2015年ネパール春調査（0-1） 国際氷河学会カトマンズ会議の報告原稿

2015年ネパール春調査（0-1）

国際氷河学会カトマンズ会議の報告原稿

　「ネパール2015年春」の主な内容は、１）カトマンズ大学の講義と２）ポカラ国際山岳博物館の展示更新ですが、講義期間中には３）国際氷河学会のカトマンズ会議参加や講義後半には４）学生たちとネパール中央部のランタン・ヒマラヤ調査を行う予定です。
　まず、上記３）の３月１日～６日の国際氷河学会カトマンズ会議の報告原稿については、下記１のように、報告書への掲載は会議で発表すること、しかし発表したからといって掲載されるとは限らないという条件で、当初は１月５日が原稿の締切日になっていました。

記１

Submissions to this issue will not be contingent on presentation at the symposium, and material presented at the symposium is not necessarily affirmed as being suitable for consideration for this issue of the Annals. Participants are encouraged, however, to submit manuscripts for this Annals volume. The deadline for receiving Annals papers is 5 January 2015.

　ところが、年末年始のホリデー・シーズンで原稿を提出できない人が多かったため、下記２のように、締切日が１月５日から１月20日へ延期されました。

記２

We are eager to maintain the pace of the publication process, but we acknowledge that the holiday season has created difficulties for many would-be authors.　We would like inform you that the deadline of Monday 5 January 2015 for submissions to Annals issue 71 is therefore extended hereby to Tuesday 20 January.

　この延期は、ぼくにとって幸運でした。といいますのは、（個人的なことですが）狭心症が昨年末18年ぶりに再発しましたので、１月初めに心臓カテーテル治療をうけ、右冠動脈４ヶ所に血管をひろげるステントを埋めこみました。そのため、狭心症再発・入院治療のため、原稿の執筆を諦めていましたが、治療後、体調はかなり回復しましたので、原稿をまとめることができ、おかげて、新たな締切日ぎりぎりにロンドンの国際氷河学会に下記３の原稿をネットで送ることができました。
　ただ、左冠動脈にも狭いところがありますので、２月はじめに再治療をうけます。したがって、当初計画の１月末のネパール出発を１ヶ月延期することにし、カトマンズ大学の講義期間は当初の４ヶ月から３ヶ月に短くなりました。

記３

Why is a large glacial lake safe against GLOF?

Hiroji Fushimi

Abstract

What is the higher risk of the GLOF? It is not a large glacial lake, but a small glacial lake. Due to the outlet erosion at the end moraine of the large glacial lake since the 16th glacial advance, the lake level has been lowering and reducing the GLOF risk. The end moraine structures of Tulagi and Imja glacial lakes are wide and strong enough to prevent the occurrence of the GLOF that is completely different from the narrow moraine of the small glacial lake with steep cliff at the upper part of the lake producing avalanches and rock falls directly dropping into the lake causing Tsunami to destroy the fragile ice-cored moraine. So, large glacial lakes such as the Tulagi and the Imja are safe against GLOF, however we must be very careful about the small glacial lakes, for example, developing in the Hong Khola around Mt. Chamlang which have steep cliff to cause avalanches and rock falls that create a Tsunami destroying the end moraine and forming the GLOF, so they must be taken to mitigate against GLOF.

Introduction

Since 1970, many glaciers are receding and glacial lakes are expanding in the Khumbu region of east Nepal, and the Mingbo　(Nare) GLOF  (Glacial Lake Outburst Flood)　occurred in 1977 (Fushimi and others,1985). After that, the　Lagmoche (Digtso)  GLOF in 1985 and the Saboi GLOF in 1998 were occurred in the Khumbu region and ａｌｌ of these valleys eroded by GLOF are clearly seen even in the satellite image (Fig.1). The reported GLOFs occurred in the region are smaller glacial lakes which area are less than 1 km² and no larger glacial lakes such as Glaciers Imja and Tso Rolpa (Fig.1) showed any GLOF phenomena. Why is a large glacial lake safe against GLOF in Nepal Himalaya?

Fig.1  GLOF map of the Khunbu region, east Nepal. Yellow circle shows the GLOF small lakes, red dashed line the large lakes and yellow dashed line the small lakes wit the higher risk of future GLOF. Aｌｌ of these valleys eroded by GLOF are clearly seen even in the satellite image.

