29

Bulletin of Glaciological Research ,. (,**1) ,3ῌ-0
῍Japanese Society of Snow and Ice

Topographical survey of end moraine and dead ice area at Imja Glacial Lake in ,**+
and ,**,
Akiko SAKAI+*, Mitsuyoshi SAITO,, Kouichi NISHIMURA-, Tomomi YAMADA.,
Yasuko IIZUKA/, Koichiro HARADA0, Shun’ichi KOBAYASHI1, Koji FUJITA+ and C. B. GURUNG2+ Graduate School of Environmental Studies, Nagoya University, c/o HyARC, Chikusa-ku, Nagoya .0.ῌ20*+, Japan, Mitsui Mineral Development Engineering, Co., Ltd.. Gatecity Osaki West Tower +2F, +ῌ++ῌ+ Osaki, Shinagawa-ku,
Tokyo +.+ῌ**-,, Japan- Department of Environmental Science, Faculty of Science, Niigata University, Ikarashi ,no-cho, 3/*ῌ,+2+, Japan. Institute of Low Temperature Science, Hokkaido University, Sapporo *0*ῌ*2+3, Japan/ Graduate School of Science and Technology, Niigata University, Ikarashi ,no-cho 2*/*, Niigata 3/*ῌ,+2+, Japan
0 School of Food, Agricultural and Environmental Sciences, Miyagi University, ,ῌ,ῌ+, Taihaku-ku, Sendai 32,ῌ*,+/, Japan
1 Research Institute for Hazards in Snowy Areas, Niigata University, Ikarashi ,no-cho 2*/*, Niigata 3/*ῌ,+2+, Japan
2 Department of Hydrology and Meteorology, Ministry of Science and Technology, Kathmandu, Nepal(Received January ,-, ,**0; Revised manuscript accepted November /, ,**0)

Abstract
A survey was carried out at the end moraine and the dead ice area of the Imja Glacial Lake in
October ,**+ and in April ,**, using a digital theodolite with a laser distance meter and di#erential
Global Positioning System (d-GPS). The survey points covered the whole dead ice area and end
moraine of the Imja Glacier, and map contours were produced at /-m intervals. This topographicalmap will be a signiﬁcant means to analyze the time variation of the dead ice area and end moraine in
future re-mapping. Changes in the lake level and the spillway on the dead ice area since +33. werealso analyzed in comparison with the measurements in +33. by Watanabe et al. (+33/).

+.

Introduction

There are many glacial lakes in the world, forexample in South America, New Zealand and Himalayas (Lliboutry, +311; Lliboutry et al., +311a; Lliboutry
et al., +311b; Kirkbride, +33-). Since there are many
debris-covered glaciers there, those glaciers can build
relatively huge moraines, and moraines tend to dam
up meltwater after decline in the glacier surface level.
There are two types of glaciers in the Himalayas(Moribayashi, +31.); one is debris-free and the other isdebris-covered. Debris-free glaciers in the Himalayas
are shrinking more rapidly than glaciers in other regions of the world (Fujita et al., +331). Moreover, theshrinkage of debris-free type glaciers accelerated in
the +33*s (Fujita et al., ,**+a; ,**+b). Debris-covered
glaciers have also been shrinking because their surfaces are declining (Inoue and Yoshida, +32*), andthere are many glacial lakes at the terminus of theglaciers. Yamada (+33-) estimated that some glaciallakes have expanded at the glacier terminus of thedebris-covered glaciers since the +3/*s or +30*s.
Those were surrounded by end and lateral morainesand dead ice area (Yamada, +332; Ageta et al., ,***;
Iwata et al., ,**,; Komori et al., ,**.). The dead ice
may have remained beneath the moraine, and thestrength of the moraine would decrease due to themelting of the dead ice.
From compared bathymetric maps made in +33,
and in ,**,, Sakai et al. (,**-) concluded that thesurface area of the Imja glacier lake in ,**, has grown
to become -*ῌ larger than in +33,. The stored water
volume has become ,2ῌ more than in +33,, although
the average depth has decreased.
Glacier ice remained between end moraine andlake at the Imja glacial lake. And the lake was dammed by the glacier ice (dead ice) directly. Based on
the survey at Imja Glacial Lake, Watanabe et al. (+33/)
reported that the lake level fell and was close to
ground level, and the possibility of outburst from thelateral moraines has diminished now from the surveyof one cross section. However, they also said that itis necessary to assess the lowering of the surface of* Research fellow (Dynamics of the Sun-Earth-Life Interactive System, No. G-., the ,+st Century COE Program, Japan)

30

Bulletin of Glaciological Research

the dead ice area since the spillway on the dead ice
area underwent a dramatic change.
In this study, ﬁeld surveys of the moraine anddead ice area have been carried out to clarify the
positional relationship among the moraine, dead ice
area and Imja Glacial Lake. The relative height between the lake level and river bed was analyzed toclarify the portion where there was the possibility ofoutburst. Change of spillway on the dead ice area
was also analyzed in comparison with the measurements in +33. by Watanabe et al. (+33/). After several
years, the variation in the lowering of the moraineand dead ice area can be clariﬁed by re-survey.

