Investigating the 7Th February, 2021 Landslide Triggered Flash Flood in the Himalayan Region Using Geospatial Techniques
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RESEARCH ARTICLE European Journal of Environment and Earth Sciences www.ej-geo.org Investigating the 7th February, 2021 Landslide Triggered Flash Flood in the Himalayan Region Using Geospatial Techniques Giribabu Dandabathula, Srinivasa Rao Sitiraju, and Chandra Shekhar Jha ABSTRACT On 7th February 2021 just before noon, news reports came in regarding a flash flood in Rishi Ganga/Dhauli Ganga River in Chamoli district of Published Online: August 28, 2021 Uttarakhand state, India. This brief report puts forth the probable causes ISSN: 2684-446X for this flash flood that has originated in the Nanda Devi Biosphere Reserve using geospatial datasets and techniques. Datasets obtained from MODIS, DOI :10.24018/ejgeo.2021.2.4.170 Sentinel-2B, SRTM, ICESat-2 and ERA5 have been effectively utilized to infer the details about this event. Slow drizzle to severe snowfall has been Giribabu Dandabathula* witnessed during 3rd to 6th February 2021 in various parts of the Himalayan Regional Remote Sensing Centre – West, region; even the Rishi Ganga witnessed a heavy snowfall during this time. National Remote Sensing Centre, Indian Data acquired on 10th February shows a scar developed due to a landslide on Space Research Organisation, Jodhpur, 342001, India. the shoulder of Ronti Mountain that was situated on the western rim of the Nanda Devi sanctuary. There was a gradual rise in temperature on 7th (e-mail: dgb.isro@ gmail.com) Srinivasa Rao Sitiraju February 2021 at the surroundings of Ronti Mountain that consequently led Regional Remote Sensing Centre –West, to a landslide. The landslide perpetuated a movement under the influence of National Remote Sensing Centre, Indian gravity from ~5900 m to ~3900 m with a mass envelope of ~0.290 km² and a Space Research Organisation, Jodhpur, velocity of 198 m/s that may have taken ~20 seconds to hit the Ronti bank. 342001 India. Due to the virtue of heat energy generated during this process resulted in Chandra Shekhar Jha contributing huge moraine filled flood water, that has accelerated towards Regional Centres Head Quarters, National the downstream of Rishi Ganga River and there after Dhauli Ganga River. Remote Sensing Centre, Indian Space Elevation profiles from the ICESat-2 and satellite imageries confirm the pre- Research Organisation, Hyderabad, 500037, India. existing conditions of landslide that is inclusive of weathering and erosion that led to the unstable condition at transportation back-slope of the Ronti Mountain. The triggering factors that influenced this landslide event and *Corresponding Author related causes were investigated in this study and reported herewith. Keywords: Dhauli Ganga, Himalayas, India, Landslide, Nanda Ghunti, Rapid Snowmelt, Rishi Ganga, Ronti Mountain. Videos circulated on WhatsApp showing water gushing I. INTRODUCTION downstream was reported through the media. There were Himalayas, stands as the highest mountain peak on Earth videos that represented the ground events like a wall of water where snow and icy environmental conditions stands rival to breaking the bridge and eventually surging into the dams, those existing at the Polar Regions that are known as the roof were widely circulated. Initially, various reasons like glacier of the world, the third pole and water tower of Asia [1]-[3]. break, avalanche and glacial lake outburst floods were The third pole region influences social and economic suspected to have caused this flash flood. The flash flood has development pertaining to the countries that depend on it like resulted in human loss, sweeping away a 35 MW India as it plays a pivotal role in ‘water-ice-air-ecosystem- Hydroelectric project (Run of River type) at the downstream human’ interactions [1]. In recent times it was reported that of Ronti Gad (or Raunthi/Rounti Gad), severely damaging an there has been a significant loss in snow cover change, under construction Vishnugad hydropower project of 520 permafrost degradation, changing stream flows, decreased MW in the Tapovan area on Dhauli Ganga River and a large- precipitation, glacier shrinkage, expansion of glacier lakes scale devastation along the downstream of Rishi and increased frequency of extreme climatic events in the Ganga/Dhauli Ganga. It was later reported that water levels Himalayan region; all these consequences were attributed to have significantly reduced in the later part of the day. climate change and anthropogenic activities [4]-[10]. Mal et This brief report outlines the probable causes for the flash th al. [11] and Kumar et al. [12] have quantified the amount of flood that took place on 7 February 2021 in the Rishi Ganga/ recent glacier changes using remote sensing methods. Dhauli Ganga River streams that had originated in Nanda On 7th February 2021 just before noon, news reports Devi Biosphere Reserve after investigation had been carried emerged regarding a flash flood in Rishi Ganga/Dhauli out by using various geospatial data. Ganga River in