Objectives and Prevention Methods for Glacier Lake Outburst Moods (GLOFs)

Transcription

1 Snow and Glacier Hydrology (Proceedings of the Kathmandu Symposium, November 1992). IAHS Publ. no. 218, Objectives and Prevention Methods for Glacier Lake Outburst Moods (GLOFs) W. E. GRABS 1 & J. HANISCH 2 1 Teamleader, Snow and Glacier Hydrology Project at the Department of Hydrology and Meteorology, Kathmandu German Agency for Technical Cooperation (GTZ), Bergstr. 41, D Bullay/Mosel, Germany 2 Engineering Geological Advisor to the Department of Mines and Geology, Kathmandu, Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-3000 Hannover 51, Germany Abstract Glacier lake outburst floods (GLOFs) have claimed numerous lives and have inflicted high costs in destroyed infrastructure and property in Nepal. Studies on GLOFs in Nepal have so far concentrated on an inventory of glacier lakes, documentation of previous GLOFs and largely qualitative descriptions of glacier lake properties. The strategy towards GLOFs discussed in this paper is first to identify indicators to assess the risk of glacier lake outbursts and then to prevent outburst floods by controlled lake drainage and to make use of the water stored in glacier lakes. Several techniques for glacier lake drainage are discussed and compared with the advantages of the Hydraulic Syphon Technique (HST) which is described in this paper in terms of system design, installation, operation and constraints of operation. Necessary preconditions for the use of this technique are detailed geotechnical investigations for the glacier lakes to be stabilized and the installation of a monitoring system for water level fluctuations and possible rapid mass movements into the lake. This requires the close cooperation of experts and institutions in the fields of snow and glacier hydrology, geotechnical and civil engineering. INTRODUCTION Glacier Lake Outburst Floods (GLOFs) are amongst the most serious natural hazards in the Himalayas. Their unpredicted catastrophic occurrence, their path of destruction which can exceed 100 km downstream the outburst location (Post & Mayo, 1971) and their potential for destruction of settlements and infrastructure have been the subject of numerous papers. Most of the literature concentrates on aspects of historic GLOF events, glacier lake inventories, scientific aspects of GLOF processes and damage mitigation techniques viz: evacuation of populated areas and planning or re-location of infrastructure (roads, bridges). Since 1935, a total of 14 GLOFs have been reported, which either occurred in or extended into Nepalese territory. Most of these GLOFs are well

2 342 W. E. Grabs & J. Hanisch Table 1 Examples of recent GLOFs (Data from WECS, 1987; Vuichard & Zimmermann, 1987; Chaohai & Sharma, 1988). Year Location of GLOF Mingbho Valley Zhangzangbo Lake Langmoche Glacier Water volume released (m 3 ) 4 X x X 10 6 Max. s->) discharge (m i Reach of destruction (km) documented; however, records of GLOFs from the West, Far West and Far East of Nepal, where GLOFs are also likely to have occurred, are lacking. Only the available data from three recent GLOFs are cited (Table 1). In most papers, risk is assessed on the anticipation that an outburst will occur from a given glacier lake. Therefore, risk is assessed in terms of the magnitude of outburst floods and their possible damage downstream from the outburst location. But, with all likelihood, flood damage mitigation by relocation or reinforcement of infrastructure, river bank stabilisation, relocation of small hydel plants and even evacuation and resettlement of villages is much more costly than the application of methods to prevent outburst floods by controlled lake drainage. OBJECTIVES FOR GLOF CONTROL The overall objective for GLOF control is defined here as: Glacier Lake Outburst Floods are prevented using methods for the controlled drainage of dangerous glacier lakes. To achieve this objective, several results have to be obtained. 1. An inventory of glacier lakes is prepared and those lakes identified for further investigations which do pose a danger for downstream villages, power stations and infrastructure as well as agriculture. This inventory can be prepared using aerial photography and, where applicable, other remote sensing information. 2. A set of indicators has been agreed upon to classify GLOF risk using geomorphological, geotechnical, glaciological and hydrometeorlogical information gathered on field expeditions and quantitative on-site investigations. 3. Methods for controlled lake drainage are compared with respect to on-site feasibility, effectiveness and cost. 4. A lake monitoring system is designed and implemented. The monitoring system is used to operate the lake drainage system and the management of the lake which can be considered as a reservoir. 5. Depending on the site and downstream conditions, necessary ancillary structures for the use of the lake water are designed.