,.

Study area

A survey was carried out on Imja Glacial Lakelocated in the Khumbu region of the Nepal Himalayas
as shown in Fig. +. The lake is located 0 km south of
Mt. Everest (Sagarmatha in Nepalese; Qomolungma in
Chinese) and the south foot of the Mt. Island Peak asshown in Fig. ,. The lake formed at the terminus of
Imja Glacier. The Imja Glacier is a debris-covered
type glacier and the Lhotse Shar Glacier has con-Fig. +. Location of Khumbu region (above). Topographical map of the basin of Imja Glacial Lake(below), modiﬁed from map produced by the Survey Department of His Majesty’s
Government of Nepal in co-operation with the Government of Finland (Sheet No. ,120 *.,SAGARMA
…THA…). Map compiled from +: /* *** scale aerial photograph of +33,. Field
veriﬁcation done in +330.)

Sakai et al.

Fig. ,.

Photograph of Imja Glacial Lake from left side, BM ,*. ‘Island Peak’ can be seen in front.

verged with it. The Ampbu Glacier also convergedwith the Imja Glacier, but now no longer is in contact
with it.
Watanabe et al. (+33/) reported that glacier ice
was exposed in +33. between the end moraine andImja Glacial Lake, which they termed “Dead ice” intheir paper. Therefore, we call this the “dead icearea” between the end moraine and Imja Glacial Lake.
There were several supra-glacial ponds in +3//
near the terminus of Imja Glacier (Watanabe andHammond, +33.; Yamada, +33-), and they became one
huge lake from +31/ to +32. based on aerial photographs (Quincey et al., ,**/). Yamada (+33-) measured the depth distribution of the lake and made abathymetric map in +33,. Watanabe et al. (+33/) carried out a survey of the cross sections at the dead ice
area and moraines. They concluded that the possibility of outburst from the lateral moraine has diminished based on their survey of the relative height
between the lake level and ground level at the side ofthe lateral moraine. But, they reported another possibility, namely that the outburst will occur by meltdown of the dead ice area, particularly along thespillway.

-.

31

SurveySurveys of the dead ice area and end moraine ofImja Glacial Lake were carried out from ,0 October to
0 November, ,**+ and from 1 to 2 April, ,**,. A
digital theodolite with the laser distance meter (SOKKIA SET,+**) was used in ,**+, and a di#erential
Global Positioning System (d-GPS) (CMC Co., Ltd.) wasused for the survey in ,**,.
The error of the digital theodolite with the laser
distance meter was῍(,ῌ, ppm῎D) mm in distance,
where D is the measured distance, and the accuracy of
azimuth and elevation angles were , seconds. All
measured distance was less than +*** m, so the error
was less than +* mm. However, the error of measurements taking into account the mirror setting andnetwork of benchmarks would be῍*.+ meter.
The digital theodolite with the laser distance meter was settled at a benchmark and measured theazimuth, elevation angle and distance of the measurement point on the basis of another benchmark. We
did not measure survey points on the basis of magnetic north since it is di$cult to ﬁt the digital theodolite with the laser distance meter to magnetic northwith su$cient accuracy. Therefore, the measured
data were based on benchmarks with no concern withdeviation from magnetic north.
The survey using the digital theodolite with thelaser distance meter was conducted as follows: One
person engaged in operation of the digital theodolitewith the laser distance meter set one of the benchmarks, and the other one or two persons, who carrieda reﬂector, decided the points to be measured, moving
along on the dead ice area, end moraine and at the footof the moraines. Measured points included remarkable topographical features such as peaks, cols, ridges,
bottom lines of trenches, pond edges, and edges ofspillways.
A Global Positioning System (GPS) measures absolute positions on the earth with low accuracy (morethan +* meters), although the accuracy of a relative
position between ﬁxed master receiver and remotereceiver (mobile station) using d-GPS is on the order of
cm. If the master station is removed once, it is necessary to measure the benchmark again.
Instrumental error of the d-GPS depends on the
distance between the base station and the reference
station. The maximum distance was less than + km,
so the error was within *., m since the error was *., mwhen measured distance was +km according to CMC
Co., Ltd.. Those measurement errors within *., m areaccurate enough to discuss the variation in topography of the dead ice area and end moraine in future
measurement, and those data can be treated as thesame data observed by a digital theodolite with a laser
distance meter.
One GPS receiver was ﬁxed at the top of the polein the Base Camp as a reference station, and the otherwas the mobile station in the backpack of the person
who walked on the dead ice area and moraine. The
data logging interval was one second. Therefore, thetrail of a person with a mobile GPS receiver can bemeasured with high density. It can not be measured

