC was also seen to of the highly localized nature of cloudburst, antecedent soil moisture be increasing (Fig. 7d). Thus, storm intensification was promoted from conditions play a little role in determining the flood severity. It is 16 June 2013 1200UTC onwards, which was subsequently suppressed often suggested that a cloudburst occurs in a catchment with steep to- due to higher convective inhibition. pography (Das, 2005). Such steep topography could also be a factor in determining the flood hydrograph, including time to flood peak and 3.1.2. Cloudburst floods flow velocity. A detailed study of the cloudburst in the Leh region in Floods that originate from precipitation associated with mesoscale 2010 (Thayyen et al., 2013) showed that cloudbursts could result in a convective events may differ from those related to large-scale mon- very high peak discharge ranging from 545 ± 35% m3/s from a soonal flows in terms of scale, concentration time, and duration. Though 2.83 km2 catchment to a 1070 ± 35% m3/s from a 64.95 km2 catchment. the scope of geographic impact may be limited, floods generated by It was reported that the flood duration under such conditions was as high-intensity short-duration cloudburst events can be devastating low as 01 h. The study pointed towards limited possibilities for the suc- and very difficult to predict or prepare for. Cloudburst is one of the cess of forecasting such events. Even with the state-of-the-art Doppler least known mesoscale processes of the Himalayan region (Das et al., radars, forecasting cloudburst locations would be a big challenge. A 2006; Shrestha et al., 2015) whose occurrence is seemingly increasing cloudburst often impacts the mountain slopes of the first order stream over this region. Table 1 shows the cloudburst events reported for the catchments with settlements, and the mountain population becomes Himalayan region in the recent past. These storms and associated de- at more risk from cloudburst floods than the floods of torrential rain or- struction have generated more attention to their corresponding physi- igin. The control of land surface/topography on the formation of cloud- cal processes these days. According to the India Meteorological burst cells has been discussed in recent studies (Chevuturi et al., 2015). Department (IMD) a cloudburst features a very heavy rainfall over a lo- There could be differences in the cloudburst flood hydrograph charac- calized area at a very high rate of the order of 100 mm/h with strong teristics in an arid bare rock exposure region like Ladakh and a region winds and lightning. It is a remarkably localized phenomenon affecting with thick vegetation like Uttarakhand. The Ladakh region seldom 108 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Fig. 8. The varied hydrograph signatures of Himalayan floods. (a, b) torrential rain flooding in Chenab, India; cloudburst floods in upper Indus basin, (c) at Aichi, Leh (observed) and (d) in Sabu River and tributaries (simulated), monsoon flood (e, f) in Brahmaputra (Bahadurabad, Bangladesh) and Ganga (Farraka, Bangladesh) rivers during record flood years, and (g, h) GLOF floods, Dhauliganga River, India (simulated as a singular outburst flood event and as a hybrid event with a 100-year rainfall). experiences antecedent precipitation but the rest of the Himalayas could facilitate massive floods due to the occurrence of landslides into under the monsoon regime have hardly any soil moisture deficiency the river leading to river blockage. These types of floods are known as during the cloudburst period. Manifestations of these factors in the Landslide Dam Outburst Floods (LDOF) or Landslide Lake Outburst flood hydrograph characteristics are least known land surface controls Floods (LLOF). Some of the worst floods in the region are caused by on the cloudburst floods. Repeated occurrences of cloudbursts on a par- LDOF. River Indus experienced such a flood in 1841 known as the ticular catchment clearly point towards the control of the mountain “Great Indus Flood”.AsdetailedinMason (1929), the landslide was trig- slopes on the cloudburst formation (Thayyen et al., 2013). Hence, the di- gered by an earthquake in December 1840 or January 1841 on the west saster risk reduction option in response to the cloudburst floods is lim- side of the Lechar spur of Nanga Parbat. By the beginning of June 1841 ited to the settlement management. It is noticed that most of the the water probably reached the top of the earthen dam, forming a causalities in such cases occur over settlements in the paleoflood/dry 64 km long and 300 m deep lake. According to Mason (1929), the lake channels (Thayyen et al., 2013). emptied in 24 h, devastating hundreds of villages and killing thousands of people. At Attok, more than one thousand kilometers downstream, 3.2. Floods of geologic origin: Landslide Dam Outburst Flood (LDOF) the flood was about 92 ft high, whereas normal high flood level is 42 ft (Falconer, 1841). Geologic and geomorphic processes associated with steep land- The Upper Ganga basin under the monsoon regime is more prone to scapes of the Himalayan region, which is tectonically highly active, landslides and quite a few LDOF's have been reported from two major A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 109

Himalayan tributaries of Bhagirathi and Alaknanada. The earliest report 3.2.1. River avulsion of the LDOF in the upper Ganga basin dates back to 26 August 1894 Foreland basins of the Himalayan rivers often take the brunt of (Holland, 1984; Rana et al., 2013). A landslide lake of approximately floods generated by high mountains. River avulsion is one such phe- 270 m high and 3 km wide at the base was formed over Birahi nomenon which could increase the disaster potential of a flood. Avul- Ganga River, which is a tributary of Alaknanada River, on 6 September sion occurs when an aggrading river leaves its channel to pursue a 1893. It is reported that this flood was managed by diverting the pe- lower-elevation course, thus producing a sudden flood scenario in the destrian routes and removing 08 suspension bridges on lower reaches. otherwise safe low lying areas away from the normal river course. Avul- The Alaknanada valley again experienced another major flood after sion is often triggered at the flood stage (Jones and Schumm, 1999)and 76 years in July 1970. It is believed to be initiated by a cloudburst be- flood damages that occur when a river rapidly changes its course can be tween Joshimath and Chamoli (Rana et al., 2013). The 1970 flood re- severe. Many Himalayan rivers have long histories of channels shifting sulted from the failure of a persistent lake formed in the 1893 through avulsion, especially in the alluvial foothill and plains' reaches. landslide at Gohana. The 1970 flood washed away 30 buses and 13 Heavy sediment loads from actively weathering high-altitude zones bridges downstream, besides destroying roadside settlements. The are carried by high-gradient, powerful mountain rivers to deposit Bhagirathi River and its tributary Kanoldiya stream were blocked by where valleys widen and channel slopes diminish. In these alluvial a massive slide called Dabrani landslide on 6 August 1978, forming reaches, active aggradation associated with such deposition creates haz- two lakes (Dimri and Pandey, 2014). The bursting of these natural ardous conditions for avulsion-related flooding. Due to the high fre- dams later caused an immense loss of life and property. Though not quency of such events relative to other regions, some Himalayan much mechanism associated with these historical flood events, how- rivers are considered to be ‘hyperavulsive’ (Jain and Sinha, 2004). ever, a brief discussion is provided due to the nature of their The alluvial reaches of Kosi River, which has many times devastation. transitioned between channels in a wide alluvial fan, is a well-known Another