3 Prevention of glacier outburst floods Upon the method chosen, the site investigations are completed, the lake drainage designed and implemented. The activities which are necessary to implement a lake drainage project can only be outlined here in a general way; priority is given to discussion of conventional drainage methods and the presentation of a new technique: the introduction of the Hydraulic Syphon Technique as a method to prevent GLOFs. TYPES AND STABILITY OF GLACIER LAKES The assessment of the stability of glacier lakes is an important criterion for the selection of glacier lakes where flood mitigation and/or outburst prevention techniques are to be executed with priority. Characteristically, glacier dammed lakes can be classified as moraine dammed (with or without ice-core) and glacier dammed lakes - dammed either by active glacier masses or debris-covered icemasses with or without contact with the active part of the glacier. In reality, many combinations are observed. The geomorphological setting of glacier lakes and their hydrological behaviour are important general indicators for glacier lake stability. Glacier lakes, which occupy old glacier tongue basins of retreating glaciers, through valley glaciers and cirque glaciers with old, stabilized end-moraines formed during the Pleistocene period are relatively stable and outbursts less likely. On the other hand, glacier lakes which are in contact with an active glacier and which are dammed by unconsolidated moraines resulting from periods of recent glacier advances in the last 300 years, especially the last "Little Ice-Age" are potentially unstable and more prone for outbursts. These recent moraines have usually steep slope angles and, especially the lateral moraines, slope angles that may exceed the theoretical internal friction value of the moraine materials. The recent terminal moraines have mostly narrow crests and may contain ice lenses or cores. MECHANISMS OF OUTBURSTS FROM GLACIER LAKES The mechanisms that lead to an outburst flood can be divided into: (a) the triggering event, and (b) the mechanism of dam failure. Triggering events By far the most common trigger event for a GLOF is a surge wave caused by mass movements like large ice falls, avalanches, slope failures of lateral moraines, rock falls or debris flows into the glacier lake. This surge wave leads

4 344 W. E. Grabs & J. Hanisch to the dam failure. The Sino-Nepalese investigation of glacier lake outburst floods (Chaohai & Sharma, 1988) has stated that bursts of glacier lakes in the Himalayas are mainly caused by the sudden collapse of glacier tongues which result in the instantaneous release of large ice masses into the glacier lake with resultant surge waves and lake overflows. Other triggering events are: overtopping of the dam crest due to lake overflow, intense seepage and piping as a result of a rise in lake water level or melting of stagnant ice lenses in the moraine body. It is apparent that triggering events are related to the hydrometeorlogical conditions in the area: seasonal rise in temperature and high radiation which increase the melt rate and thus the lake water level. Meltwater also affects the pore water content of unstable masses that may cause slope failures. Meltwater inflow into glacier crevasses with deep crevasse penetration lowers the plasticity of the ice and produces an ice-slide surface at the interface between the glacier and the bedrock. Mechanisms of dam failure Only mechanisms of the most common dam failures can be discussed here: site investigations and aerial photos of burst lakes tend to indicate that for most lakes, the surge waves have not directly caused the failure of the dam. Rather than this, they have overtopped the crests of the dams, deepened the existing outflow channel, and then started to erode the leeward side of the moraines. In this way, a channel is formed which, due to the steep slopes, rapidly extends backward by regressive erosion (cf. Lliboutry et al, 1977). After reaching the lake itself, a great part of the moraine collapses and will be washed away instantly: an outburst is generated and the corresponding flood starts to surge down the valley picking up increasing amounts of debris. More gradual rises of the water levels, e.g. during melting periods or glacier advances, may lead to increased seepage and piping which can weaken the dam structure and subsequently can result in a local dam failure. This has been reported from the Zhangzangbo glacier lake (XuDaoming, 1988, cit. Mool, 1992). RISK ASSESSMENT OF GLOFS Unfortunately, there exists no commonly agreed set of indicators to assess the risk of GLOFs from young, moraine dammed glacier lakes. Qualitative methods include risk assessments from aerial photography and field surveys. From empirical evidence with GLOFs in the Himalayas, a set of indicators for visual surveys of high-risk glacier lakes can be outlined: lake dammed by young, unstable and unconsolidated moraines; - lake is in contact with active ice body of a glacier;