Fig.2  1977 Mingbo GLOF. In the left photo, Mingbo valley eroded, ice cored moraine and newly formed lake, and in the right figures, a map of  the vacant lake and Dudh Kosi hydrograph are shown.

GLOF characteristics of the small glacial lakes

The Mingbo glacial lake was located in the south of Mt. Ama Dablam, Khumbu region (Fig. 1and 2). On 3 Sept., 1977, the lake caused the GLOF when we had been making glaciological surveys in the region. So, we went up along the eroded Minbo valley caused by the GLOF (Left photo in Fig.2) and observed the vacant lake (Upper right in Fig.2) 300 m long, 200 m wide and 30 m deep. There was an ice-cored moraine (Left photo in Fig.2) and the terminal moraine was destroyed (Upper right in Fig.2). Due to the large amount of debris caused by the GLOF, a new lake was formed in the Imja Khola near Pangboche (Lower left photo in Fig.2), while the river level of Dudh Kosi raised abruptly about 1 m at Raswa Hydrological Station (Lower right in Fig.2). So, the GLOF disasters, such as destruction of roads, bridges and houses near the river bed, were occurred along Dudh Kosi (river).

Fig.3   Lagmoche (Digtso) glacial lake near Thame in the western part of the Khumbu region after the 1985 GLOF. In the right photos, end-moraine was violently destroyed at the GLOF and the GLOF disaster was reported along the Dudh Kosi (river).

Fig.4   Hink Khola and Saboi glacial lake before the 1998 GLOF in the left photo. A steep clif in the upper part of the lake before (Lower right photo) and after the GLOF (Upper right phot).

The　Lagmoche (Digtso) glacial lake (Fig.1) near Thame in the western part of the Khumbu region has the steep cliff (Fig. 3) causing avalanches and rock falls directly into the lake and huge waves (Tsunami) possibly destroyed  the end moraine (Upper right photo in Fig.3)and villages along the river (Lower right photo in Fig.3) by the 1985 GLOF. The Saboi glacial lake (Fig. 1 and 4) is in the south-eastern part of the Khumbu region has also the steep cliff in the upper part of the lake (Right photos in Fig.3) was thought to have destroyed the end moraine by a kind of Tsunami due to the avalanche or rock fall from the steep cliff at the time of the 1998 GLOF.

Fig.5  Gaptse glacial lake is located at the south face of Mt. Annapurna. Gapche Glacier is fed by avalanche which causes a large wave called  “Tsunami” to form GLOF that happened in 2003, 2005 and 2009.

The Gaptse glacial lake (Fig, 5) is located at the south face of Mt. AnnapurnaⅡ in the central Nepal Himalaya. It is reported that the lake caused the GLOF in 2003 and 2004, “however the field verification was not done regarding the nature of glaciated region” (Manoj and others, 2005). So, the field observation was carried out in May 2012 and we found that it is the lowest glacial lake with altitude of 2,500 m a.s.l. in Nepal Himalaya and there is a quite large cliff, which altitude difference is about 4000 m, in the upper part of the glacial lake. We noticed frequent avalanches from Mt. AnnapurnaⅡand Lamjung Himal (Upper right photo in Fig. 5), and such avalanches and rock falls create a Tsunami to cause GLOF when they fall directly into the lake. The GLOF disaster occurred along the Madi Khola in 2003, 2005 and 2009 according to the local residents.

Moraine structure of the large glacial lakes

   The Tulagi glacial lake is located at the upper part of the Dana Khola, one of the Marshangdi’s tributary river in the west of Mt. Manaslu and Mt. Peak 29 (Fig. 6 and 7). 

Fig.6   The Tulagi glacial lake is located at the upper part of the Dana Khola (river), one of the Marshangdi’s tributary river in the west of Mt. Manaslu and Mt. P29. There is a huge debris coverd glacier in the upper part of the lake.

   The changes of the Tulagi glacial lake and its glacier terminus were determined by air photos and field surveys. The lake was 0.5 km wide, 3 km long in 2008 and  expanded at yearly rates of 31 m from 1975-1992, 47 m from 1992-2005, 68 m from 2005-2008, 60 m from 2008-2009, and no significant change from 2009-2014 (Fig. 8). The recent changing rate had been accelerating with active calving from 1975 to 2009, but it seems to be settled down and show no remarkable calving phenomena since 2009. As Nepal Department of Hydrology and Meteorology made a glaciological survey in 1996 (DHM, 1997), the topographic characteristics were compared with that of 1996 and 2009 in the lower part of the lake, and the lowering of the lake level is recognized and a clean pond is newly formed due to the sedimentation of glacial clay (glacier milk) at P point in Fig. 9. The shore line without vegetation is 2.5 m above the present lake level (Lower photo and upper right in Fig. 10) and the 1996 water gauge has been left higher than the lake level (Upper left photo in Fig. 10).