recent and well-documented LDOF is the Pareechu land- example of avulsion phenomenon (Wells and Dorr, 1987). Most re- slide dam in Tibet on 25 June 2005. The lake was created by a landslide cently, following heavy monsoon rains in 2008, the Kosi River broke that occurred on 08 July 2004 (Gupta and Shah, 2008). Generally, the through man-made embankments to flow out of its channel (Sinha, areas of Tibet bordering India do not receive heavy rainfall and the land- 2009). It started flowing through an old channel 120 km to the east, in- scape is completely barren. Hence, the trigger for this landslide is not undating cropland and towns and villages with 1.2 million inhabitants, very clear. Most recently, the Phutkal River in Zanskar got blocked by mostly in Bihar province of India. Over 30 million people were affected a heavy landslide that occurred on 15 January 2015 in a comparable by this one avulsion related flood event. In this case, heavy monsoon arid topography (www.greaterkashmir.com). The location of the land- precipitation triggered avulsion in the Kosi channel, yet the influence slide lake was about 90 km interior of the road head and monitoring of river engineering works also contributed to the avulsion (Sinha and managing this lake was a challenge. The National Remote Sensing et al., 2008; Sinha, 2009). As the origins of this event included interac- Centre (NRSC), India, has carried out routine monitoring of this lake tions of atmospheric, hydrologic, and geomorphic variables and was (www.nrsc.org). The lake had busted on 07 May 2015, affecting around likely influenced by human engineering of the river channel, traditional 40 villages. These two LDOFs in the cold-arid region suggest a need for methods of flood forecasting would have been unlikely to predict such better understanding of the trigger of landslides in such areas. The an occurrence. Paleochannels formerly occupied by aggrading rivers role of permafrost thaw and snow melt water seepage in triggering may be particularly hazardous. landslides in such areas needs to be studied. The floods in the Seti River in Nepal had been suggested to be triggered by a rock slope failure 3.3. Floods of cryospheric origin at an altitude of 6700 m at Annapurna-IV which travelled through the southwest slope up to the river confluence (Bhandary et al., 2012). 3.3.1. Glacial Lake Outburst Floods (GLOF) The LDOFs occurred in monsoon and non-monsoon flood zones of the Non-monsoon floods in the Himalayas are mainly of cryospheric or- Himalayas indicating diverse landslide triggering mechanisms. Hence, igin and spread across the trans-Himalayan region where monsoon a better understanding of factors triggering the mass movement in the penetration is least and at the higher altitude glacial regimes 3500 m Himalayas, especially under the climate change scenario, is needed for above sea level in the monsoon zone (Fig.1). Floods of cryospheric origin managing these floods. in the Himalayas mainly occur due to Glacial Lake Outburst Flood The control of sediment supplies to rivers draining the Himalayas is (GLOF), glacial surge dam outburst flood and snow avalanche dam out- highly influential to drainage network dynamics and channel morphol- bursts. The GLOF is the major flood generating mechanism of this genre. ogy. Dominant autocyclic processes include both changes to channel A glacial lake is defined as water mass existing in a sufficient amount morphology through flood cycles and stochastic land-forming events, and extending with a free surface in, under, beside, and/or in front of a such as gravity-driven mass wasting and dam break floods. Addition- glacier and originating from glacier activities and/or retreating pro- ally, allocyclic events, notably related to seismicity, influence the peri- cesses of a glacier. Retreating glaciers in the Himalayan region have fa- odic sediment supply. Though the channel responses and the cilitated the formation of proglacial glacial lakes in the region. In geomorphic effects vary spatially, the most pronounced geomorphic ef- recent years accumulation of water in these lakes has been increasing fects of monsoon floods are over the Indian Himalayan region (Kale, rapidly in the Himalayas (Ives et al., 2010). The glacial lakes are classi- 2003). There are numerous instances of flood-induced dynamic changes fied into Erosion (Valley trough and Cirque), Ice Blocked, Moraine in the channel dimensions, position and pattern. As Himalayan rivers Dammed (Lateral Moraine and End Moraine Dammed lakes), and often carry vast quantities of sediment and debris (Kale, 2003), flows Supraglacial lakes (Campbell and Pradesh, 2005). Inventory of glacial laden with material can be highly destructive and may require special lakes in the Himalayan countries has revealed the existence of a large preparedness measures for risk reduction. Additionally, generation of number of glacial lakes in the region. In the case of GLOF, the probability debris-laden flows entails different processes and conditions than of occurrence mainly depends on the expansion rate of the glacial lakes, those driving floods of atmospheric origin. Thus, forecasting such as well as the probability of ice/rocks falling into the lake (Richardson flows may require different approaches to monitor and model than and Reynolds, 2000). The stability of damming moraines (Clague and those commonly applied to riverine flooding. Along the continuum Evans, 2000) and the failure of damming moraine (Fujita et al., 2013) spanning end members of pure water flows and those containing little are essential for the release of glacial lake water. water, such as landslides, researchers and managers classify flows of A GLOF is created when the water dammed by a glacier or a moraine variable proportions of water and other materials (Postma, 1986; is released suddenly. Some of the glacial lakes are unstable and most of Hungr et al., 2001). them are potentially susceptible to sudden discharge of large volumes of 110 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 water and debris which cause floods downstream. These floods pose se- flood was the most devastating of this kind that has occurred in this re- vere geomorphological hazards and can wreak havoc on all manmade gion. The state of the Kumdan dam in 1928 (Ludlow, 1929)anditssub- structures located along their path. Much of the damage created during sequent burst in 1929 is well documented (Mason, 1930). It has been the GLOF events is associated with large amounts of debris that accom- reported that the Kumdan Lake was 16 km long, 120 m deep and stored pany the floodwaters. The lakes at risk are invariably situated in remote around 1.5x109 m3 of water blocked by 2.4 km ice barrier. After the and often inaccessible areas. When they burst, the devastation to local breach, the lake water was evacuated in 48 h with an estimated peak communities could be tremendous. Due to extreme hazard potential, discharge of 22,650 m3/s (Hewitt, 1982). This flood resulted in a huge it is necessary to take into account GLOF, while planning, designing devastation in the Shyok valley and Indus up to 1194 km downstream and constructing any infrastructure, especially water resources projects, (Attock) of its origin. bridges and culverts as they are located on the path of flood wave and Glacial surges are not random events but cyclic phenomena, proba- would be the prime target for catastrophe. bly driven by the frozen and unfrozen conditions of the glacier bed It is observed that the number of GLOFs has increased in the (Murray et al., 2000; Fowler et al., 2001). Hence, surge dam outburst Himalayas through the last century (Richardson and Reynolds, 2000). floods are a perennial threat for the regions inflicted by the glacier Previous studies have shown already that the risk of lake development surge phenomena. However, proper monitoring of these areas can re- is