5 Prevention of glacier outburst floods 345 glacier is advancing; glacier tongue is steep; glacier is dissected by many deep crevasses; unconsolidated, high and steep lateral moraines; evidence of general slope failures above the glacier lake; high lake water levels and strong dam seepage; evidence of recent multiple small outbursts. Qualitative risk assessment is useful for afirstapproach to the problem and a theoretical categorization of high risk lakes. As has been mentioned above, the most important indicator for a risk assessment is the stability of the dam itself. For this, detailed hydrological and geotechnical investigations are necessary. As surge waves are the most common cause for GLOFs in the Himalayas, the probable mass wastage into the lake must be estimated and the resulting height and energy of the surge wave quantified (cf. Damen, 1992). This is important to calculate the desirable water level to which the lake has to be drained so that crest overtopping by surge waves is prevented. The dam itself must be geotechnically investigated to quantify its composition (large ice bodies, rock boulders, sand and gravel lenses) to calculate its structural stability against surge wave impact, abrasion of dam crest, seepage and piping, and structural failure due to possible melting of stagnant ice bodies inside the moraine dam. A set of quantitative indicators can then be set up for high risk glacier lakes: high probability of slope or ice failures with sudden quantified mass wastage into the lake; kinetic energy and height of surge wage overtops the moraine crest; excessive seepage and piping occurs during lake water level increases; large ice bodies and/or sand/gravel lenses occur in moraine dam which are likely for melting or being washed out during excessive seepage. It is evident that the risk of GLOF is also time-dependant as melting processes, lake water level fluctuations, seepage, glacier movements etc. are varying with seasonal climatological and hydrological conditions. If a glacier lake has therefore been selected for the application of outburst prevention techniques, the lake must be continuously monitored as well as glacier observations made and the geotechnical stability of the dam monitored. The risk assessment for downstream GLOF impacts is not discussed in this paper. It is mainly a function of the volume of water discharge, the amount of morainic debris and of material picked up downstream. GLOF PREVENTION METHODS Previous methods Apart from various kinds of precautions in order to mitigate the effects of

6 346 W. E. Grabs & J. Hanisch potential GLOFs in the areas downstream of the glacial lakes, a series of methods has been proposed to prevent the outburst of GLQF-prone lakes. All these have the aim to lower the water level in the glacier lakes. The main purpose for this is not only the reduction of the water volume but to reduce the hazard of overtopping the morainic dam by surge waves. For water level lowering of glacier lakes the most common techniques are: 1. Artificial deepening of the natural spillway. This most utilized method consists of the cautions digging down of a breach which enables the water to run off. The hazard of this method is that the run-off of the water can start to erode the spillway and thus get out of control. A disastrous case has been reported by Lliboutry et al. (1977). During the lowering works a huge ice block fell into the lake. The tongue of the glacier had formed a subtle apron sustained by the buoyancy forces of the water. After the lowering, the ice block broke down and the subsequent surge waves swept over the artificial trench eroding rapidly the outside slope of the dam. The resulting GLOF devastated the downstream valley and more than 200 people were killed. 2. Blasting the moraine dam. In urgent cases, even blasting can be envisaged to cut a trench into dam. Naturally, this can only be done after a series of precautions and after the total evacuation of the downstream area. This can be an interesting solution during an early stage of glacial lake formation. 3. Inclined drilling through the moraine dam. Instead of digging a trench into the dam, the lowering of the water level can be achieved by the installation of a pipe in the moraine dam. This drilling, however, is rather problematic due to the extremely unsorted and poorly consolidated mass. Blocks of up to several metres in diameter may be encountered and the risk of piping along the casing installed during the drilling is high. To prevent this the surroundings of the pipe should be carefully injected with cement grout. The transport of such equipment to the high and remote areas of the Himalayan glaciers is difficult and expensive. 4. Driving a tunnel from an adjacent deeper lying valley (Norwegian Method). This very elaborate and safe method has been successfully used, especially in Norway. If the morphology of the surroundings of the glacier lake are appropriate such a solution may be appropriate. In remote areas like the Himalaya with missing infrastructural requirements, however, this system is prohibitively costly. In Norway, the tunnels are lined with concrete and used as race tunnels to existing hydropower systems. The hydraulic syphon technique (HST) All methods and techniques mentioned above suffer from unpredictable risks,