32

Bulletin of Glaciological Researchby d-GPS in a dangerous area, such as a steep slope
since a person with a mobile receiver can not walkthere.
Data processing of d-GPS measurement fromthose data obtained at removal and reference stations
was performed using GrafNav software (Waypoint
Consulting Inc.). Processed data are represented by
the Universal Transverse Mercator system (UTM)(Zone: ./): north, east, and altitude. Measured data

can not be processed when the number of receivable
Satellites becomes less than .. Some measured dataat the foot of the end moraine could not be processedusing GrafNav software because of the insu$cient of
number of data received from satellites. On the other
hand, all data measured on the dead ice area could beprocessed.
Nine benchmarks were set on and around theImja Glacier to measure the topography of moraine

Fig. -. Location of benchmarks on and around dead ice area and end moraine.
Coordinate is represented with BM1 as zero.

Table +.

Locations of benchmarks in the Universal Transverse Mercator (UTM) (Zone ./).

UTM position (zone: ./)Projection: Transverse Mercator,
Datum: India-India,
Ellipoid: Everest +3/0

Relative location from BM1

East (m)North (m)Elevation (m)East (m)North (m)Elevation (m)BM+.3+/3.4,3-*2/3104--/*-+433

2+/4/-+2-4++

014+1

BM.

.3++-24+1-*2/232410/*+242--/34.++*/4/.

2.40.

BM0.3+*,34/,-*2/2+34/1/*+/41/,/*410

,04-0

/*43-

BM1

.3*112410-*2/13-4,,.30.42,

*4**

*4**

*4**

BM,*

.3*31-43/-*2/,/.4..

/+.+412+3/4+3

ῌ/-2412+10430BM,+.3+**.4,+-*20++*4.+/*-34,1,,/4./-+14+3

1.4./

BM,,

.3++*043+-*2/-12400/*0/4**-,24+/

ῌ.+.4/0+**4+2BM,-.3+/2/4.,-*2/3,24/2/*,243-2*0400+-/4-0

0.4++

BM,/

.3+.*,4-.

-*20/0*43//*.34.0

0,-4/210141.

++340-

Sakai et al.

and dead ice area in ,**+. Locations of all benchmarks in the Universal Transverse Mercator (UTM)(Zone: ./) projection were summarized in Table + andshown in Fig. -. The relative distance and directionof each measurement point from the network of benchmarks were calculated. BM1, BM,- and BM. were
measured again by d-GPS in ,**,. Then, the survey
data using the digital theodolite with the laser distancemeter obtained in ,**+ overlapped the data surveyedusing d-GPS in ,**, by the Universal Transverse Mercator system. As mentioned above, the accuracy of
the relative position was within *., m, although theaccuracy of the absolute position was several meters.
Therefore, relative positions of all points from thebenchmark, BM1, are also listed in Table +.
The four benchmarks of BM+, ., 0, and ,-, were on
the dead ice area or moraine. Therefore, there is thepossibility that those . benchmarks will be removed
in the future by melting glacier ice under debris. On
the other hand, presumably there is no ice under BM1,,*, and ,,, so, those - benchmarks can be used as ﬁxed
points in the re-survey.
Measurements were done at +,--2 points and.+,/.1 points by laser distance meter and by d-GPS,
respectively. Contours of the map were created using Geographical Information System (GIS) software(TNTmips ver. 0.1; Micro Images Inc.) based on theobserved data. The Minimum Curvature method
was applied to surface ﬁtting. This method applies a
two-dimensional cubic spline function to ﬁt a smoothsurface to the set of input elevation values. The
computation requires a number of iterations to adjust
the surface so that the ﬁnal result has a minimum
amount of curvature.
The west side of the shoreline of Imja Glacial
Lake was also surveyed in ,**+ using digital theodolite with the laser distance meter in order to connect
the bathymetric map and the map of end moraine anddead ice area. The spillway on the dead ice area has
been surveyed at more than /* points and by photographed in order to be incorporated into the topographical map.

