Glacial Flooding & Disaster Risk Management Knowledge Exchange and Field Training July 11-24, 2013 in Huaraz, Peru HighMountains.org/workshop/peru-2013
Reducing Lake Water Level in Thorthormi Glacial Lake in Bhutan: An Adaptation Project to the Adverse Impact of Climate Change Karma Department of Geology and MinesBhutan Introduction Bhutan, a tiny little country located in the eastern part of the Himalayas surrounded by China in the north, India to the east, west and south (fig.1) has 677 glaciers and more than 2500 glacial lakes (ICIMOD 2001). Out of 2500 glacial lakes 24 of them has been classified as potentially dangerous lakes which has the potential of outburs. Later in 2006, Department of Geology and Mines (DGM) again in collaboration with ICIMOD has found an additional lake which has capacity to outburst in the future, thus taking the number of potentially dangerous lake to 25. Subsequent studies carried out by DGM in collaboration with the scientists from University of Vienna, Austria and Nagoya University, Japan thereafter in Lunana region in northern part of the country (fig.1) found out that the risk of GLOF from Thorthormi lake is ever increasing. Accordingly, during the formulation of first National Adaptation Plan of Action (NAPA) in Bhutan, a proposal to reduce the risk of GLOF from Thorthormi Tso was proposed. Considering the facts based on the results of intensive scientific investigations, the proposal received the highest priority and was approved for funding under the UNFCCC funding scheme. The project had three main components: reducing water level Thorthormi lake, installation of GLOF Early Warning System along the river Pho Chu and Public Awareness. The first component was implemented by Department of Geology and Mines, second by the Department of Hydromet Services and the third component by Department of Disaster Management respectively. As the main implementing agent for the first component, this paper is focussed primarily on reducing water level of Thorthormi Lake.
Figure 1: Location map of Bhutan and Project Site Glacial Lake Outburst history in Bhutan Before 1950 there are no information on any incidences of GLOF happenings but field evidences suggest that there had been several GLOF incidences in different river basins. Through some verbal communication with the elders in villages there were cases of GLOFs in 1957 along Punakha-‐Wangdue valley in which the historical Punakha dzong was partially damaged. Again in 1960 an outburst from a lake in eastern lunana damaged the same historical structure partially. There were cases of damages by flood in Punakha, Thimphu and Paro dzongkhags in 1967. The most recent and well documented GLOF event occurred on 7th October 1994 from a glacial lake called Lugge in Lunana region along Pho chu river in Punakha-‐Wangdue valley. This GLOF has caused severe damages to infrastructures such as houses and bridges, agricultural land, livestocks and 17 human lives were lost. The project The project was designed for four years with four working seasons (4 months) per year. This is mainly based on the accessibility in the project site. Thorthormi Lake is located in Lunana which takes 9 walking days from the nearest motorable road and need to cross three high passes, the highest being Gangla Karchung with a height of 5220 masl. Since these passes will not be accessible till April-‐May, the working season was proposed from June-‐September coinciding with the peak monsoon season which makes it a challenging one. The objective of the project is to reduce the water level in this growing Thorthormi lake by 5 m. In doing so approximately 17 million cubic meters of water was being release artificially from the Lake Thorthormi. This volume is almost equivalent to the 1994 GLOF which was 18 million cubic meters of water.