7 Prevention of glacier outburst floods 347 are too expensive or are too complicated for the harsh Himalayan conditions. Any optimized method should, therefore, cover the following requirements: preservation of the physical and structural integrity of the moraine dam, thus minimizing the GLOF risk; simplicity and robustness in design and operation; independence from permanent power supply; low installation and operating costs. The well-known hydraulic syphon method, used in many fields of hydraulics, seems to fulfil perfectly these requirements and combines their advantages. Physical principles and constraints of HST A hydraulic syphon takes advantage of the difference of a hydraulic potential between the inlet and the outlet of a water-filled pipe. It is able to suck water to a certain vertex from where it runs to the deeper lying outlet (Fig. 1). The maximum possible suction head is a function of the following factors: atmospheric pressure (dependent on the altitude, temperature, and its variability due to météorologie processes); vapour pressure of the water (dependent on the temperature); specific weight of the water (dependent on the temperature and on the latitude and altitude of the site); internal friction of the pipe system. Fig. 1 Schematic diagram of the hydraulic syphon for GLOF hazard mitigation.

8 348 W. E. Grabs & J. Hanisch Technical principles and design of a HST system Most of the Himalayan glacier lakes are situated at levels between 4200 and 5000 m. Therefore, in the appendix, an example is given assuming an altitude of 4500 m. Considering all known factors, a theoretic suction head of 5.5 to 5.75 m for this altitude is the result, depending on the air temperature. Since most of the melting water enters the glacier lake during the warm season, the latter value should be taken for the technical design. Including the factor of atmospheric pressure changes of +10%, a lowering of a lake under the above mentioned conditions by about 5 m from its spill point level seems to be realistic. Before any further steps of technical design and installation, this value should be tested on each specific site. For this a simple flexible tube can be used; it must be filled with water, closed at both ends and then laid across the crest of the dam near the spill point. The upper end is put into the lake to the calculated depth and the other end situated several metres below the inlet level. Provided that the tube is free of air and no cavitation occurs due to too high suction heads, the water will flow out of the lower pipe end once the caps have been removed. To adapt the HST system to a specific site, some more factors must be considered which influence the design and the desired lowering of water level. Among these, the possible height of surge waves originating from ice or rock fall is one of the most important. Installation and operation of the HST systems The details of the technical design of the proposed HST method are not within the scope of this paper. For this, a close collaboration of engineering geologists, civil engineers and experts in snow and glacier hydrology must be sought. Nevertheless, some principles are discussed here. The dimensioning of the pipe system depends on the volume of water desired to be drained from the lake and has its limits in the possible effects of air intake in the outlet, and the resulting cavitation. A major problem, at first view, seems to be the filling of the system. Conventional suction of the pipe has to be ruled out because of the required large diameter. Therefore, a special filling system with two valves at the ends of the tube is proposed. A separate small water pump would fill the tube after closing both ends. A special ventilation valve installed at the vertex of the system would enable the air to escape totally. After closing thefillinginlet and ventilation valve, the two major valves are opened and the water column will start draining the water from the lake. To prevent hazardous erosion of the moraine material, the site of the outlet of the tube must be selected carefully; it must lie outside of the morainic dam, best discharging the water on solid sound rock or a specially reinforced concrete apron (Fig. 1). The amount of drainage water can be controlled by the outlet valve. In this way, the desired lowering of the lake level can be achieved and the system can