highest where the glaciers have a low slope angle and a low flow ve- duce the disaster potential of such lake outbursts considerably, as locity or are stagnant (Quincey et al., 2007; Reynolds, 2000). Whether surge initiation, lake formation and its eventual failure give ample glacial lakes become dangerous depends largely on their elevation rela- time for a calibrated response. tive to the natural spillway over the surrounding moraine (Benn et al., 2000; Sakai et al., 2000). Triggering events for an outburst can be mo- 3.4. Floods of hybrid origin raine failures by the melting of ice cored moraine or increased water pressure, or a rock or snow avalanche slumping into the lake causing Flood events in the Himalayas often are the product of multiple in- an overflow (Buchroithner et al., 1982; Ives et al., 2010; Vuichard and teractive geneses. Floods in the hybrid category are produced by rain Zimmermann, 1987). or snow events, rain over glacial/periglacial lakes, earthquake-induced Factors contributing to the hazard/risk of moraine-dammed glacial GLOF or LDOF, or earth burst - a phenomenon of sudden outflow of lake include (a) large lake volume; (b) narrow and high moraine dam; water from the earth usually along the mountain slope which is accu- (c) stagnant glacier ice within the dam; and (d) limited freeboard be- mulated over days of continuous rain over the region during the mon- tween the lake level and the crest of moraine ridge. Potential outburst soon and is a relatively unknown phenomenon. flood triggers include avalanche displacement waves from (a) calving One of the most devastating recent examples of a hybrid flood is the glaciers; (b) hanging glaciers; (c) rock falls; (d) settlement and/or pip- June 2013 flood of the upper Ganga basin, including Kedarnath area of ing within the dam; (e) melting ice-core; and (f) catastrophic glacial Uttarakhand. During the Kedarnath floods, the entire upper Ganga drainage into the lake. In the Himalayas GLOF events are reported basin from Haridwar to Kedarnath experienced floods during 15–17 from Nepal, Bhutan (ICIMOD, 2011) and Ladakh Himalayas (Thayyen June 2013. These floods were one of the worst in this region, causing se- et al., 2010). Though these flood events are reported but still needed vere loss of life and property-killing around 6000 people and damaging to have a more comprehensive understanding of the associated intricate innumerable roads and bridges and adversely affecting 30 odd hydro- processes. power plants (Allen et al., 2015). Widespread rains across the state of Uttarakhand and nearby plains caused by an anomalous advancement 3.3.2. Glacial surge dam burst floods of ITCZ associated with ISM that led to high-intensity torrential rains. Surging glaciers flow at exceptional rates during limited durations This extreme precipitation was caused by pulsed extension of the mon- ranging from a few months to a few years resulting in sudden advances soon trough with interim support from an existing unusual western dis- in the front (Meier and Post, 1969; Hewitt, 1969). The surging of glaciers turbance, which was the key driver of accelerated northward advance of is not a regular phenomenon to be associated with all the glaciers sys- the ISM convergence zone (Chevuturi and Dimri, 2015). The rainfall, re- tems in the Himalayas or elsewhere. It is limited to certain mountain corded during 14–18 June 2013 period, averaged over a wide swath of ranges or areas. Hewitt (1969) suggested that around 90% of world's the region, was 364 mm (Durga Rao et al., 2014). The highest recording surging glaciers lie in the Alaska-Yukon and Karakorum region. There- station in the area above Kedarnath at 3820 m above sea level recorded fore, in the Himalayan region, manifestations of the surging glaciers 325 mm rainfall in 24 h interspersed between 15 and 16 June 2013 are so far limited to the Karakorum region. Some of the exceptional gla- (Dobhal et al., 2013). The widespread sustained rainfall caused floods cier advances have been reported to be from the Karakorum region in the downstream reaches of the Alaknanada and Bhagirathi Rivers. ranging from around 3.2 km in 08 days for Yengutz Har glacier in Hispar The Kedarnath temple, one of the most revered Hindu pilgrim sites at valley (Hayden, 1907) to 12 km in 02 months for Kutiah glacier in Stak 3800 m above sea level, was hit by the floods firston16June2013eve- valley (Desio, 1954). Apart from these, Hassanabad, Aktash, Kichik ning caused by the torrential rain falling over the extensive snow cover Kumdan and Chong Kumdan are some of the well-known surging gla- (Dobhal et al., 2013). This rain on snow event fed huge quantities of ciers in the Karakorum region. Surging glaciers lead to catastrophe water into the periglacial Chorabari Lake situated at 3960 m above sea when they advance across the major headwater streams blocking and level near the Chorabari glacier which resulted into the bursting of damming the river. Hewitt (1982) enumerated the occurrence of thirty this lake during the early morning hours of 17 June 2013, killing scores five destructive outburst floods in the Karakorum region in the past two of pilgrims and devastating the Kedarnath temple and downstream hundred years. He also states that thirty glaciers are known to have ad- areas. One of the factors that worsened this extreme event was the an- vanced across major headwater streams during this period. Most au- tecedent precipitation/snow melt which ensured the slope saturation thentic records of the glacier surge dam outburst floods dating back to before the actual event (Allen et al., 2015; Durga Rao et al., 2014). This the 18th century come from the headwaters of Shyok River, as it served event has demonstrated extreme possibilities of hybrid Himalayan as a route to central Asia from India. At this location three surging gla- floods and their disaster potential. ciers, Chong Kumdan, Kichik Kumdan and Aktash, are responsible for the surge dam burst floods. Mason (1929) presented a detailed analysis 3.4.1. Earth bursts of the historical records on the snout position of these glaciers and Water is the principal medium of flow and the geomorphic evidence floods that originated from the river blockade by these glaciers. Accord- is extremely close to the cloudburst. Hence this phenomenon is usually ing to Mason (1929) there were a number of such floods spanning (mis)interpreted as a cloudburst or as debris flow in the Himalayan re- through 1780, 1835, 1839, 1842, 1873, 1903, 1926 and 1929. The 1929 gion. The genesis of such events are often revealed by the villagers living A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 111 close to the earth burst zones who are witnesses to the water bursting precise location of highly convective cloudburst events (Das et al., out of the earth. One of the known events of earth burst occurred in 2006). In addition, the spatial resolution of topographic data across the Hinval catchment in the middle Himalayas of Uttarakhand on 15 the Himalayas is limited to 30 × 30 m further limiting the application Aug 2014. The event, earlier thought to be a cloudburst flood, turned of 2-dimensional flood modeling. into an earth burst event after surveying the people living close to the Ground-based precipitation radar in the Himalayas is constrained site. Such events are common along the Western Ghats of Kerala and due to the steep terrain, and precipitation radar from space holds prom- there are some studies detailing these phenomena (Kuriakose et al., ise to overcome this obstacle for making regional-scale precipitation ob- 2006). However, the occurrence of such events is not common in the servations. However, the uncertainties of precipitation radar monsoon dominated regions of the Himalayas. This indicates that the observations are dissimilar across the orography (Duan et al., 2015; lack of understanding of the