Exploring the History of Alaknanda - Patalganga Tragedy of 1970 & Possibility of its Recurrence and Impacts on Patalganga Basin - A GIS and Remote Sensing Based Study
Dr. Kishor Kumar
ScientistGeotechnical Engineering Division
Central Road Research Institute
Email: Kishornhrm@yahoo.com
Dr. D. Sati
Scientist
Geotechnical Engineering DivisionCentral Road Research Institute
Abstract
Patalganga, a moderate Landslide extends for about 260m along the toe of the river Patalganga and 350m along the slope, covering an area of 0.91 sq km situated adjacent to National Highway 58, at km 256 between longitude 79 29 15E and 79 29 30E and Latitude 30 29 25N and 30 29 15N.
The flashflood of July 1970 in the Alaknanda, which brought in loss to the life and property of an unprecedented magnitude, was a rare catastrophe for the Himalayan region. Belakuchi hamlet, which happened to be the centre for regulating the one-way traffic for the Joshimath-Niti and Badrinath bound vehicles was completely washed away, wherein about 15 vehicles and 35 lives, staying there on the fateful night of 20th July, also added to the loss. Nearly 15 km of Joshimath - Rishikesh road between Pipalkoti and Langsi was also breached, resulting in de-linking the life-line of the highlanders including the large Defense outposts located in the boarder areas of district Chamoli.
The tremendous run-off in the river Patalganga, a tributary of Alaknanda originating in Kunwarikhal catchment was the real culprit. The continuous and heavy downpour starting from 16th to 20th July flooded the streams in the upper catchments. Stretched between the highest point Kunwarikhal (4200m) and lowest point i.e. mouth of the valley (confluence) at 1200m, the drainage basin has a short mean stream length (approximately 14km) having relatively steep gradient (…. o) punctuated by two major bottleneck areas along the river course. The upstream of these bottlenecks the valley produced wide flood plains. Besides this, a lake was noticed by the natives in the upstream of Ganesh Ganga (tributary of Patalganga from the northeastern side). The Ganesh Ganga Lake burst triggered the sudden upsurge in the volume of stream water. The geomorphic feature of the catchment, which has large number of landslides and debris cones, facilitated the stream to erode these loose masses and the sideways terraces and thus huge debris carried along was dumped in the river Alaknanda at the confluence. The process builds a nearly 30m high dam and the water level was about 5m above the road alignment. As a result the Tapon - Langsi Bridge located at less than 500m northwest of the dam was lifted up (due to buoyancy) and thrown in the upstream direction by the backwaters. Due to continuous increase in volume of impounded water the dam could not bear the thrust and thus breached with sudden gush of water and debris down stream. Belakuchi, which was located just about a km downstream of the dam, was completely wiped-off within seconds along with the stretch of the road.
Patalganga landslide was developed in 1970 at a place where a small hamlet was situated during the 1970 tragedy. Since then the landslide is repeatedly occurring during every monsoon. The recurrence, although of low magnitude, reminds the public about the 1970 tragedy when Patalganga was blocked at a narrow constriction became the cause of the fateful event of 1970. Similar narrow constriction exists at the toe of the Patalganga landslide, which is fared to become dangerous, if the same amount of rain falls with in 24 hours as fell during 20th August of 1970.
The above facts brought forth the need of a fresh look on the Patalganga basin and Patalganga landslide to look at the current situation and comparing it with the one existed during 1970. This article highlights, in brief, the work carried out in the Patalganga basin and on Patalganga landslide.
The chief objective of the study was first to prepared base maps of Patalganga landslide in 1:500 scale and Patalganga basin in 1:12500 scale using topographic maps, satellite data and extensive field investigations. The base map were then to be utilized for preparation of other thematic maps such as geology, landuse, landcover, geomorphology, drainage, drainage basins, landslide and many other related out put maps using GIS and Image Processing software. GIS as well as Image Processing software provided excellent prospects to overlay, clip, join various thematic layers and draw out new groupings of the layers and their relationships to understand various factors, their interrelationships and the mechanism of the slope processes.
Both the base maps as targeted were prepared followed by the preparation of all the required thematic layers. The overlay, clipping, joining etc were carried out as per the requirement of the function of each layer and their attributes. The output of the study can be mentioned, in brief, as subsequent.
1) The overburden material in Patalganga basin is of Periglacial origin - the remnants of the last major glaciation event in the Himalaya in the Late Pleistocene - Holocene time.
2) The thickness of the overburden as roughly estimated ranges from a few meters to 500m and in a few places might be even more. Such type of thick overburden when exposed to weathering and erosion may develop a huge landslide during high precipitation.
3) The type of glacial material indicates towards the sensitivity of tectonic activeness of the region because most of the material encountered was brought down from the highly sheared and pulverized MCT zone and Higher Himalaya.
4) The flat stretches in the valley having huge thickness of periglacial deposit formed ideal places for agricultural fields and habitation.
5) For the last 30 years of Patalganga basin development it was noticed that around 357 - 1st order streams, 86 - 2nd order streams and 18 - 3rd order streams were added. The increasing numbers of streams add to an overall length of the streams in each order and therefore proportionately increasing the potential for weathering, erosion and landsliding. At present there are 386 landslides within the Patalganga basin and most of them are either along the streams or adjacent of them.
6) A few landslides are of moderate size as Patalganga landslide. The other minor landslides will also grow in its size and may reach to the size bigger than the existing moderate one if no precautionary measures are taken.
7) The situation in the valley is alarming because apart from 386 landslides almost whole basin is under the grip of the intense weathering and erosion.8) The landslides distribution in the basin indicate that the area is influenced by the major dislocation structures such as MCT, Gulabkoti Thrust and other faults/ lineaments including master joints. The Semkura Anticline in the middle of the basin has also been responsible for the extensional regime and consequent landsliding.During the presentation of the paper the above points will be discussed in detail.
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