Patrick Robichaud

Increased Risk of Flooding from South Lhonak Lake

At an altitude of 5,200 meters in the Himalayan region of Sikkim, the South Lhonak Lake is fed by the retreating South Lhonak Glacier. Around midnight on the 3rd of October, a part of the left lateral moraine (sediment that is transported by glaciers) failed and fell into the South Lhonak Lake causing a huge wave of water to overtop the natural (moraine) dam that was at the end of the glacier lake. This event was the start of the Teesta Flood in Sikkim and West Bengal, resulting in the destruction of the Teesta III dam and causing many casualties downstream.

Photos of South Lhonak Lake showing collapsed moraine. In first photo moraine dam is shown on right side of lake (Source: Rajeev Rajak).

The Teesta flood is a unique event that started off as a Glacial Lake Outburst Flood (GLOF), but then led to the failure of the Teesta III Dam and transformed into a glacial lake outburst combined with a dam failure flood. The South Lhonak Glacial Lake had been identified by the government and academics as one of the fastest growing lakes from increased glacial melt in the Himalayas and was at high risk of a GLOF event. As such, there had been previous attempts to decrease the risk associated with potential outburst flooding. For example, a 2016 project that was led by Sikkim State Council of Science and Technology in collaboration with the Land Revenue and Disaster Management Department and a Ladakh-based NGO, Students' Educational and Cultural Movement of Ladakh (SECMOL), attempted to lower the water level in the lake by siphoning water over the dam. Additionally, there were efforts in developing an early warning system for such GLOF event. However, the system was not operational when the lake outburst occurred last month.

Rajeev Rajak, a PhD student at Sikkim University who studies glacial lakes and associated hazards, explained that there were three hypotheses as to the cause of the moraine failure. The failure could have been initiated by a heavy precipitation event, such as a cloudburst, thawing of permafrost, or seismic activity. Seismic triggering as a cause has been largely discarded by the scientific community considering no earthquakes were recorded in the region at the time of the dam failure. However, it could easily have been a combination of these causes or other factors as well. Understanding the triggers that caused the moraine failure will require field based scientific research.

Once the wave of water had overtopped the lake’s dam, it raced down rivers to the town of Chungthang, the first major habitation along the Teesta. The sudden flooding of the river surprised the large number of citizens residing on its banks at Chungthang where the Teesta III dam formally stood.

Impacts to Communities and Infrastructure

When the flood waters reached the Teesta III hydropower dam further downstream it overwhelmed the dam structure, completely washing away the concrete-face rock-filled dam. This event added a large volume of water and sediment to the outburst flood, combing to create a hybrid glacial lake outburst – dam outburst flood. The floodwaters then flowed rapidly downstream washing away multiple bridges and causing damages to the highways. This wall of water then partially damaged the next two dams downstream, at Dikchu and Shirwani, as they were overcome by the sheer volume of water. The spillways of these two dams were overtopped, and their entire reservoirs were drained. However, the structures were not washed away. As the floodwater reached gentler slopes, the large amount of sediment that was being carried by the river started to be deposited. Singtam was hit by the flood in the early hours of 4th October, when the swollen Teesta reclaimed its banks, washing away a major bridge and depositing large volume of sediments. Luckily, many lives in the area were saved through messages on the mobile network and the swift action of the local authorities who had started evacuation procedures before the floodwaters reached that area.

The official accounts put the death toll at around 100. However, it is estimated that this number could be much more. The number of people killed who resided in informal settlements along the riverbanks may not have been reported correctly.

During our visit to Rangpo, which is a major town at the border of Sikkim and West Bengal, we witnessed meters of sediment that had been deposited, including in buildings where the sediment had fully engulfed multiple stories. Many houses were destroyed, and many others were severely damaged by the deposited sediment. Since the flood, communities have been working hard to remove sediment from the streets and their homes. In Rangpo, an army of dump trucks and excavators work daily to remove these mountains of sediment and then dump them by the river. The sediment deposits and lack of basic water and other facilities have forced many of the residents to either shift to government run shelters or to move into the homes of nearby relatives, while they rebuild their homes. But for the unfortunate few who have lost their homes completely, there is little to look forward to.

Trucks moving sediment deposited from the flood in Rangpo.

Stories from Teesta Bazar

Further downstream, at only 90 meters of altitude, Teesta Bazar is a town located alongside the Teesta River near the border of Sikkim and West Bengal. On the 1st of November, we arrived at the refugee camp in Teesta Bazar, where families who have recently lost their homes in the flood have been staying. Most of the men that live at the refugee camp were away, either at work or helping with the recovery efforts of clearing up the debris. Consequently, those we encountered were primarily women, children, and a few elderly men.

Women we met in Teesta Bazar who have lost their homes to the Teesta flood.

A man in the Teesta Bazar refugee camp.

We interviewed four women who narrated the events of the night of the flood. They told us that around 2 am, they were notified that a flood was coming but did not perceive it as a serious threat. They explained that they receive multiple such notifications for floods annually and that these floods are usually minor and do not cause any significant destruction. Due to this, they remained awake but did not move any of their possessions or themselves to higher ground. It was only when the flood waters reached the area, that they realised the severity of the flood and managed to escape, but only with the clothes on their back. All their belongings and houses were washed away by the flood.

The retelling of their stories was emotionally charged. Their tales narrated a similar event, but each one showed a personal struggle. After narrating these stories, the women took us to see what was left of their homes. By the river, we only saw the flat floodplain and a destroyed football pitch while women pointed to the deserted sand by the river where their houses once stood. One of them showed us the foundation of her bathroom, the rest had either been washed away or covered in sediment.

A woman stated, “we need a house”. They wanted us to photograph what was left of their houses. One woman pointed out a dead natural Christmas tree in the sand, once valued, now a symbol of her loss. As we walked away from the river, we realized the danger the Teesta can pose.

A woman on the foundation of her home.