Wednesday, January 24, 2024

AFTER THE FLOOD - By Pemzang Tenzin (Mangan, North Sikkim)


Zanak, at the base of the aptly named Sentinel Peak, is the last outpost on the lesser used but religiously and historically important Chorten Nyima La route to Tibet.  The ITBP personnel stationed there first raised the alarm on the South Lhonak Glacial Lake Outburst Flood (GLOF) and possibly saved the lives of many people in the path of the ravaging flood.

 South Lhonak Lake, like many other glacial lakes, is meltwater stored behind a fragile dam created by debris and ice left behind by the snout of the retreating glacier.   The 1950 Swiss Foundation map of Sikkim, based on earlier Government of India surveys and still used today by trekkers and mountaineers, does not show a lake at this location.  The lake has grown from nothing to its present dimensions of more than 200 standard football pitches within a span of less than a hundred years.  The glacier is expected to recede further and the size of the lake will increase in the very near future. The volume of water stored is thirteen times the storage of the erstwhile Teesta Urja reservoir.  It is estimated that half this volume spilled out during the October GLOF event.

This is a story repeated again and again in others parts of the high mountains as the ice cap starts melting at an accelerating rate due to the effects of climate warming.  The Eastern Nepal, Sikkim and Bhutan regions are considered a hotspot for GLOF activities.  In Sikkim, the total area covered by ice caps is larger than the size of some of the districts.  The Swiss Consortium, in partnership with National Disaster Management Authority (NDMA) is studying some of these lakes.  They have compiled a list of twenty five lakes in Sikkim that have been red-flagged by different agencies as being potentially dangerous.  Twenty four of these are located in North Sikkim, almost equally distributed along the Lachen and Lachung axes and one in West Sikkim.  With increased melting and the recession of glaciers, more lakes are expected to form or increase in size, making new areas in other areas of Sikkim vulnerable to GLOF.

Sikkim’s limited experience with mitigation measures for dangerous lakes, like siphoning of the same South Lhonak in 2016 and controlled breaching of Mantam in 2017, has met with at best limited success. The NDMA-Swiss Consortium project involves studying the South Lhonak Lake and the Shako Cho as exemplars to suggest mitigation measures or to design an Early Warning System.  Without ground access, the Swiss experts had already completed the desktop studies based on satellite imagery.  Due to security reasons, the team could only make its first field visit in September of 2023 but before the team could even take stock of their visit, South Lhonak Lake surprisingly burst on October 3rd.

 It is difficult to create an accurate time line for the GLOF event and the arrival/peak flows at different towns as both public and official reports vary considerably.  This is due to the fact that the flood happened in the middle of the night and it builds up gradually from first arrival to peak flood levels over  a matter of hours.  The ITBP camp noticed the level rise in the Goma Chu, the effluent stream from South Lhonak at about 10.30 at night (the Print), though the triggering landslides and the breaching of the lake must have happened at least hours earlier.  In India, Central Water Commission (CWC) monitors flows, including GLOF, in all the significant rivers through a network of instrumentation.  Hydrologist/activist Himanshu Thakkar writes in his blog that the automatic stations upstream of Dikchu, including Lachen stopped reporting sometime before 10.30 pm on October 3rd and, in his opinion, these stations were probably non functional.  Downstream of Dikchu, instrumentation show the river rose by about the height of a three storey building at Khanitar (near Manipal) and by about a five storey building at Melli where the CWC stations are located.  At these locations, the floods first arrived at around midnight but took a couple of hours to build up to maximum flows. 

 The actual levels recorded at Khanitar were more than twice as high as predicted in  the CWC simulation for South Lhonak GLOF published in their advisory in 2016.  Gazoldoba near Siliguri, located a couple of hundred kilometres downstream of the GLOF source, recorded high flows (more than 7000 cumecs above pre flood flow) despite the flood flowing through a number of manmade and natural impedances on its long journey downstream.  The flood marks recorded at RangRang also augments this observation that the flows on October 3rd/4th were much higher than those simulated both by CWC and the Swiss consortium.  The flood marks upstream and downstream of Chungthang could also provide an estimate of the effect of the collapse of the dam on flood flows.

 The dam failure at Chungthang has highlighted the vulnerability of power projects and the inadequacy of the existing safety protocols in the face of abnormally high flood flows associated with GLOFs.  The dams are not designed for overtopping and the amount of water it can manage are conventionally calculated from hypothetical rainfalls in the catchment area.  While repairing and rebuilding the damaged or collapsed dams, GLOF has to be factored in.   There is a need to have a relook at the hydrological design and flood response protocols of all existing dams in Sikkim and elsewhere in the Himalayas.  It is reasonable to question whether the reservoirs were operating at minimum levels in October (monsoon period) as required by safety protocols and whether the gates were working properly.  There is a need for more oversight from dam safety committees in the state (if it exists) and the centre at both the design stage and during operations.

 Chungthang is built on a triangular river terrace at the confluence of the Lachen and Lachung rivers.  River terraces are ephemeral geographical features which seem doubly vulnerable to both GLOF and the shaking of earthquakes.  The October 3rd/4th GLOF seems to have severely damaged or wiped out most of the river terraces upstream of Dikchu along with infrastructure and property built on them.  Parts of Toong and Rel villages built on Talus slopes on the Teesta a few kilometres downstream of Chungthang are collapsing due to undercutting by the flood.  Here, the houses on and below the highway have toppled while those above the highway have also developed cracks and may not survive the next monsoon.  Reports of cracks in structures in other areas situated on hill slopes above the banks of the marauding flood also needs to be investigated empathically.

 Whether it is landslides, GLOF or any other natural disaster, the best mitigation measure is avoidance of the vulnerable area.  Past experience shows that once the disaster is temporarily abated, people double down and start rebuilding in the vulnerable areas.  Due to ad hoc decision making and paucity of funds, Government departments usually put promises of resettlement in the back burner with the passage of time and slowly forget about it.  With our increasing populations and increased exposure to multiple hazards, there is a need to legally prevent development and houses in the designated disaster prone zones and to formulate comprehensive and humane disaster rehabilitation policies. 

 Beyond activism and partisan politics, we need to rationally study and understand these constantly evolving and growing natural threats to our society and learn to cope with them.

Pemzang Tenzing