Climate Change and Water Resources in South Asia - Chapter 6 ppt

6.2 GLOF HYDROLOGY As implicit from the above discussion, GLOF is a catastrophic discharge of water from the glacier lakes due to failure or breach of ice or moraine dam formed at the en

Climate Change and Glacier Lake Outburst

Floods and the Associated Vulnerability in

Nepal and Bhutan

6.1INTRODUCTION

Natural disasters have become a part of the worldwide spectacle of a globalize media The Glacier Lake Outburst Floods (GLOF) induced disaster has also become a part of it, for its impact and risk on the people and economy of the mountain cannot be in no way underestimated The GLOF events in the Himalayas have been occurring since long as evidenced by many landform features downstream For example the GLOF event of the Barun Khola in Nepal was not known, but the accumulation of debris along the river valley

is an indication GLOF event Similarly, the debris accumulation in Pokhara Valley gives clues to the GLOF event some 450 years ago in the Seti due to collapse of moraine of a glacier lake in Machhapuchhare range in the Himalayas In Bhutan, several evidences also show that GLOF event have been the common phenomenon, although the past events are not recorded in the modern chronicle or many events are unknown to people A Swiss, Geologist Augusto Gansser, during his expedition to Bhutan Himalayas in the 1960s and 1970s had an opinion about the 1957 Punakha flood He felt that it was due to an outburst

from Taraina Tso in Western Lunana (Gansser, 1970 quoted in Mool et al., 2001).

A GLOF event could be traced back to the 1935 event in Sun Kosi Basin which destroyed cultivated land and livestock in Nepal Several catastrophic GLOF events that originated in China or Nepal in 1964, 1977, and 1980 (Yamada, 1993) were also experienced, but these events were not considered seriously A GLOF event of 1981 in the Boqu River (Sun Koshi) in China was one that slightly raised the brows of development planners and policy makers, for it had destroyed a large section of the China-Nepal road as well as the Friendship Bridge, and impacted 30 km downstream in Nepal But it was in

1984 an outburst of glacier lake (Dig Tsho, below Langmoche Glacier in Khumbu) that caused a severe disaster to lives and property downstream has strongly revealed the stakeholders its disaster potential The lake was drained suddenly and sent a 10 m to 15 m high surge of water and debris down the Bhote Koshi and Dudh Koshi Rivers, for more than 90 km An estimated 1 million m3 of water was released, creating an initial peak discharge of 2,000 m3/sec; two to four times the magnitude of maximum floods due to heavy monsoon rains This spectacular natural event destroyed the nearly completed Namche Small Hydel worth NRs 40 million It eliminated all the bridges, for 42 km downstream;

four or five people lost their lives (Fushimi et al., 1985; Galey, 1985; Ives, 1986).

MOTILAL GHIMIRE

GLOF events in Bhutan are also common due to similar Himalayan environment as

in Nepal Awareness about its potential threat goes back to 1970s as evidenced by a brief study done by a joint team of experts from Indo-Bhutan on possible outburst of a glacier lake in Lunana region Recently, Bhutan experienced a GLOF event on October 7, 1994, the event that partially bursted from Lugge Tsho located in Eastern Lunana (Watanabe and Rothacher, 1996) It had incurred loss of lives and huge property along Punakha Wangdue Valley Since then concern about GLOF and its threat to development effort seemed to scale up among the stakeholders which is indicated by consequent several studies and

mitigation effort that were carried out (Mool et al., 2001).

6.2 GLOF HYDROLOGY

As implicit from the above discussion, GLOF is a catastrophic discharge of water from the glacier lakes due to failure or breach of ice or moraine dam formed at the end of these lakes that may cause a huge disaster Mool (1993) defined as “the flood due to sudden bursting

of glacier lakes which are ice dammed or moraine dammed” It is a phenomenon of glacier lakes, a ponding of glacier melt water in depression area of glaciers surrounded by the lateral and end moraines or may formed at the side of lateral moraine of the extended glaciers due to interception of the tributaries by its lateral moraine (Yamada, 1993) The

first type of glacier lakes, are called moraine dammed glacier lake, while the second type

of glacier lakes are referred as ice dammed glacier lakes Almost all types of glacier lakes

in Nepal are moraine dammed, it is because of the fact that the Himalayan glaciers produce very rich debris that make relatively large lateral and end moraine compared to others glaciers in the world Ice dammed lakes are very rare, and are considered less dangerous than a moraine dammed lake According to Yamada (1993), apart from the above reasons,

a pond may be formed on, in or under glacier under some critical condition; such glacier lakes might be negligible to consider being flood hazard But such small ponds called