9 Prevention of glacier outburst floods 349 be closed before the critical suction height is reached. If this should happen, the tube would empty and the system would have to be filled again for renewed operation. On the other hand, the possibility to control the outflow gives the opportunity to adapt the discharge volume to the actual inflow of melting water or rainfall. For reasons of freezing during winter time, the tubes must be resistant enough to withstand forces from ice expansion and ice drift. Since in the Himalaya the winter coincides with low inflow into the lake, no dangerous increase in lake water level it expected. Therefore, no operation during this period is required and the system can be emptied. Aspects of lake drainage and system monitoring As has been outlined above, the water level of a glacier lake should be lowered only to a predetermined safe level to prevent overtopping and bursting by surge waves. A fast lowering of the lake water level or a complete drainage of the lake is not desirable. Especially the lateral moraines are highly instable. If the water level sinks too fast, the pore water pressure in the moraines may provoke massive slope failure. The masses sliding into the lake can cause dangerous surge waves leading to the collapse of the glacier tongue as a result of too fast or excessive draining (see case study in Lliboutry, 1977). From an economic point of view, the complete drainage of a lake should not be attempted: aside from technical constraints to drain a lake completely, the cost-benefit ratio of lake drainage may be negative if a lake is drained much below the determined safety level. Also, the lake may serve as a reservoir for other uses of the stored water as described below. Ecologically, the continuous lake discharge is important for the local water balance as it buffers seasonal discharge fluctuations. It thus maintains a steady flow which serves as a water source for downstream settlements, field irrigation and livestock. Both the safety aspect of glacier lake drainage and the maintenance of the local water balance require a monitoring of the glacier lake and its environment before and during the lake drainage. The monitoring system must include at least the following observations and calculations: System monitoring (observations and calculations) Slope or glacier tongue failures; drainage Seepage and piping processes (with sediment observations) System control (activities) Drainage may be too fast: slow down lake drainage Assess moraine dam stability; if necessary, increase lake drainage velocity Meteorological observations Basic data for drainage control on the basis of (precipitation, temperature, radiation, lake inflow and operation constraints of the pressure) HS-Technique (Atmospheric pressure)

10 350 W. E. Grabs & J. Hanisch Calculation of meltrate of snowfields Forecast of lake inflow and outflow; and glaciers adjustment of drainage velocity to lower or maintain lake level Hydrological observations (lake water level fluctuations, lake discharge, natural and artificial) Calculation of lake water balance Basic data for drainage control and verification of meltrate calculations; principal reservoir management tool; optimization of reservoir operation Overall reservoir monitoring especially, when use of lake water is desired UTILIZATION OF DRAINAGE WATEE Once the glacier lake has been stabilized against GLOF by controlled drainage, the use of the remaining lake water may be considered. If there is still a difference between the necessary draw-down and the maximum possible suction head, this water volume can be used as a seasonal reservoir. Once the safe water level has been reached, the water flowing into the lake can also be utilized. Possible uses are: micro-hydel generation, irrigation, drinking water supply for people and livestock. The anticipated use of the water will also influence the dimensioning of the pipes and the regulating system to control the drainage volume. CONCLUSION The improved management of water resources in the Nepal Himalaya must also consider the risks associated with GLOFs. Glacier lake inventories show that there are numerous potentially dangerous lakes in Nepal where a GLOF seems highly probable. There is an acute danger for downstream villages and infrastructure, considering that GLOFs can reach more than 100 km downstream of the outburst site. To ward off GLOF hazards, the controlled drainage of dangerous glacier lakes is proposed. A set of indicators is introduced to assess the risk of moraine dam failure by surge waves. The implementation of classical methods of lake drainage in the Nepal Himalaya is burdened by unpredictable risks, high costs or simply being not feasible in the high altitude, remote regions. Therefore, a new concept is proposed for a controlled continuous glacier lake drainage without the danger of moraine dam destabilization. Its main element is the well-known hydraulic syphon technique which can be adapted to the specific conditions of the high Himalaya. The lowering of lake water levels in the magnitude of 5 m seems to be possible. This can be sufficient to stabilize glacier lakes against outbursts as a result of surge wave action. The hydraulic syphon can be installed relatively easily and does not require permanent power supply for its operation. It must be