various earth flood generating mechanisms Wilson and Schreiber, 2015). Furthermore, precipitation radar observa- is limiting the scope of effective flood management in the Himalayas. tions are not available in real time. The sub-grid spatial distribution of precipitation is wider for gridded precipitation observations and is 4. Challenges and pathways to monitoring, predicting, and prepar- poorly defined. These sub-grid uncertainties are dependent on the at- ing for Himalayan floods mospheric and land surface processes. Detailed analyses, which include ground-based precipitation observations to identify sub-grid spatial and The varied nature of flood hazards in Himalayan Rivers necessitates temporal characteristics of rainfall across the orography, would be fruit- a multi-faceted and multi-scaled action towards preparedness. The pro- ful for the development of uncertainty estimates. There have been a lim- cesses and phenomena need to be monitored, predicted, and dissemi- ited number of short-term basin scale studies across the Himalayas in nated. These actions need to transcend traditional disciplinary which a dense number of precipitation stations were deployed (Barros boundaries, requiring unified and collaborative efforts across various et al., 2000). levels of government and administration between scientists, decision Many Himalayan rivers are transboundary watercourses and flood makers, and civil society. The relevant scales of Himalayan hazards hazards often cross national boundaries and affect more than one coun- trend towards global extremes, for instance, in terms of precipitation in- try. Hence, cooperation regarding water-related disaster risk reduction tensities, frequencies of active tectonics, flood discharges, sediment and communication of hydrometeorological conditions between ripar- yields, land areas inundated, and number and vulnerability of peoples ian river basins must be a shared priority. As of now, minimum ad- affected. Despite the high impact of Himalayan flood hazards, geo- vanced warning of hazardous conditions at upstream is being graphic and topographic constraints impede robust monitoring. In the provided for hazard mitigation efforts in the downstream countries. case of transboundary rivers, lack of robust mechanisms for monitoring However, formally-agreed procedures for sharing detailed ground and and sharing of hydro-meteorological data between the nations hamper satellite based hydrometeorological data in real time are likely to gener- affirmative actions. ate the most impactful and just outcomes with regard to advanced pre- The remote, rugged, and high-altitude source areas of many flood di- paredness. When river basin riparians lack procedures for direct sasters (GLOF, LDOF, cloudbursts) confer limited access for ground- exchange of observed hydrometeorological data, forecasters may face based data collection and thus have traditionally been some of the great uncertainty in the ‘geopolitically ungauged’ catchment areas out- most poorly monitored places on the Earth. In-situ precipitation and side their administration. Programs intended to enhance basin wide co- river gauging stations are limited across the rugged terrains of the operation, including data sharing, in Himalayan river basins, such as the Himalayas, especially along the steep ridges of valleys where intense Hindu Kush-Himalayan Hydrologic Cycle Observing System (HKH- convection is likely to occur (Thayyen et al., 2013). Over recent decades, HYCOS) are notable pathways to remove geopolitical barriers to flood satellite observations and coupled hydro-meteorological modeling have risk reduction. improved characterization of flood hazards and understanding of inter- Basin scale flood source area zonation has a great value for disaster active precipitation and topographic mechanisms over the Himalayas. risk reduction efforts, including flood forecasting. Among the major An extensive inventory of benign and potentially dangerous glacial types of Himalayan floods listed earlier, cloudbursts, GLOF, glacier lakes across the Himalayas is synthesized based on satellite information surge dam outburst flood, debris flows, and earth bursts are bound to (ICIMOD, 2010). Furthermore, a better understanding of the meteoro- impact the first order streams of the Himalayan slopes (Fig. 9). Design- logical and atmospheric mechanisms generating extreme monsoonal ing Disaster Risk Reduction (DRR) strategies to deal with such floods are precipitation events causing floods is provided (Barros et al., 2004; the major challenges due to very limited lead time and unknown source Barros and Lang, 2003; Das et al., 2006; Houze, 2012). In addition, areas, especially for cloudbursts and debris flow including earth bursts. there are a number of landslide hazard assessment tools which use Potentially dangerous glacial lakes can be identified, but extremely remotely-sensed data to predict potential LDOF (Pardeshi et al., 2013). short lead time in a GLOF situation (Fig 7g and h) and challenges asso- Despite the promise of remotely-sensed data and modeled forecasts, ciated with regular monitoring of such high altitude lakes make fore- without observed data to anchor predictions, such tools may be limited casting a big challenge. The middle reach of the Himalayan trunk in their application. Without robust ground-based data upon which channel is among the safest areas as far as the flood is concerned, as it remotely-sensed data or hydrological models may be calibrated or val- flows through deep gorges most of the time (Fig. 9) away from idated, uncertainty associated with remote rainfall observation and hy- human settlement. The downstream floods mainly generated from the drological prediction will be high. Flood modeling and management torrential rains over a large area of the mountain can be forecasted ef- applications are severely limited by the coarse spatial and temporal res- fectively, as it provides a significant lead time (Fig. 8a and b). olution of remotely-sensed datasets. Only a few gridded remotely- The topographic control over the orographically uplifted monsoon sensed datasets (e.g. TRMM 3B41RT) have sub-daily, 3-hourly, temporal rains provide another opportunity to constrain the monitoring efforts coverage, and most are limited to a 0.25 × 0.25° spatial coverage. These along the two highest precipitation zones assisted by the two-step to- products are spatially and temporally resampled from finer resolved pography. In a study carried out in the Chenab basin, this important as- satellite data. Using hydrologic reanalysis in the Ladakh region, pect is demonstrated. Chenab basin comes under the one step Thayyen et al. (2013) estimated a storm depth of 320 (±35%) mm topography regime where monsoonal precipitation occurs along the which took place in 8.8 min in a small catchment (0.842–1.601 km2) first orographic barrier in the basin. Rainfall is monitored in the Chenab of the Ladakh region. Harris et al. (2007) revealed the limitations of basin at 21 stations and there are 09 discharge stations in the basin. real time gridded satellite rainfall products in dynamic 1-dimensional Studies have shown that the source areas of all the major floods in the flood modeling of small catchments due to its course grid size. Further- Chenab basin is restricted to lower 7269 km2 area (Fig. 10). It is ob- more, numerical weather prediction models are unable to capture the served that the floods are actually gaining strength while passing 112 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Fig. 9. Flood impact zones in the upper Ganga basin. Recent cloudburst/lake burst often impact the first or second order streams in the mountain slopes, mainly impacting the settlement along the valley floor. Floods from torrential rains impact the foothill zones. The trunk river in the mountain reach often flow through deep gorges posing limited risk.