“supra glacier” formed within a glacier may eventually grow and connect each other to form a large lake which might be potentially dangerous A history of past GLOF events of moraine dammed lakes indicates that they are initially derived from supra glacier lakes

(Mool et al., 2001).

Glacier lakes may not remain as they are all the time, a glacier lake may disappear, once the dam is destroyed or sedimentation fills the lake completely They are formed and maintained in certain stages of glacier activities corresponding to climatic variation (Yamada, 1993) One theory states that during the so-called “Little Ice Age” which lasted until 1850, glaciers were extensive and due to gradual change in climate since mid 19th century, majority of mountain glaciers has been thinning and retreating which resulted in the formation of glacier lakes behind the end moraines of these retreated glaciers (Röthlisberger and Geyh, 1985) The recent global warming phenomenon have ushered scientist to consider the expansion of glacier lakes in recent times and their outburst as an affect of warming trend

Glacier lakes formation and their outburst are conducive in the High Mountain of Tropics and Sub-Tropics in the Himalayas and Andes since snow in these areas is very sensitive to small change in temperature Any small rise in temperature would cause a retreat of glacier and enhances the formation of glacier lakes at the earlier toe of the glacier behind the end moraine dam It is reported that glaciers in the Himalayas are in the order of

10 km-25 km in length, generally are longer than in the Alps, and have a relatively flat longitudinal profile in their lower part Often they have a large end moraine, which enhances the formation of lakes behind these dams during climatic changes The gradual

rise in the glacier lake may lead to glacier lake outburst flood by over powering the dam due to increasing water pressure The rises in water level in the glacier lakes are generally

attributed to (Mool et al., 2001):

• Warming of temperature,

• Intensive precipitation events,

• Decrease in seepage across moraine due to sedimentation,

• Blocking of an outlet by an advancing tributary glacier,

• Melting of ice-core moraine wall or subterranean thermal activities,

• Inter-basin sub-surface flow of water from one lake to another lake due to height difference and availability of flow path, and

• Others local specificities

It is quite clear that a climate change and variability is one of the causes of rise in water level in the lakes There are many hypotheses about the bursting mechanism which are presented in the chart in Figure 6.1 (WECS, 1994)

6.3 STUDIES ABOUT GLACIER LAKES AND THEIR OUTBURST EVENTS IN

NEPAL AND BHUTAN

Works on glacier inventory in Nepal began in the late 60s, the initiation was made by the Japanese team (Japanese Glacier Research Group (1968-1973) and Glaciological

Expedition Nepal: GEN (1973-1974) (Higuchi et al., 1978) But these studies virtually did

not describe about glacier lakes and their outburst events Some historical data on glacier lake outburst data was offered by the Chinese Investigation Team (1973-1974) in its interior report (Yang, 1982 quoted in Xu and Quingua, 1994) In Nepal, the catastrophic outburst of Dig Tsho Lake in Eastern Nepal on 4 August 1985, after similar events in 1977 and 1981 (Xu, 1985; Galey, 1985; Ives, 1986) made outburst events serious disaster and environmental issue to national and international community as well This concern

heralded a series of studies on glacier lakes in Nepal Fushimi et al (1985); Galey (1985);

Xu (1985); Vuichard and Zimmerman (1986, 1987); and Ives (1986) highlighted on the past or recent GLOF events in Nepal and Tibet and their threat to people and infrastructures at downstream Government agencies like Water and Energy Commission Secretariat, Nepal Electricity Authority and Chinese counterpart, Lanzhou Institute of Glaciology and Geocrylogy (1988) made a preliminary assessment of glaciers and glacier lakes in the Pumqu (Arun) and Pioque (Bhote Kosi) River basins in both China and Nepal