Identification of risk on Thorthormi Lake Several factors were considered in identifying GLOF risk from Thorthormi glacial lake. The following are some of the main factors and scenarios considered: a. Rapid expansion of Thorthormi supra glacial ponds on the glacier
Photo 1: Recent and rapid expansion of supra glacial ponds on Thorthormi glacier
Thorthormi glacier has been melting at an accelerated rate since 1950s. The average annual rate of glacier retreat in Bhutan for the debris covered glacier was reported as high as 30-‐35 meters per year (Ageta et al 2001). Photo 1 shows the changing surface of Thothormi glacier from 2008 to 2012 and drastic expansion in terms of surface area of the water bodies (lake) can be observed. b. Size of glacier and future glacial lake
Photo 2: Size of probable future lake on Thorthormi glacier
The above satellite image shows the most probable size of lake to be developed on Thorthormi glacier. If whole glacier ice melts on Thorthormi glacier a lake with a surface area of 3.42 square kilomenter will be formed. During a bathymetric survey in 2008, it was found that the depth of supra glacier ponds varies from 23 m to 76 m. The average glacier ice thickness was found to be about 150 m. c. Weakened moraine wall on the left lateral moraine of Thorthormi glacier
Photo 3: weakened left lateral moraine of Thorthormi glacier and direction of 1994 GLOF (black arrow) During the 1994 GLOF from Lugge lake located in the upstream of Thorthormi glacier, the left lateral moraine of Thorthormi was severely damaged due to erosion on the outer side (photo 3, marked by red boundary). Due to this, the left lateral moraine wall has become so thin that at some point on the moraine crest people can hardly walk across. In case a full lake develops inside this moraine by melting of ice, this stretch of moraine wall may not be able to stand the hydrostatic pressure of the water in the lake.
d. Seepage from the lake
Photo 4: Seepages from Thorthormi Lake The above satellite image (photo 4) shows several seepage points around the lake. Atleast there are 4 seepage points which are confirmed to be coming out from the lake. e. Active sliding on the moraine wall separating Thorthormi and Rapstreng lake
Photo 5: Slide on both sides of maraine wall between Rapstreng and Thorthormi lake The two lakes Rapstreng and Thorthormi are separated by a complex moraine wall formed by left lateral moraine of Rapstreng and right lateral moraine of Thorthormi glacier. As the wall facing both the lakes have high gradient, active sliding is a common
phenomenon. This sliding activities on both sides as shown in photo 5 is resulting in thinning of this moraine barrier between the two lakes which ultimately will result in weakening of the moraine wall surrounding Thorthormi lake. f. Rock and snow avalanches
Photo 6: snow avalanche from the accumulation zone of Thorthormi glacier Due to high slope in the accumulation zone of Thorthormi glacier, avalanches which take place frequently can directly fall into the lake which can generate a surge wave. Apart from that with the retreat of glacier more rock faces are exposed which are quite fragile (brittle) and result in mass movement and impact the lake directly in generating surge waves from the lake. g. Ice cored moraine barrier between Rapstreng and Thorthormi Lakes The moraine barrier between Rapstreng and Thorthormi lake is not only getting thinner due to mass movement but as per the result of the resistivity carried along this, shows a presence of huge ice body inside at a depth of around 80 m from the surface (photo 7). Now considering the increasing global temperature, the ice body inside the moraine has a high chance of melting faster which may result in destabilizing the whole moraine wall. This can again result in weaken of the moraine wall surrounding the future Thorthormi lake.
Photo 7: Result of the resistivity profile along the moraine barrier between Rapstreng and Thorthormi lakes. The Risk Considering the above 7 factors, the risk in the present context is shown in the following section diagrammatically. The total volume estimated in this scenario is 53 million cubic meters of water which is approximately 3 times the one occurred in 1994.
Failure of moraine barrier between Rapstreng and Thorthormi
Over topping of water from Thorthormi lake into Rapstreng lake. Water level in Thorthormi lake is higher than Rapstreng lake by 74 m
Finally resulting in an outburst with combination of water from Thorthormi & Rapstreng. Estimated volume is 53 million cubic meter.