Fig. 10. Floods in the Chenab basin is generated from one step topography. (a) Hydrograph of some major floods in the Chenab river at Akhnoor (b) generated from the lower most zone constitutes Shivaliks mountain acting the first orographic barrier for monsoon winds blowing from the south-west. (c) distribution of seven rainfall measuring stations in this lower zone and location of representative station of Paoni. A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 113 through the last three discharge stations (Fig. 10a and b). Seven rain imperative for developing a robust flood management system for the gauge stations in this constrained area are found to be representing Himalayan region. the extreme torrential rainfall responsible for the floods and the studies even narrowed down to one station representing the regional extremes of the rainfall (Fig. 10c). This suggests that the spatial data generation, Acknowledgments research and mapping of flood source zones can be very useful for fore- casting and managing the monsoon floods from torrential rains in the Authors acknowledge the funding from Indo-US Science and Tech- “ region. Disaster potential of floods generated by the torrential rains nology Forum (IUSSTF) to organize Indo-US Bilateral Workshop on ” – over the mountains extends to the foothill zones further downstream Modeling and Managing Flood Risks in Mountain Areas during 17 19 (Fig. 9) (Ref. Koshi flood, Brahmaputra floods). Floods in the Brahmapu- Feb. 2015 Folsom, Sacramento, CA, USA. The paper is part of the out- tra River bring in huge devastation in Assam and Bangladesh further come of workshop deliberations. downstream and has been generated by torrential rains over steep mountain slopes and surrounding areas. References Catastrophic flooding in the downstream reaches of Brahmaputra River was caused by the simultaneous peaking of floods in the Ganges Adhikari, P., Hong, Y., Duglus, K.R., Kirshbaum, D.B., Gourley, J., Adler, R., Brakenridge, G.R., and Brahmaputra Rivers (Fig. 8e and f), providing the best example of 2010. A digitized global flood inventory (1998–2008): Compilation and preliminary – fl results. Nat. Hazards 55, 405 422. complexities associated with the downstream ooding in the big basins Allen, S.K., Rastner, P., Arora, M., Huggel, C., Stoffel, M., 2015. Lake outburst and debris with significant mountain area. Due to higher transit time, these floods flow disaster at Kedarnath, June 2013: hydrometeorological triggering and topo- can be effectively forecasted, if robust forecasting systems are in place graphic predisposition. Landslides 1–13. fl Annamalai, H., Hamilton, K., Sperber, K.R., 2007. The south Asian summer monsoon and and minimize the ood risk with effective intervention by the local gov- its relationship with ENSO in IPCC AR4 simulations. J. Climate 20, 1071–1092. ernance. Understanding of the seasonal distribution of floods across the Baldauf, M., Seifert, A., Forstner, J., Majewski, D., Raschendorfer, M., et al., 2011. Opera- Himalayan arc is also important for effective flood management by the tional convective scale numerical weather prediction with the COSMO model: de- fl scription and sensitivitites. Mon. Weather Rev. 139, 3887–3905. administration. In the western part, oods from cloudbursts are ex- Barros, A.P., Lang, T.J., 2003. Monitoring the monsoons in the Himalayas: observations in pected in the month of July and August and the GLOF's probability is Central Nepal, June 2001. Mon. Weather Rev. 131, 1408–1427. high during August and September months. In the Kashmir region Barros, A.P., Kim, G., Williams, E., Nesbitt, S.W., 2004. Probing orographic controls in the most of the major floods in the River Jhelum have occurred in the Himalayas during monsoon using satellite imagery. Nat. Hazards Earth Syst. Sci. 4, 9–51. month of May and September. Of late, more cloudbursts events are re- Barros, A.P., Joshi, M., Putkonen, J., Burbank, D.W., 2000. A study of 1999 monsoon rainfall ported from the Kashmir region as well as experienced during the July in a mountainous region in central Nepal using TRMM products and rain gauge ob- – and August 2015. In the central and eastern Himalayas, flood manage- servations. Geophys. Res. Lett. 27, 3683 3686. Benn, D.I., Wiseman, S., Warren, C.R., 2000. Rapid growth of a supraglacial lake, ment effort can be focused during the monsoon months of June, July Ngozumpa glacier, Khumu Himal, Napal. Int. Assoc. Hydrolo. Sci. 264, 177–185. and August. Bhandary, N. P., R. K. Dahal, and M. Okamura (2012). Preliminary understanding of the Seti River Debris-Flood in Pokhara, Nepal on May 5th 2012. A report based on a quick field visit program. ISSMGE Bulletin, 6(4), 8–18. Blanford, H.F., 1884. On the connexion of the Himalaya snowfall with dry winds and sea- 5. Conclusions sons of drought in India. Proc. R. Soc. Lond. 37, 3–22. Boers, N., Bookhagen, B., Barbosa, H.M.J., Marwan, N., Kurths, J., Marengo, J.A., 2014. Pre- fl fl diction of extreme oods in the eastern central Andeas based on a complex network Himalayan oods are generated from various atmospheric, approach. Nat. Commun. 5, 5199. http://dx.doi.org/10.1038/ncomms6199. cryospheric and geologic processes and are highly influenced by the to- Bookhagen, B., 2010. Appearance of extreme monsoonal rainfall events and their impact pography. Floods in the Himalayas have varying genesis and scale, on erosion in the Himalaya. Geomat. Nat. Haz. Risk 1 (1), 37–50. Bookhagen, B., Burbank, D.W., 2010. Toward a complete Himalayan hydrological budget: linked to atmospheric, cryospheric and geological processes. Most fre- spatiotemporal distribution of snowmelt and rainfall and their impact on river dis- quent flood events in the Himalayas are associated with monsoon pre- charge. J. Geophys. Res.: Earth Surface (2003−2012) 115 (F3). cipitation, either from torrential rains or from local convective events. Bookhagen, B., Burbank, D.W., 2006. Topography, relief, and TRMM-derived rainfall vari- Associated precipitation mechanisms and manifestations of these two ations along the Himalaya. Geophys. Res. Lett. 33 (8). Bookhagen, B., Strecker, M.R., 2008. Orographic barriers, high resolution TRMM rainfall, rainfall types are entirely different from each other. The Himalayan arc and relief variations along the eastern Andes. Geophys. Res. Lett. 35, 1–6. orography plays a very dominant role in defining