Fig.7   The Tulagi glacial lake and the terminus of Tulagi glacier in the west of  Mt. P29.

Fig.8  The changes of the Tulagi glacial lake and its glacier terminus. The yellow dotted lines show glacier termini determined by air photos and field survey. The lake was 0.5 km wide, 3 km long in 2008 and  expanded at  yearly rates of 31 m from ’75-’92, 47 m from ‘92-’05, 68 m from ’05-’08, 60 m from ‘08-’09, and no significant changes from ‘09-’14. The recent rate had been accelerating with active calving from 1975 to 2009, but it seems to be settled down and show no remarkable calving phenomena after 2009.

   The lowering of the lake level was also found by checking both the GPS trail and Google Image (Fig. 11), the 2009 GPS trail runs within the lake parallel to the lake shore of the 2005 Google Image (Upper right photo in Fig. 11). However, the 2009 trail coincides with the shore line of the 2011 Google Image (Lower left photo in Fig. 11) and this indicates the lowering of the lake level continued from 2005 to 2011.

Fig.9   The recent lowering of Tulagi lake level form 1996 to 2009. The yellow arrows in the above right photo show the lowering of Tulagi lake level form 1996 to 2009. The clean pond was newly formed at P point in the former glacial lake shown in the left photo.

Fig.10  The shore line without vegetation is 2.5 m higher than the present lake level (Lower and Upper right photos) and the 1996 water gauge is left above the present lake level (Upper left photo).

     The Tulagi glacial lake (Fig. 12) was formed after the glacial advance in 16th century (Fushimi, 1981) and there is no evidence of the GLOF occurrence indicated by a newly formed river terrace with an eroded valley topography along Dana Khola. At the  end-moraine of Tulagi glacial lake between the lower part of the lake and the most upper part of Dana Khola, the river mouth (outlet) is eroded about 30 m from the top of the 16th end moraine (Fig. 13). So, the water level of Tulagi glacial lake has been lowered at the average annual rate of 5 cm due to the outlet erosion at the end moraine since the 16th glacial advance that indicates to lower the GLOF risk. At the same time, the lake level also continuously lowers in recent years since 1990’s.

     There are no avalanches and rock slides directly falling into the Tulagi glacial lake to create a Tsunami, but a small wave with 30 cm height occurred when the glacier terminus collapsed as calving phenomena and it will be one of agents to cause an erosion at the river mouth (outlet) between the glacial lake and the upper part of the down-stream river. The Tulagi glacial lake has the huge debris covered glacier in the upper part where the avalanches and rock falls occur (Fig. 6), which is completely different from the small glacial lakes having the avalanches and rock falls directly dropping into the lake.

 Fig.11   Comparison of the GPS trail and Google Image. The 2009 trail runs within the lake parallel to the lake shore of the 2005 Google Image, but it coincides with the 2011 Google Image shown in the lower left photo.

Fig.12  The Tulagi glacial lake was dammed up by the moraine formed at the time of the glacial advance in 16th century.

Fig.13 The end moraine has been eroded about 30 m from the top of the end moraine formed in the 16th　century and this indicates that the lake level has been continuously lowered at an annual rate of 5 cm.

Fig.14  The end moraine structure of Tulagi glacial lake is wide and strong enough to prevent the occurrence of the GLOF.

