How Sikkim Flash Flood Happened: Explained In Graphics

The Sikkim flash flood, in which 14 people have been killed and 102 others are missing, has been so intense because it was the result of a cloudburst causing a glacial lake to overflow. A glacial lake outburst flood, or GLOF, usually results in more damage and destruction than a flood caused by excess rainfall alone.

Here's how the events played out:

Around 1 am on Wednesday, the South Lhonak Lake, a glacial lake in the upper reaches of Sikkim, burst its banks following a cloudburst in the area. The lake is around 17,000 feet above sea level and satellite images showed that nearly 65% of its water was drained, flooding the Teesta River. 

All of this water flowed downhill, leaving destruction in its wake, and reached Sikkim's biggest hydropower project - the Teesta-III project in Chungthang - about 5,000 feet above sea level. The water, with all the debris it picked up along the way, rammed into the dam, causing parts of it to give way.

This led to massive flooding downstream. The Central Water Commission said the Teesta was flowing below the danger mark before the dam burst and then breached the mark in a matter of six hours. 

Severe damage was reported in all downstream areas of the river, including Mangan, Dikchu, Singtam and Rangpo. Bardang, the town where the 23 Army personnel were washed away and went missing, is also on the banks of the Teesta river. 

One of the personnel was rescued on Wednesday and a search is on for the rest.

Road links to Gangtok have been cut off because sections of National Highway 10 have been washed away. 

Destruction, Relief Efforts

As many as 14 bridges have been washed away in the state and over 3,000 tourists are reported to be stranded.

Mobile networks and broadband connections have been disrupted in Chungthang and most of North Sikkim due to fibre cable lines being destroyed by the flash floods in Sangkalan and Toong in Mangan district. 

The Army is laying down Bailey bridges so that essential items can reach all affected areas. The state government has also set up relief camps in various places, including Singtam, Rangpo, Dikchu and Adarsh Gaon.