the associated dynam- Buchroithner, M.F., Jentsch, G., Waniverhaus, B., 1982. Monitoring of recent glaciological ics of precipitation mechanism. Steering of atmospheric flow due to the events in the Khumbu Area (Himalaya, Nepal) by digital processing of Landsat MSS data. Rock Mech. 15 (4), 181–197. variable topography and heterogeneous land use determines the scale Campbell, J.G., Pradesh, H., 2005. Inventory of Glaciers, Glacial Lakes and the Identification and quantum of the precipitation. Hence, the Himalayan arc geomor- of Potential Glacial Lake Outburst Floods (GLOFs) Affected by Global Warming in the phology is one of the key parameters determining the flood type and se- Mountains of India, Pakistan and China/Tibet Autonomous Region. International Cen- fl tre for Integrated Mountain Development, GP O. Box, 3226. verity. A composite understanding is required for ood management Chevuturi, A., and A. P. Dimri (2015). Investigation of Uttarakhand (India) Disaster - 2013 but is lacking in the region. Using Weather Research and Forecasting Model. Nat. Hazards (in revision). Since a large population lives in the foreland basins of the Himalayas, Chevuturi, A., Dimri, A.P., Das, S., Kumar, A., Niyogi, D., 2015. Numerical simulation of an – fl intense precipitation event over Rudraprayag in the Central Himalayas during 13 14 socio-economic impacts of the large Himalayan oods are huge. Hima- September 2012. J. Earth Sys. Sci. (accepted). layan floods often lead to the loss of life, property, livelihood, health Clague, J.J., Evans, S.G., 2000. A review of catastrophic drainage of moraine-dammed lakes and epidemic and leave deep scars in the social and economic fabric. in British Columbia. Quat. Sci. Rev. 19 (17), 1763–1783. Das, S., 2005. Mountain weather forecasting using MM5 modeling system. Curr. Sci. 88 While forecasting capability of extreme events associated with large- (6), 899–905. scale monsoon flow is increased in tandem with the advancements in Das, S., Ashrit, R., Moncrieff, M.W., 2006. Simulation of a Himalayan cloudburst event. the weather satellite technology, ground based flow monitoring, flood J. Earth Sys. Sci. 15 (3), 299–313. forecasting remains as the area of huge concern in the Himalayas. Fur- Desio, A., 1954. An exceptional advance in the Karakoram-Ladakh region. J. Glaciol. 2, 383–385. ther, lack of understanding of the cloudburst processes, tracking and Dimri, A.P., 2004. Impact of horizontal model resolution and orography on the simulation monitoring facilities and capabilities remain a huge challenge. It is ob- of a western disturbance and its associated precipitation. Meteorol. Appl. 11 (2), – served that the cloudbursts are only recorded or reported when they 115 127. Dimri, A.P., 2013a. Interannual variability of Indian Winter Monsoon over the Western cause damages to life and property and occur in the vicinity of a popu- Himalaya. Glob. Planet. Chang. 106, 39–50. lated area. Hence, it is impossible to assess whether such incidents are Dimri, A.P., 2013b. Intraseasonal oscillation associated with Indian Winter Monsoon. increasing in the recent past due to changing climate. This seriously im- J. Geophys. Res.-Atmos. 118 (3), 1189–1198. Dimri, A.P., Niyogi, D., 2012. Regional climate model application at subgrid scale on Indian pacts our understanding of the causative factors of such extreme local- winter monsoon over the western Himalayas. Int. J. Climatol. http://dx.doi.org/10. ized events. Hence concerted efforts to assimilate these disciplines are 1002/joc3584. 114 A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115

Dimri, A.P., Niyogi, D., Barros, A.P., Ridley, J., Mohanty, U.C., Yasunari, T., Sikka, D.R., 2015. Joseph, P.V., Sijikumar, S., 2004. Intra seasonal variability of the low level jet stream of the Western disturbance: a review. Rev. Geophys. 53. http://dx.doi.org/10.1002/ Asian summer monsoon. J. Clim. 17, 1449–1458. 2014RG000460. Joseph, S., A. K. Sahai, S. Sharmila, S. Abhilash, N. Borah, R. Chattopadhyay, P. A. Pillai, M. Dimri, V.P., Pandey, A.K., 2014. Centre for Himalayan Study in Uttarakhand. Curr. Sci. 107 Rajeevan and A. Kumar (2014). North Indian heavy rainfall event during June 2013: (10), 1647. diagnostics and extended range prediction. Clim. Dyn., doihttp://dx.doi.org/10.1007/ Dobhal, D.P., Gupta, A.K., Mehta, M., Khandelwal, D.D., 2013. Kedarnath disaster: facts and s00382-014-2291-5. plausible causes. Curr. Sci. 105 (2), 171–174. Kale, V.S., 2003. The spatio-temporal aspects of monsoon floods in India: implications for Doms, G., Schaettler, U., 2002. The nonhydrostatic limited area model LM-Part 1: dynam- flood hazard management. Disaster Management. University Press, Hyderabad, ics and numerics. Scientific Documentation. Deutscher-Wetterdienst, Ofenbach, pp. 22–47. Germany, p. 140 (Online at http://www.cosmo-model.org). Kim, Y.J., Marshall, W., Pal, I., 2014. Assessment of infrastructure devastated by extreme Duan, Y., Wilson, A.M., Barros, A.P., 2015. Scoping a field experiment: error diagnostics of floods: a case study from Colorado, USA. Civ. Eng. 167 (CE4), 186–191. TRMM precipitation radar estimates in complex terrain as a basis for IPHEx2014. Kripalani, R.H., Kumar, P., 2004. Northeast monsoon rainfall variability over south penin- Hydrol. Earth Syst. Sci. 19, 1501–1520. sular India vis-à-vis Indian Ocean dipole mode. Int. J. Climatol. 24, 1267–1282. Durga Rao, K.H.V., Venkateshwar Rao, V., Dadhwal, V.K., Diwakar, P.G., 2014. Kedarnath Krishanmurti, T.N., Ardanuy, P., Ramanathan, Y., Pasch, R., 1981. On the onset of the sum- flash floods: a hydrological and hydraulic simulation study. Curr. Sci. 106 (4), 598–603. mer monsoon. Mon. Weather Rev. 109, 344–363. Falconer, H., 1841. Letter to the Secretary of the Asiatic Society, on the recent cataclysm of KrishnaKumar, K., Rajagopalan, B., Cane, M.A., 1999. On the weakening relationship be- the Indus. J. Asiat. Soc. Bengal 10 (1841), 615–620. tween the Indian monsoon and ENSO. Science 284 (5423), 2156–2159. Findlater, J., 1969. Inter-hemispheric transport of air in the lower troposphere over the KrishnaKumar,K.,Rajagopalan,B.,Hoerling,M.,Bates,G.,Cane,M.,2006.Unraveling western Indian Ocean. Quart. J. Royal Meteorol. Soc. 95, 400–403. the mystery of Indian monsoon failure during El Niño. Science 314 (5796), Fowler, A.C., Murray, T., Ng, F.S.L., 2001. Thermally controlled glacier surging. J. Glaciol. 