It was a first step for Nepal to join the research of GLOF In 1990 and 1991, with support from Japan International Cooperation Agency (JICA), WECs have carried out several inventories of glacier lakes in the Arun, Honku Drangka, Hinku Drangka, Dudh Koshi, Lantang Khola, Chilime, and Marsyangdi Basins through flight observation The flight observation report recommended detailed examination of the dangerous glacier lakes by site visit As a result, several lakes such as Lower Barun, Imja, Thulagi, Dig Tsho, and Tam Pokhari glacier lakes were studied The general features of these potentially dangerous lakes are presented in Table 6.1

Much of the studies in the later years were carried on these lakes by (semi)-government institutes including professional consultancies, individual, and students For instance, the description about the Imja Lake in Khumbu region is found in the studies

of Hammond (1988), Yamada (1993), Watanabe et al (1994), Watanabe et al (1995),

Kettelmann and Watanabe (1998) From the study of topographic maps, aerial photos, and

MOTILAL GHIMIRE 139

Causes of Outburst For GLOFs

Tunnel

under ice

caused by upper glacier calving, ice fall or rock fall

Ice melts

glacier

retreats

Over topping caused by upper glacier calving, ice fall or rock fall

Ice core melts resulting in piping

Over topping caused by upper glacier calving, ice fall or rock fall

Fig 6.1 Process of Glacier Lake Outburst Floods (GLOFS).

Table 6.1 Some features of studied glacier lakes in the Nepal Himalayas

Source: Mool et al., 2001.

Barun

Pokhari

Depth (m)

Average

Maximum

-

50

118

- 47.0

99

- 55.1

131

- 41.8

81

-

20

-

45 left after GLOF

the imageries, the development of Imja Glaciers have been reconstructed (Yamada, 1993) The lake has increased in size from 0.03 km2-0.60 km2 during 1955-1992 (Fig 6.2) A recent study warns this lake to be potentially dangerous as it is in contact with the tongue of

glacier (Mool et al., 2001) which is likely to increase water volume and pressure, and trigger

lake outburst

Fig 6.2 Glacier lake development process.

About Tsho Rolpa Glacier Lake’s geomorphology, lake development process, hydro-meteorology, and hazard assessment could be found in the studies of Damen (1992), Modder and van Olden (1995, 1996a, 1996b, and 1996c), WECS (1993a), Reynolds

Geosciences Ltd (1994), Mool (1995a), Budhathoki et al (1996), Chikita et al (1997),

Yamada (1993, 1998), DHM (1997c, 1998b, and 2000) Monitoring the development process of Tsho Rolpa Lake from the study of maps, aerial photos and imageries it has been reported that during 1957-1992 period the lake has increased from 0.23 km2 to 1.37 km2 (WECS, 1993a) and by 2000 the lake has grown to 1.55 km2 (Mool et al., 2001).

Studies in Lower Barun Glacier Lakes were done by WECS (1993a, 1997), and NEA (1995) WECS (1993a) recommended that the lake is increasing and is associated with larger mother glacier So any project downstream of Lower Barun Glacier Lake requires detail investigation of the lake and downstream valley

Similarly, WECS (1995c), DHM(1997c), Hanisch et al (1998) had studied Thulagi

Glacier WECS (1995c) study reveals the gradual increase of lake during the last 45 years; comparing the maps of 1958 and field work in 1992, it revealed that the lake has increased from 0.22 km2-0.76 km2 and the glacier has retreated by 1.37 km within the last three decades However, the study by DHM (1997b) sees no danger from the lake in foreseeable future because it is dammed by extended ice bodies which can neither be rapidly breached

by lake water pressure or by erosional forces of river It can only be removed by large scale melting of ice core which requires a period of hundreds to thousand years

Imja Glacier Lake, 1955-63

(Yamada, 1993)

a 1955-63

f 1979

October 1988 (MOS-I) 1956-58

(Toposheet): 1968

1967 (Gansser, 1970)

1989-90 (DGM, 1996I)

December 1994 (Spot 3)