Need for lake mitigation work
Figure 2: Settlement map of Bhutan Fig. 2 shows settlement pattern of Bhutan and it can be seen from the figure that major settlements are all along major river basins. Infact, as per the survey it is said that about 70% of settlements in Bhutan are located along the fertile valley bottoms where major river systems run through. With the settlements comes the developmental activities. Along Puna Tsang Chu basin in which Thorthormi Lake drains, there are number of mega hydro power projects coming up which are still in constructions phases. Beside, Khuruthang town of Punakha district is located right on the bank of this river. Therefore, the mitigation work was required considering the following: 1. There are vast areas of agricultural land with number of settlements located close to the river. 2. Many developmental activities are taking place along this river including construction of mega hydro power plants. 3. Numerous historical monuments are located on the low lying bank of this river such has Punakha Dzong (old fortress) Method Adopted to lower lake water level in Thorthormi lake Basically three methods were considered for lowering water level in Thorthormi lake. The three methods are
1. Construction of spillway channel (through the natural existing outlet channel) 2. Siphoning 3. Pumping Considering number of uncertainties, workability of equipments, environment and location of lake Thorthormi, transportation of heavy equipments for 9 days on foot, siphoning and pumping were considered not feasible. The best option was found to be construction of spillway channel along the existing outlet channel by deepening and widening the existing natural outlet channel. The width of the finished channel was designed to be 10m. A 2% bed gradient was adopted for the final channel bed which is based on the existing channel gradient. Constraints Implementing such a huge project in a place which is located 9 days trek from the nearest motorable road head, situated at an altitude of about 4300 m is a challenging task. Due to harsh weather conditions and high working environment there were various difficulties faced by the project management team. The following are some of the main problems encountered during the implementation phase of the project. a. Unpredictable weather system As the working season coincides with peak monsoon season in Bhutan, weather conditions can be an important impeding factor for the project. Most of the time the workers had to work in wet condition where the water temperature can be near to freezing point. Combining this with torrential down pour all day impacts work output to a great extent. Not only the excavation work at the site is affected but really bad weather can also impact the transportation of rations on the way due to landslides and swelling rivers blocking them from reaching the project site on time. b. Health Issues (medical issues) To reach the project site the workers need to trek for 9 days crossing high passes. This makes them vulnerable to high altitude sickness. Although there is a medical team to cater to such situation but due to accessibility reason, providing medical attention to those needed on time is a huge challenge. During the entire four years of the project, three lives were lost to high altitude sickness. Incase of emergencies, evacuation from the project site by helicopter was a big problem mainly due to weather condition and also beauracratic procedure involved in arranging the helicopters. c. Transportation of materials In each phase of the project approximately 60 MT of materials including rations need to be transported at the site by horses and yaks. The whole logistic arrangement for the transportation of such quantity of material on yaks and horses was a challenging task. d. Recruiting and managing large workforce
In a place like Lunana which is located in a remote corner of Bhutan, 9 days trek from any place with modern amnesties, recruiting and managing about 400 people is a big task. First, in the later part of the project period it was difficult to recruit workers despite paying about 4 times the normal daily wage and other incentives like food and cloth. Even if enough work forces are found, taking them to the project site involves number of life threatening risk (on the way due to mountain sickness). Once they are at the project site, various management issues comes up starting from maintaining discipline in the camp and relation with the local communities. The management had to face unnecessary social issues from a simple brawl in the camp to serious problems with the locals. Acknowledgement I would like to acknowledge the support I and my other colleagues in the multidisciplinary team received from the following individuals, organizations, agencies and institutions during the entire project period including the opportunity given to me to attend the workshop in Peru. Firstly, I on behalf of the project management team like to extend my gratitude to the main donor of the project which is UNFCCC/LDCF and other co-‐financers such as Austrian Government, WWF, UNDP and Royal Government of Bhutan for their generous contribution in terms of budget for this important project. Secondly, I would like to thank the Royal Bhutan Army for their kind support in whatever we requested from them especially for their readiness to participate in the last phase of the project in 2012 when the project management had huge problem in recruiting work force. Thirdly, the project management would like to acknowledge the kind support received from Gasa Dzongkhag in providing the logistic support especially transportation of project materials using horses and yaks. More importantly, the kind support and cooperation received by the project team from the local communities including the gup of lay and lunana are also highly acknowledge. Lastly, our sincere gratitude goes to UNDP Bhutan for their unfailing support throughout the project including the opportunity and support provided to Mr. Karma, Chief Glaciologist from DGM to attend an important workshop in Peru.
Published on Sep 25, 2013
Bhutan, a tiny little country located in the eastern part of the Himalayas surrounded by China in the north, India to the east, west and sou...