47 115–119. (159), 527–538. Kumar, A., Houze Jr., R.A., Rassmussen, K.L., Lidard, C.P., 2014. Simulation of flash flooding Fritz, H.M., Blount, C.D., Thwin, S., Thu, M.K., Chan, N., 2009. Cyclone Nargis storm surge in storm at the steep edge of the Himalayas. J. Hydrometeorol. 15, 212–228. Myanmar. Nat. Geosci. 2, 448–449. Kumar, P., Rupakumar, K., Rajeevan, M., Sahai, A.K., 2007. On the recent strengthening of Fujita, K., A. Sakai, S. Takenaka, T. Nuimura, A. B. Surazakov, T. Sawagaki, and T. the relationship between ENSO and northeast monsoon rainfall over South Asia. Clim. Yamanokuchi, T. (2013). Potential flood volume of Himalayan glacial lakes. Nat. Haz- Dyn. 28, 649–660 L02706, doi: 10.1029/2005GL024803. ards Earth Syst. Sci., 13(7), 1827–1839. Kuriakose, S.L., Lele, N.V., Joshi, P.K., 2006. Fractional vegetation cover estimation from Goswami, B.N., Ajaya Mohan, R.S., 2001. Intraseasonal oscillations and interannual vari- MODIS NDVI data – a comparison of methods and application in dynamic numerical ability of the Indian summer monsoon. J. Clim. 14 (6), 1180–1198. modelling for debris flow (Urul Pottal) initiation prediction. Indian Geographical So- Gupta, V., Shah, M.P., 2008. Impact of the trans-Himalayan landslide lake outburst flood ciety Conference Publication, International Conference on Natural Hazards and Disas- (LLOF) in the Satluj catchment, Himachal Pradesh, India. Nat. Hazards 45 (3), ters – Local to Global Perspectives, November 25th to 27th, Anantapur, India. Sri 379–390. Krishnadevaraya University, Anantapur and The Indian Geographical Society Univer- Gupta, V., Dobhal, D.P., Vaideswaran, S.C., 2013. August 2012 cloudburst and subsequent sity of Madras, Chennai, India (ISSN No. 0973-5062). flash flood in the Asi Ganga, a tributary of the Bhagirathi river, Garhwal Himalaya, Laat, A.T.J., Lelieveld, J., 2002. Interannual variability of the Indian winter monsoon circu- India. Curr. Sci. 105 (2), 249–253. lation and consequences for pollution levels. J. Geophys. Res. 107 (D24), 4739. http:// Harris, A., Rahman, S., Hossain, F., Yarborough, L., Bagtzoglou, A.C., Easson, G., 2007. dx.doi.org/10.1029/2001JD001483. Satellite-based flood modeling using TRMM-based rainfall products. Sensors 7, Lau, W.K.M., Kim, K.M., 2012. The 2010 Pakistan floodandRussianheatwave: 3416–3427. teleconnection of hydrometeorological extremes. J. Hydrometeorol. 13, 392–403. Hartmann, D.L., Michelsen, M.L., 1989. Intraseasonal periodicities in Indian rainfall. Ludlow, F., 1929. The Shyok Dam in 1928. Himalayan J. 1. J. Atmos. Sci. 46, 2838–2862. Maharana, P., Dimri, A.P., 2015. Study of intraseasonal variability of Indian summer mon- Hayden, H.H., 1907. Notes on certain glaciers in Kashmir, Lahaul and Kumaon. Records, GS soon using a regional climate model. Clim. Dyn. 2015. http://dx.doi.org/10.1007/ 35, 123–148. s00382-015-2631-0. Hewitt, K., 1969. Glacier surges in the Karakoram Himalaya (central Asia). Can. J. Earth Sci. Mason, K., 1929. Indus floods and Shyok glaciers. Himalayan J. 1, 10–29. 6(4),1009–1018. Mason, K., 1930. The Shyok flood: a commentary. Himalayan J. 2, 40–47. Hewitt, K., 1982. Natural dams and outburst floods of the Karakoram Himalaya. Hydrolog- Meier, M.F., Post, A., 1969. What are glacier surges? Can. J. Earth Sci. 6 (4), 807–817. ical Aspects of Alpine and High Mountain Areas (Proceedings of the Exeter Sympo- Murray, T., Stuart, G.W., Miller, P.J., Woodward, J., Smith, A.M., Porter, P.R., Jiskoot, H., sium, July 1982). IAHS, pp. 259–269. 2000. Glacier surge propagation by thermal evolution at the bed. J. Geophys. Res. Hobley, D.E., Sinclair, H.D., Mudd, S.M., 2012. Reconstruction of a major storm event from 105 (B6), 13491–13507. its geomorphic signature: the Ladakh floods, 6 August 2010. Geology 40 (6), Nandargi, S., Dhar, O.N., 2011. Extreme rainfall events over the Himalayas between 1871 483–486. and 2007. Hydrol. Sci. J. 56 (6), 930–945. Holland, T.H., 1984. Report on the Gohna Landslip, Garhwal, Selections from the Records Pal, I., Lall, U., Robertson, A.W., Cane, M.A., Bansal, R., 2013. Predictability of Western Hi- of the Government of India in the Public Works Department: CCCXXIV(324), Office of malayan river flow: melt seasonal inflow into Bhakra Reservoir in northern India. the Superintendent of Government. Printing, Calcutta. Hydrol. Earth Syst. Sci. 17 (6), 2131–2146. Hong, C.C., Hsu, H.H., Lin, N.H., Chiu, H., 2011. Roles of European blocking and tropical Pardeshi, S.D., Autade, Sumant E., Pardeshi, Suchitra S., 2013. Landslide hazard assess- extratropical interaction in the 2010 Pakistan flooding. Geophys. Res. Lett. 38, ment: recent trends and techniques. Springer Plus 2 (523). http://dx.doi.org/10. L13806. http://dx.doi.org/10.1029/2011GL047583. 1186/2193-1801-2-523. Houze Jr, R. A., (2012). Orographic effects on precipitating clouds. Rev. Geophys., 50, Postma, G., 1986. Classification for sediment gravity-flow deposits based on flow condi- RG1001, 47 pp., doi:http://dx.doi.org/10.1029/2011RG000365. tions during sedimentation. Geology 14 (4), 291–294. Houze Jr., R.A., Wilton, D.C., Smull, B.F., 2007. Monsoon convection in the Himalayan re- Pruppacher, H.R., Klett, J.D., 2010. Microphysics of clouds and precipitation. Atmos. gion as seen by the TRMM precipitation radar. Quart. J. Roy. Meteor. Soc. 133, Oceanographic Sci. Lib. 18. http://dx.doi.org/10.1007/978-0-306-48100-0. 1389–1411. Quincey, D.J., Richardson, S.D., Luckman, A., Lucas, R.M., Reynolds, J.M., Hambrey, M.J., Houze Jr., R.A., Rassmussen, K.L., Medina, S., Brodzik, S.R., Romastshke, U., 2011. Glasser, N.F., 2007. Early recognition of glacial lake hazards in the Himalaya using re- Anomolous atmospheric events leading to the summer 2010 floods in Pakistan. mote sensing datasets. Glob. Planet. Chang. 56 (1), 137–152. Bull. Of Amer. Meteoro. Soc, Mar, pp. 291–298. Raju, P.V.S., Potty, J.R., Mohanty, U.C., 2012. Simulation of severe tropical cyclone Nargis Hoyos, C.D., Webster, P.J., 2007. The role of intraseasonal variability in the nature of Asian over the Bay of Bengal using RIMES operational system. Pure Appl. Geophys. 169, monsoon precipitation. J. Clim. 20, 4402–4424. 1909–1920. Hungr, O., Evans, S.G., Bovis, M.J., Hutchinson, J.N., 2001. A review of the classification of Rana, N., Singh, S., Sundriyal, Y.P., Juyal, N., 2013. Recent and past floods in the Alaknanda landslides of the flow type. Environ. Eng. Geosci. 