Discussion

   The lake level of the Tulagi glacial lake has been continuously lowered by the outlet erosion at the end moraine to decrease the GLOF risk and it is occurred without having a man-made canal. The Tulagi glacial lake is thought to have a kind of an autonomous　property to prevent the GLOF. The end moraine structure of Tulagi and Imja glacial lakes are wide and strong enough to prevent the occurrence of the GLOF (Fig. 14 and 15) that is completely different from the narrow moraine of the small glacial lake with steep cliff at the upper part of the lake producing avalanches directly dropping into the lake causing Tsunami to destroy the fragile ice-cored moraine found in the end-moraine of the Mingbo (Nare) glacial lake (Fig. 2). The Imja glacial lake expanded from 1975 to 2002 (Right and upper left photo in Fig. 16), but the water level lowered in 2013 (Lower left photo in Fig. 16), so the large glacial lakes such as the Tulagi and the Imja are safe against the GLOF, as the ICIMOD (2011) reported “Imja Tso (glacial lake) has less likelihood of outburst than Tulagi lake”. However, we must be very careful about the small glacial lakes, for example, developing in the Hong Khola region around Mt. Chamlang (Fig. 1) which have steep cliff in the upper part of their accumulation area, so they must be taken to mitigate against GLOF.

Fig.15  The end moraine structure of Imja glacial lake is also wide and strong enough to prevent the occurrence of the GLOF.

Fig.16  This shows the Imja glacial lake taken in 1975 by plane, and in 2002 (Upper left photo) and 2013 (Lower left photo) on the field. The Imja glacial lake expanded from 1975 to 2002, but the water level lowered in 2013.

   Since the United Nations Development Programme (UNDP) and Nepal government made an agreement to implement the Community Based Flood and Glacial Lake Outburst Risk Reduction Project (CBFGLOF) by building a man-made canal (The Himalayan Times, 2013), I recommend that we must make necessary field observations before taking such kind of the project to construct a man-made canal in the pristine nature of the Himalayas. Finally, I would like to make it sure whether it is appropriate or not to have already constructed the man-made canal at Tso Rolpa glacial lake in the west of Khumbu region, east Nepal.

Conclusions

    All of the reported GLOFs occurred in Nepal Himalaya are smaller glacial lakes which area are less than 1 km² and no larger glacial lakes such as Glaciers Tulagi and Imja showed any GLOF phenomena. Due to the outlet erosion at the end moraine of the larger glacial lake since the 16th glacial advance, the lake level has been lowering and reducing the GLOF risk.Why is a large glacial lake safe against GLOF in Nepal Himalaya? What is the higher risk of the GLOF? It is not a large glacial lake, but a small glacial lake. The lake level of Tulagi glacial lake has been continuously lowering by the outlet erosion at the end moraine and it is occurred without having a man-made canal. The end moraine structures of Tulagi and Imja glacial lakes are wide and strong enough to prevent the occurrence of the GLOF that is completely different from the narrow moraine of the small glacial lake with steep cliff at the upper part of the lake producing avalanches and rock falls directly dropping into the lake causing Tsunami to destroy the fragile ice-cored moraine found in the end-moraine of the Mingbo (Nare) glacial lake. So, large glacial lakes such as the Tulagi and the Imja are safe against GLOF, however we must be very careful about the small glacial lakes, for example, developing in the Hong Khola around Mt. Chamlang which have steep cliff to cause avalanches and rock falls that create a Tsunami destroying the end moraine and forming the GLOF, so they must be taken to mitigate against GLOF.

Acknowledgements

I am very much grateful to Ski Expediton Party to Mt. Everest 1970, Glaciological Expedition of Nepal Himalaya (GEN) from 1973 to 1978 and Japan International Cooperation Agency (JICA) from 2008 to 2010 to have given me chances for making field surveys in Nepal Himalaya.

References

Fushimi H (1981) Glacial history in the Khumbu region, Nepal Himalayas in relation to upheavals of the Great Himalayas. Symposium on Qinghai-Xizang (Tibet) plateau (Beijin, China), 2, 1641-1648.

Fushimi H, Ikegami K, Higuchi K and Shankar K (1985) Nepal case study: Catastrophic Flood. Techniques for prediction of runoff from glacierized areas, International Association of Hydrological Sciences, 149, 125-130.

His Majesty's Government of Nepal Ministry of Water Resources, Department of Hydrology and Meteorology (DHM) (1997) Thulagi Glacier Lake Study-Final Report-. (unpublished).

ICIMOD (2011) Glacial lakes and glacial lake outburst floods in Nepal. Kathmandu, Nepal: ICIMOD.

Manoj Kr. G, Shreekamal D and Subhrant K. C. (2005) Glacial study in Madi watershed with special reference to GLOF of 2003. Journal of Nepal Geological Society, Vol. 32 (Sp. Issue), p48.

The Himalayan Times (2013-07-15). Nepal, UNDP ink deal on cutting flood risk.