Chamoli district of Uttarakhand state, India. DOI: http://dx.doi.org/10.24018/ejgeo.2021.2.4.170 Vol 2 | Issue 4 | August 2021 75 RESEARCH ARTICLE European Journal of Environment and Earth Sciences www.ej-geo.org II. STUDY AREA Bethartoli, Trishul, Dakshni (South) Nanda Devi and Ronti. Nanda Devi group of glaciers falls within the upper Rishi Glacier systems containing North Nanda Devi, South Nanda Ganga catchment (a tributary of Dhauli Ganga river), Central Devi, Changabang and Ramani give rise to streams thereby Himalaya, covering an area of ~690 km2. Yuji [13] and Jain leading to the formation of the Rishi Ganga River. With the et al. [14] have provided extensive reports pertaining to the Nanda Devi as a peak (7817 m, 7434 m East) in the region, geological details of the Nanda Devi region; the area is the other peaks like Ronti, Nanda Ghunti and Trisul lies on composed of rocks belonging to the centre crystalline of the the western-south rim of the Nanda Devi; from here, the Ronti Vaikrita Group like garnetiferous mica schist and garnet mica Gad stream forms a sub catchment of the Rishi Ganga. Rishi schists which are well exposed along the Rishi Ganga Ganga River enters the lower gorge to merge with the Dhauli catchment. The catchment is dominated by Higher Ganga near Rini village. Fig. 1 shows the extent of Ronti Gad Himalayan rugged topography with high elevation ridges sub-watershed boundary, Rishi Ganga catchment, relief map adjacent to deep glacial valleys. Prominent glaciers in this of Nanda Devi Biosphere Reserve and the location of dams th area are Uttari (North) Nanda Devi, Changbang, Ramni, that were damaged due to the flash floods on 7 February 2021. Fig. 1. Location of Ronti watershed, Rishi Ganga catchment and the dams that got damaged due to 7th February 2021 flash floods: (a) Map of India highlighting Uttarakhand state. (b) Uttarakhand state map highlighting the Rishi Ganga catchment. (c) Ronti Gad watershed boundary, Rishi Ganga catchment, dam locations and area where landslide happened shown in a relief map (generated from SRTM) and overlaid with OpenStreetMaps. DOI: http://dx.doi.org/10.24018/ejgeo.2021.2.4.170 Vol 2 | Issue 4 | August 2021 76 RESEARCH ARTICLE European Journal of Environment and Earth Sciences www.ej-geo.org Earlier scholars like Aitken [15], Bisht [16] and Kapadia [17] have briefly mentioned the record of historical summiteers to Nanda Devi region and emphasized the difficult topographic conditions in this area in terms of accessibility. Dr. T.G. Longstaff is credited as the first explorer in this region during 1905 and was later followed by the surveyors Tilman and Shipton in 1934 [18]. Shipton [19] has mentioned the experiences of his survey emphasising the interiors of Rishi Ganga catchment. Lamba [20] and Bisht [16] have elaborated the details of flora and fauna of Nanda Devi National Park. Emmons [18], Luvkumar [21] and Lamba [20] during their independent surveys in this region have observed collapsed heaps of rubble and glacial debris at the lower stretches of Ronti glacier giving a hint about the retreating of the glaciers. Lavkumar [21] mentioned the existence of coniferous forests growing in the lower stretches of Ronti valley which is open to moisture-bearing winds from the lower Dhauli gorge. III. DATASETS AND METHOD OF INTERPRETATIONS Multi-platform and multi-sensor satellite data, open source Digital Elevation Models (DEM), space-borne Laser Altimeter and reanalysed weather data that are disseminated from various web portals have been used in this study for identifying the areas of damage, moraine accumulation and the extent of area where the actual event happened. Table I Fig. 2. Associated datasets and the method of interpretations were done to shows the summary of datasets that were used in this analyse the disaster event occurred on 7th February 2021 in the region of investigation and Fig. 2 represents the method of Nanda Devi glaciers, Chamoli district, Uttarakhand state, India. interpretations. TABLE I: DETAILS OF DATASETS USED TO STUDY THE LANDSLIDE EVENT OCCURRED ON 7TH FEBRUARY 2021 IN THE NANDA DEVI BIOSPHERE REGION OF HIMALAYAS. S. No. Dataset Specifications Remarks 1 Moderate Resolution Imaging MOD09QG daily data with 250 Data pertaining to pre and post events were used to Spectroradiometer (MODIS) m spatial resolution which visually interpret and to assess the snow conditions provides surface spectral from 2nd to 8th February 2021. reflectance corrected for atmospheric conditions in Red and NIR bands 2 Sentinel-2B Visible and NIR bands @ 10 m Used to assess the areas that got damaged and moraine spatial resolution accumulation due to the landslide event. Data acquired on 1st January 2021, 16th January 2021 and 10th February, 2021 were used in the study. Additionally, time series analysis has been done using Sentinel-2B data from the year 2017 till 2020 to understand the peak seasons for accumulation and ablation process as well as to understand the snowline conditions 3 SRTM 30 m Digital Elevation Model Used to understand the topographic features like elevation, slope and aspect of the study area.