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Newsletter No. 38 |
V.C.Thakur, Wadia Institute of Himalayan Geology, Dehradun, India
(wihg@sirnetd.ernet.in)There are numerous instances of disasters caused by cloudburst floods and flash floods from landslides and debris flow dam failures in the Indian Himalayas. On average, at least one event is reported every year from some part of the Himalayas. Cloudbursts are common in the Higher Himalayan region and are mainly confined to narrow valleys. Selected cases of cloudbursts and flash floods and breaching of landslide-debris flow dams in the Indian Himalayas are given below.
Cloudburst Floods
In 1963, a cloudburst struck the meadow of a tourist resort in Pehalgam, 68 km from Srinagar, in Jammu and Kashmir. It uprooted a hotel and some large trees, killing several people and causing a flash flood in the Lidar River. In September 1995, heavy incessant rain in the upper reaches of Bias River and a cloudburst on the southern slope of Rohtang in the Higher Himalayas formed a small dam from debris flow north of Manali. The bursting of the dam generated a flash flood, causing a sudden increase in runoff that swelled into the dry paleochannel of the Bias River. This swept away several establishments, including a hotel, tourist resort, and government installations that were located on the old paleochannel of the river. At the same time, downstream near Kulu, a landslide on the road cutting near the river killed 60 people.
In Chirgoan, tehsil (district) headquarters of Simla district in Himachal Pradesh, a tragedy struck on 11th August 1997. In 10 minutes, a massive flash flood swept away civil and government buildings, including a 15 MW powerhouse, taking over 200 human lives. A cloudburst struck Andhra- Neogli
Khad watershed, leading to heavy discharge of debris flow that took the residents of the Chirgoan market area by surprise.
Debris Flow Dam Failure
On 20 July 1970, a flash flood on the upper Alaknanda Valley in Garhwal wiped out the village of Belakuchi situated on the bank of the Alaknanda River. It also washed away a 7km stretch of road and 15 vehicles and killed about 100 people. The retreating flood deposited up to a one-metre thick column of silt on the road and on the low-lying houses in Srinagar town. The upper Ganga canal downstream from Hardwar was closed for 75 days due to heavy silting caused by the flood. The cause of this tragedy was the damming of the Alaknanda River at the confluence of Patalganga tributary, which brought a vast amount of debris and landslide material from eroded river terraces. A cloudburst and heavy rain in Kumarikhal area were responsible for carrying the debris flow to dam the river.
On 10th August 1978 a flash flood from Bhagirathi River caused extensive damage between Gangnani, 60 km from Uttarkashi, and Tehri in the Bhagirathi Valley in Garhwal. The motorable road between Bhatwari and Tehri was breached in several places. Damage was caused to Maneri Bhali hydroelectric project, part of Uttarkashi town, and Bhatwari village. Human lives were not lost because of an early warning received by the people about the impending disaster. The flash flood was caused by damming of the Bhagirathi River at the confluence of Kandaliya gad(stream), a small tributary to the river. A cloudburst in the upper reaches of Kandaliya gad brought down a debris flow and created a dam, but this dam was breached and carried a vast amount of debris to the confluence of the Bhagirathi River, forming another dam across the main river. All this happened from 4th to 10th August, but the dam finally failed on 10th August causing flash floods downstream. Carving out a channel in the dam and draining out the excess flow reduced the destructive power to some extent. The Sutlej Valley in Himachal has had four flash floods as a result of cloudburst and landslide debris flow dam failure in the last 12 years.
On 29 September1988, a cloudburst in Soldan stream generated a flash flood that brought a huge amount of sediment to its confluence with Sutlej River, partially blocking the river and creating a reservoir. The cloudburst washed away 15 houses and an apple orchard: 32 people were killed. The flash flood downstream washed away a 2km stretch of road and a bridge over the national highway. On July 31st 199,1 a flash flood occurred in the Maling nallah, a tributary to the Spiti River, in upper Sutlej Valley. The flash flood was caused by a glacial lake outburst in the headwater region on Maling nallah(stream). Vast amounts of sediment generated by the flash flood were deposited at the confluence of the Maling and Spiti rivers. The failure of the temporary dam across the Spiti River was responsible for damage to a road section, a bridge, and farm land downstream.
On February 24, 1993, a major landslide near Jhakri in Sutlej Valley totally blocked the river. This blockade created a lake about 12-km long and 15 to 20-km wide. However, timely action next morning, which consisted of cutting a channel through the dam drained the stored flow out and thus averted a flash flood downstream. On July 8th 1993, a second blockade caused by a massive fall of rock occurred at Nathpa near Bhabanagar in the Sutlej Valley, the site proposed for a dam for the Nathpa Jhakri hydroelectric project. A temporary natural dam was created with the formation of a 6-km long lake. The raised water level entered some hydroelectric power installationst, which remained inoperative for three months because of damage caused by the increase in water level. The blockade was partially removed and the water level lowered during the winter of 1993 by using divers to blast under water and thus avert a flash flood downstream. The most recent flash flood was in the Sutlej River Valley on July 30th 2000. This flood washed away many bridges and a national highway in several places and killed about 200 people.
Lessons Learned and Suggestions
a) Monitoring of weather parameters and runoffs from streams and rivers in disaster prone areas
b) Reducing the capacity of weathering and erosion by afforestation of hill slopes and control of anthropogenic activities
c) Installation of early warning systems in disaster-prone areas
d) Avoiding construction of installations, houses, and other structures on the old paleochannel of a river
e) Development of technology and training people to blast dams and carve out channels
f) Geo-environmental appraisal of the area in terms of slope stability, likely debris flow material, and likely sites of glacial lake outbursts before making a decision to install infrastructure such as dams, bridges, roads, and townships