7 (3), 221–238. valley: causes and consequences. Curr. Sci. 105 (9), 1209–1212. India Meteorological Department (IMD), 2010. Cloud Burst Over Leh (Jammu and Kash- Ranalkar, M.R., Chaudhari, H.S., Hazra, A., Sawaisarje, G.K., Pokhrel, S., 2015. Dynamical mir). Ministry of Earth Sciences, p. 4 (Available online at www.imd.gov.in/doc/ features of incessant heavy rainfall event of June 2013 over Uttarakhand. India. Nat. cloud-burst-over-leh.pdf). Hazards 1-23. International Centre for Integrated Mountain Development, Kathmandu, (ICIMOD), Rasmussen, K.L., Houze Jr., R.A., 2011. Orogenic convection in South America as seen by 2011K. Glacial Lakes and Glacial Lake Outburst Floods in Nepal. Kathmandu, p. 99. the TRMM satellite. Mon. Weather Rev. 139, 2399–2420. International Centre for Integrated Mountain Development, Kathmandu, (ICIMOD), Rasmussen, K.L., Houze Jr., R.A., 2012. A flashflooding storm at the steep edge of high ter- 2010K. Formation of Glacial Lakes in the Hindu Kush-Himalayas and GLOF Risk As- rain: disaster in the Himalayas. Bull. Am. Meteorol. Soc. 93 (11), 1713–1724. sessment. p. 58. Reynolds, J.M., 2000. On the formation of supraglacial lakes on debriscovered glaciers. Ives, J.D., Shrestha, R., Mool, P., 2010. Formation of Glacial Lakes in the Hindu Kush- IAHS Publ. 264, 153–161 (debris-covered glaciers). Himalayas and GLOF Risk Assessment. ICIMOD, Kathmandu, pp. 10–11. Richardson, S.D., Reynolds, J.M., 2000. An overview of glacial hazards in the Himalayas. Jain, V., Sinha, R., 2004. Fluvial dynamics of an anabranching river system in Himalayan Quat. Int. 65, 31–47. foreland basin, Baghmati river, north Bihar plains, India. Geomorphology 60 (1), Romatschke, U., Houze Jr., R.A., 2011. Characetristic of precipitating convective systems in 147–170. the southasian monsoon. J. Hydrometeorol. 12 (1), 3–26. Jhajharia, D., Singh, V.P., 2011. Trends in temperature, diurnal temperature range and Saha, S.K., Halder, S., Rao, A.S., Goswami, B.N., 2012. Modulation of ISOs by land-atmo- sunshine duration in Northeast India. Int. J. Climatol. 31 (9), 1353–1367. sphere feedback and contribution to the interannual variability of Indian Summer Jones, L.S., Schumm, S.A., 1999. Causes of avulsion: an overview. Fluvial Sedimentology VI. Monsoon. J. Geophys. Res. 117, D13101. http://dx.doi.org/10.1029/2011JD017291. Vol. 28, pp. 171–178. Sakai,A.,Chikita,K.,Yamada,T.,2000.Expansion of a moraine-dammed glacial lake, Joseph, P.V., Raman, P.L., 1966. Existence of low level westerly jet stream over peninsular Tsho Rolpa, in Rolwaling Himal, Nepal Himalaya. Limnol. Oceanogr. 45 (6), India during July. Indian J. Meteorol. Hydrol. Geophys. 17, 407–410. 1401–1408. A.P. Dimri et al. / Science of the Total Environment 556 (2016) 98–115 115

Shrestha, Arun B., Wake, Cameron P., Mayewski, Paul A., Dibb, Jack E., 1999. Maximum Vuichard, D., Zimmermann, M., 1987. The 1985 catastrophic drainage of a moraine- temperature trends in the Himalaya and its vicinity: an analysis based on tempera- dammed lake, Khumbu Himal, Nepal – cause and consequences. Mt. Res. Dev. 7, ture records from Nepal for the period 1971–94. J. Clim. 12 (9), 2775–2786. 91–110. Shrestha, P., Dimri, A.P., Schomburg, A., Simmer, C., 2015. Improved understanding of an Webster, P.J., Toma, V.E., Kim, H.M., 2011. Were the 2010 Pakistan flood predictable? extreme rainfall event at the Himalayan foothills-a case study using COSMO. Tellus Geophys. Res. Lett. 38, L04806. http://dx.doi.org/10.1029/2010GL046346. 67 http://dx.doi.org/10.3402/Tellusa.v67.26031. Wells, N.A., Dorr, J.A., 1987. Shifting of Kosi river, Northern India. Geology 15, 204–207. Singh, P., Ramasastri, K.S., Kumar, N., 1995. Topographical influence on precipitation dis- Wilson, J.W., Schreiber, W.E., 2015. Initiation of convective storms at radar-observed tribution in different ranges of Western Himalayas. Nord. Hydrol. 26, 259–284. boundary-layer convergence lines. Mon. Weather Rev. 114, 2516–2536. Singh, R., Singh, D., Gokani, S.A., Buchunde, P.S., Singh, R.P., Singh, A.K., 2015. Climate, to- Wu, T.W., Qian, Z.A., 2003. The relation between the Tibetan winter snow and the Asian pographical and meterological investigation of the 16–17 Jun 2 2013 Kedarnath summer monsoon and rainfall: an observational investigation. J. Clim. 16 (12), (India) disaster causes. Nat. Haz. and Ear. Sys. Sci. Diss. 3, 941–953. 2038–2051. Sinha, R., 2009. The great avulsion of Kosi on 18 August 2008. Curr. Sci. 97 (3), 429–433. Wulf, H., Bookhagen, B., Scherler, D., 2010. Seasonal precipitation gradients and their im- Sinha, R., Bapalu, G.V., Singh, L.K., Rath, B., 2008. Flood risk analysis in the Kosi river basin, pact on fluvial sediment flux in the northwest Himalaya. Geomorphology 118 (1–2), north Bihar using multi-parametric approach of Analytical Hierarchy Process (AHP). 13–21. Indian Journal of Remote Sensing 36, 293–307. Yadav, R.K., Ramu, D.A., Dimri, A.P., 2013. On the relationship between ENSO patterns and Steppler, J., Doms, G., Chattler, U.S., Bitzer, H., Grassmann, A., Damrath, U., Gregoric, G., winter precipitation over North and Central India. Glob. Planet. Chang. 107, 50–58. 2003. Meso-gamma scale forecasts using the nonhydrostatic model LM. Meteorog. http://dx.doi.org/10.1016/j.gloplacha.2013.04.006. Atmos. Phys. 82, 75–96. Yadav, R.K., Rupa Kumar, K., Rajeevan, M., 2007. Role of Indian Ocean sea surface temper- Thayyen, R.J., Dimri, A.P., Kumar, P., Agnihotri, G., 2013. Study of cloudburst and flash atures in modulating northwest Indian winter precipitation variability. Theor. Appl. floods around Leh, India during August 4–6, 2010. Nat. Hazards 65 (3), 2175–2204. Climatol. 87, 73–83. http://dx.doi.org/10.1007/s00704005-0221-5. http://dx.doi.org/10.1007/s11069–012-0464-2. Yasunari, T., 1976. Seasonal weather variations in Khumbu Himal. Seppyo 38, 74–83. Thayyen, R.J., Goel, M.K., Kumar, N., 2010. Estimation of aerial and volumetric changes for Yasunari, T., Kitoh, A., Tokioka, T., 1991. Local and remote responses to excessive snow selected glaciers in Jammu & Kashmir. Project Report. National Institute of Hydrology, mass over Eurasia appearing in the northern spring and summer climate. J. Meteor. WHRC,Jammu,p.92. Soc. Japan 69, 473–487. Turner, G., Slingo, J.M., 2011. Using idealized snow forcing to test teleconnections with the Indian summer monsoon in the Hadley Centre GCM. Clim. Dyn. 36, 1717–1735.