December 1993 (SPOT XS)

a 1957-59

b 1960-68 g 1983-84

h 1988-90

i 1993

j 1999

k 2000

1.40 km 2

1.55 km 2

1.37 km 2

1.27 km 2

1.16 km 2

c 1972

d 1974

e 1975-77

0.80 km 2

0.78 km 2

0.62 km 2

0.61 km 2

b 1975

c 1984

c 1992

4 km

Tsho Rolpa Lake, Rolwaling Nepal, 1957-2000 (WECs, 1993; Mool et al 2001)

Expansion of Raphstreng Tsho Glacier Lake from 1956-1994 (Ageta et al 1999)

The outburst of Dig Tsho Lake in 1985 and the accompanying damage set in train a lot

of field investigations to understand the glacier lake morphology and outburst mechanism (Galey, 1985; Ives, 1986; Vuichard and Zimmerman, 1986, 1987; WECS, 1987b) After outburst the lake is considered to be safe as it has been drained completely (WECS, 1987b)

However, Mool et al (2001) argues the reappearance of lake at the tongue to the glacier

poses concern and therefore, surrounding moraine and the activity of the lake should be

studied in detail Dwivedi et al (1999) reported about the bursting mechanism, discharge

of water volume and the loss/damage caused by Tam Pokhari Glacier Lake Mool et al.

(2001), compared the lakes and interpreted from the topographic maps of 1963, satellite imagery of 1992-1993, and the topographic maps of 1996 (based on 1992 aerial photo) that the lake area had increased from 0.138 km2 to 0.472 km2

In Bhutan, the first glacier expedition was briefly made in 1960s by Gansser (1970)

He identified a number of dangerous lakes, which could flood in the lower valleys He attributed 1957 flood in Punakha Wangdi Valley to the outburst from Tarina Tsho, Western Lunana In 1970s and 1980s, joint study team of Geological Survey of Bhutan (GSB) and the Geological Survey of India (GSI) carried out several investigations to assess hazard and socio-economic risk of glacier lakes in Lunana area These studies concluded that there was no danger of outburst of Lunana Lake in the near future but recommended periodic checks every 2 or 3 years due to presence of ice cores in the moraine dams After the partial outburst of Lugge Tsho located in Eastern Lunana which has affected life and damaged property along the Punakha - Wangdue Valley (Watanabe and Rothacher, 1996) Some government agencies of Bhutan carried out research on cause and effect of outburst and to recommend short- and long-term mitigation measures (Dorji, 1996a, 1996b; National Environment Commission, 1996)

Meanwhile, in 1996 after the many years gap of first glacier inventory, Phuntso Norbu, Division of geology and mines prepared an inventory of glaciers and glacier lakes which was edited and updated by Geological Survey of Bhutan (1999) On the basis of these

studies, expansion of glacier lakes were reported by Ageta et al (1999) (quoted in Mool et al., 2001; Karma et al., 2003) For instance, the area of Rapstreng Tsho Glacier

Lake was 0.15 km2 in 1960s, in 1986 the lake was 1.65 km long and 0.96 km wide and

80 m deep and in 1995 the lake had the maximum length of 1.94 km, width 1.13 km and the depth of 107 km (Fig 6.2)

Most recently in 2001, international institutes like ICIMOD and UNEP came up with inventory of glaciers and glacier lakes covering the entire part of Nepal and Bhutan from the study of topographic maps, aerial photos, satellite imagery and literature available

(Mool et al., 2001) The study made total inventory of 3,252 glaciers with total area of

5,332.89 km2 These glaciers contain 2,323 nos of glacier lakes with total area of 75.70 km2 in Nepal Out of them, 20 have been identified as potentially dangerous in Nepal (Fig 6.3) Likewise, the study found 677 glaciers with total area of 1,316.71 km2 in whole of Bhutan The ice reserve has been estimated to be 127.25 km2 The glacier lakes have been identified in the numbers of 2,674; out of them 24 lakes have been identified as potentially dangerous (Fig 6.4)

To sum up the past researches on glacier lakes were mainly focused on the following aspects:

• Inventory of glacier lakes based on topographic maps, aerial photographs, satellite images, flight observation and field data,

• Assessment of cause and impact of the recent GLOF events and the possible outburst of glacier lakes,

MOTILAL GHIMIRE 143

• Bathymetric mapping,

• Hydro-meteorological conditions using field instrumentation,

• Glaciological condition and geomorphologic analysis of moraine dams, and

• Risk of GLOF events to proposed hydropower projects

Fig 6.3 Potentially dangerous lakes of Nepal.

