Winter season flood in not a common occurrence in Himalaya. During the winter season higher reaches of Himalaya are under thick cover of snow, and melting of glaciers is insignificant. Winter season is thus a lean flow period for Himalayan rivers. No one was thus had scant idea of this winter season flood in Himalaya.
We can thus safely call this winter season flood a mystery flood because flood is caused either by heavy rainfall or dam breach as also sudden meltingof ice / snow. None of these really happened and yet the Dhauliganga valley was overwhelmed by a devastating flood on February 7, 2021.
Winter season of 2020-21 was unusual as there was little or no rain or snowfall all through in the Himalayan state, Uttarakhand. Except for November 2020 deficiency in average monthly rainfall for all the districts between September 2020 and February 2021 was between 34 and 99%. There was however slight rainfall on February 4 and 5, 2021 and higher reaches did experience snowfall. This resulted in lowering of general temperatures but it was a bright sunny morning on February 7, 2021 when suddenly Rishiganga and Dhauliganga valleys in Chamoli district of Uttarakhand were suddenly overwhelmed with debris laden floodwaters and before anyone could respond two hydropower projects in these valleys were devastated and there was no trace of more than 200 persons.
Most of you must be familiar with Ganga valley and might have travelled along the same while visiting Badrinath, Auli, Uttarkashi or Gangotri. Technically speaking it is either Alaknanda or Bhagirathi river along which you travel while going to Badrinath and Gangotri respectively and these two rivers meet at Devprayag (REF) to form Ganga river. Dhauliganga is a tributary of Alaknanda that meets it at Vishniprayag. In this valley there at two hot water springs at Bhapkund and Tapoban. Rishiganga originating from Nanda Devi massif (7,817 m) is the main tributary of Dhauliganga and has confluence with it around Rini.
Aware of high vulnerability of the low lying areas to both landslide and flood, people in Uttarakhand traditionally settled at higher elevations even though both agricultural fields and sources of water were located in middle and lower slopes of the valley. This amounted to routine extra labour for mending fields and fetching water but then these people, unlike us in present times, always accorded priority to safety over comfort. The habitations were thus safe from floodwaters but damage to 6 bridges disrupted connectivity of 13 villages. 09 persons of these villages (05 of Rini, 02 of Tapoban and 02 of Ringi) together with 02 Police personnel were amongst 204 persons that went missing in this unfortunate incidence. Many persons working in a tunnels and adits of the hydropower projects were trapped therein as these were overwhelmed with silt and debris, while the others were washed off.
A hydropower project of 13.2 MW capacity on Rishiganga river upstream of Rini was washed off by floodwaters while to the downstream of this dam axis of an under construction hydropower project of 520 MW on Dhauliganga river at Tapovan was severely damaged (Fig. 2).
The flood magnitude
With no discharge monitoring station in the catchment of Dhauliganga river increase in the discharge of Alaknanda river at Marwari, that is 18 km downstream of Tapoban, was utilised by Central Water Commission (CWC) to assess flood magnitude. The discharge of Alaknanda river at Marwari was 1,670 cumecs at 1100 hrs on February 7, 2021 as against normal discharge of around 41 cumecs. Alaknanda river crossed both danger level (1,383.00 m) and HFL (1,385.54 m) of June 28, 2013 at Marwari. It is estimated that about 6 million cu m water passed through this gauging station in one hour.
Thick pile of fluvio-glacial sediments brought down by the floodwaters was deposited all along the valley and according to the cross section measurements of the Alaknanda river carried out by CWC at Marwari on February 10, 2021 the river bed has risen by 3.09 m. Thickness of the deposited sediments increases gradually towards the upstream with the sediment thickness reaching more than 12 m at the dam site at Tapoban.