11 Prevention of glacier outburst floods 351 emphasized that the proposals made in this paper can not be a blue-print for the design and installation of such a drainage system; rather, they give new impulses to mitigate GLOF hazards and focus the discussion on GLOF prevention instead of on damage control of GLOFs. For specific site conditions, the use of the drainage water, e.g., for microhydel production may be considered. For the detailed design, testing, installation, operation and monitoring of the hydraulic syphon, the close collaboration of engineering geologists, civil engineers and experts in snow and glacier hydrology is indispensable. Acknowledgements Thanks go foremost to the Nepalese staff of WECS and their Japanese counterparts, because they laid the foundation for GLOF research in Nepal. The field expeditions of the Nepalese-Sino team to selected river basins represent also a landmark in GLOF research in the Himalaya. The work of Dr. Damen on the Tsho Rolpa lake in the Rolwaling and the recommendations made are also highly commended. All these investigations have, together with the authors of the literature cited, contributed to the proposals made in this paper. REFERENCES Chaohai, Liu & Sharma, C. K. (eds) (1988) Report on First Expedition to Glaciers and Glacier Lakes in the Pumqu (Arun) and Poiqu (Bhote-Sun Kosi) River Basins, Xizang (Tibet). China Science Press, Peking. Damen, M. (1992) Study on the potential outburst flooding of Tsho Rolpa glacier lake, Rolwaling valley, East Nepal. ITC Draft Report Enschede, The Netherlands, 58 pp. Galay, V. (1987) Erosion and sedimentation in the Nepal Himalaya an assessment of river processes. WECS Report, Kathmandu, Nepal. Ives, J. D. (1986) Glacial lake outburst floods and risk engineering in the Himalaya. ICIMOD Occasional Paper No. 5. Kathmandu, Nepal, 42 pp. Lliboutry, L., Arnao, B. M., Pantre, A. & Schneider, B. (1977) Glaciological problems set in the control of dangerous lakes in Cordillera Blanca, Peru. Part I: Historical failures of morainic dams, their causes and prevention. /. Glaciol. 18, Post, A. & Mayo, L. R. (1971) Glacier dammed lakes and outburst floods in Alaska. US Geological Survey Hydrologie Investigation Atlas HA-455, SGHP ( ) Yearbooks of the Snow and Glacier Hydrology Project. Department of Meteorology (DHM/HMG), Kathmandu, Nepal. Vuichard, D. & Zimmermann, M. (1987) The 1985 catastrophic drainage of a moraine-dammed lake, Khumbu Himal, Nepal cause and consequences. Mc untain Research and Development 7(2), WECS (1986) Preliminary study of glacier lake outburst floods (GLOFs) in the Nepal Himalaya: Phase 1 interim report. WECS Report Kathmandu, Nepal. WECS (1987) Dudh Kosi river 1985 GLOF study survey report. WECS Report Kathmandu, Nepal. Xu Daoming (1985) Characteristics of debris flows caused by outbursts of glacial lake in Boqu river in Xizang, China, Lanzhou Inst. Glaciol. Cryopedology, Acad. Sinica, 24 pp.

12 352 W. E. Grabs & J. Hanisch APPENDIX Calculation of the possible suction head of a site with the following parameters (according to charts of physical constants and data from SGHP Yearbooks ): Geographic latitude <j> = 28 Altitude h = 4500 m Atmospheric pressure Pa = to Pa (-17 to +6 C) ±10% Water temperature t = 4 C Vapour pressure at 4 C Pv = 839 Pa Density of the water = 1000 kg m" 3 Local gravity g(28 ) = (g(0 ) X 10" 6 X h) m s" 2 = m s" 2 Loss by friction f = 5 % (estimated) The formula for the maximum suction head is z = Z^(l-f) Pg 57.5Q0Pa-839Pa 1000 kg nr 3 x m s" 2 (Pa = N m" 2 ; N = kg m s" 2 ) xq95 z = kg m s' 2 m' 2 xq kg m" 3 x m s" 2 z = 5.51 m ± 10% (fort = -17 C) z = 5.75 m ± 10% (for t = +6 C)