6.4 GLOF EVENTS’ IMPACT, VULNERABILITY AND ADAPTATION

GLOF events in the Himalayas not only signify the damage or disaster from flood, but in recent times these events correlate with glacier retreat which again proximate the global warming trend The glacier retreat implies a serious concern for water availability for household, agriculture, power and industry for 400 millions living in downstream over a great Indo-Gangetic and Brahmaputra Plain The water demand for agriculture, industry and urban sector in Nepal, India and Bangladesh is progressively growing and a decline in snow cover would mean a condition of water deficit which a serious threat to food security, energy availability and industry In the High and Trans-Himalaya region the decline in snow cover would cause serious impact to mountain ecosystem and the livelihood base of the local people which based on snowmelt water fed agriculture and pasture for livestock grazing

A = Nagma Pokhari (Tamor); B = (unnamed) (Tamor); C = Lower Barun (Arun);

D = Lumding (Dudh Koshi); E = Imja (Dudh Koshi); F = Tam Pokhari (Dudh Koshi);

G = Dudh Pokhari (Dudh Koshi); H = (unnamed) (Dudh Koshi); I = (unnamed) (Dudh Koshi);

J = Hungu (Dudh Koshi); K = East Hungu 1 (Dudh Koshi); L = East Hungu 2 (Dudh Koshi);

M = (unnamed) (Dudh Koshi); N = West Chamjang (Dudh Koshi);O = Dig Tsho (Dudh Koshi);

P = Tsho Rolpa (Tama Koshi); Q = (unnamed) (Budhi Gandaki); R = Thulagi (M arsyangdi);

S = (unnamed) (Kali Gandaki); T = (unnamed) (Kali Gandaki)

Measured in the term of past damage and loss over the last half-century, the damage compared to other natural hazard is less and also less frequent (Table 6.2) However, the disaster potential of GLOF has increased in recent times due to growth of settlements along the river valleys, construction of motor roads, bridges, and canals at downstream About 1.56 million people live within the territory of Nepal (within 3 km from glacier fed rivers) downstream of the blocked or moraine dammed lakes (Fig 6.5) Several hydropower projects which are either in operation, or under construction or are proposed are associated to rivers that have moraine dammed lakes at their head (Table 6.3) It is argued that these moraine dammed lakes may develop into potentially dangerous lake About 5 existing, 1 under construction and 3 proposed hydropower projects are associated to the rivers that have potentially dangerous lakes at their sources within Nepal

(Yamada, 1993; Mool et al., 2001).

As discussed in the earlier section, investigation on glacier lakes and the attempt to identify potentially dangerous lake began since the last two decade Out of those identified

as potentially dangerous only on a few such lakes mitigation measures have been carried out In Nepal, Tsho Rolpa is the only glacier lake on which detailed study and mitigation measures are carried out A first lay man hazard assessment of this lake was done in 1992 (Modder and van Olden, 1995) and in 1993 a hydro-meteorological station was installed Later in 1994, a British study team made a scientific study on the assessment of the hazard

at Tsho Rolpa and recommended that the lake level should be reduced by at least 15 m over 3 to 5 years (Reynolds, 1994) It estimated that occurrence of a GLOF from Tsho Rolpa Lake, could damage up to 100 km downstream from the lake, threatening about 10,000 human lives, thousands of livestock, agricultural land, bridges, including some components of the Khimti Hydroelectric Project and other infrastructures As a result,

siphons and early warning systems were tested (Mool et al., 2001).

The first flood warning system in the country was installed in May of 1998 to warn the people living downstream from Tsho Rolpa Glacier Lake, in the potential GLOF hit area

Fig 6.4 Potentially dangerous glacier lakes of Bhutan.

MOTILAL GHIMIRE 145

China

India Potentially dangerous glacier lakes

Baun boundary

River

International boundary

N

B H U T A N

Table 6.2 Past GLOF events and their impact

Source: Yamada, 1993; Mool et al., 2001.