..

33

shown in Fig. / depends on the density of the measured points (Fig. .). The precision is high where themeasurement density is high, and vice versa. The area
where measurement was carried out using d-GPS wasvery high, since the logging time of the d-GPS was +second. d-GPS was used to measure the rough terrain because of impenetrability by the digital theodolite with the laser distance meter. d-GPS was also
useful in this regard since the logging interval of
d-GPS was one second, and we could observe manymeasurement points. However, one should note thebias in the measurement point density because with
d-GPS observation the measurement points concentrate along accessible area.
The roughness of the surface at the dead ice area
was high at the end moraine side and low at the lakeside from the range of the contour line. The spillway
was located at the lowest portion of the dead ice areaand moraine area, and the slope surrounding the spillway was relatively steep. There was, however, no
exposed ice at the dead ice area of Imja Glacier during
the survey period in ,**+ and ,**,. There was much
exposed ice contacting the lake water in +31-, to judge
by a photograph taken by Japanese Glaciological Expedition of Nepal mentioned by Watanabe et al. (+33.).
Exposed ice can melt very rapidly since the surface is
not insulated by rock debris (Sakai et al., ,**,). It is
important to check for possibility exposed ice on adead ice area or moraine in forthcoming studies.

Results

Figure / shows the completed topographical mapof the dead ice area and end moraine of Imja Glacier.
The scale of the original map is +: +****. The contour
intervals are / m. A bathymetric map of Imja Glacial
Lake was also combined based on d-GPS measurement at BM ., 1 and ,-.
Densities of measurement points were shown in
Fig. .. A survey using d-GPS was carried out on theright side of the dead ice area since the surface can not
be seen from all benchmarks because of the rugged
topography. Therefore, high-density areas are concentrated on the right side. The precision of the map

Fig. .. Density of points for topographic measurements of dead ice area and end moraine. Coordinate is the same as in Fig. -.

Fig. /. Topographical map of dead ice and end moraine of Imja Glacier and bathymetric map of Imja Glacier. Contour of moraine and dead ice area was at /-m intervals. Depth
contour of the lake is expressed as relative height to water surface level (/**3 m a.s.l.) at +*-m intervals.

34
Bulletin of Glaciological Research

Sakai et al.

/.

Discussion

Watanabe et al. (+33/) reported that the waterlevel declined +*.- m from +323 to +33., assuming thatthe BM-B did not move during that period. Unfortunately, the BM-B could not be found during our ﬁeld
surveys in ,**+ and ,**,. The altitude of BM-B was/*-/ῌ/ m a.s.l. as shown in Fig. 0, which overlapps
Fig. 2 in Watanabe et al. (+33/) based on the location of
the end moraine. The water level of the lake was/**3 m a.s.l. in ,**+ by our observation. Therefore,
the relative height between BM-B and the water level
was ,0ῌ/ m ((/*-/ῌ/)-/**3 m) in ,**+. Watanabe etal. (+33/) found that the relative height was ,/ m in
+33.. Then, the change in water level of the lake wasin the range of error.
The spillway in +33. (Fig. 2 in Watanabe et al.
(+33/)) also overlapped the topographical map in ,**+
as seen in Fig. 0. The spillway on the dead ice area in,**+ ﬂowed along the southern side of the cone (indicated as “C” in Fig. 0), as opposed to its northern sidein +33.. The spillway changed the path of ﬂow from+33. to ,**+. The area where there had been a spillway in +33. was at a relatively low altitude and ﬂat in,**+, as shown in Fig. 0. Unfortunately, Watanabe etal. (+33/) did not measure the cross section around thecone, ‘C’, in Fig. 0. Therefore, we have no information

35

on the topography there in +33.. There was no otherchange in the path of the spillway from +33. to ,**+.
A Glacial Lake Outburst Flood (GLOF) can occur
where the height of the lake surface is higher than theriver bed i.e., the foot of the moraine. The level ofImja Glacial Lake was /**3 m a.s.l. Thus, the areawith an altitude less than /*+* m a.s.l. was shaded inFig. 0. There was no area below the lake level on theright (north) side of the moraine of Imja Glacier as
indicated by Watanabe et al. (+33/). Therefore, there
is no possibility that a GLOF occurs by collapse at theright of the moraine at the present lake level. On theother hand, the relative height of the foot of the end
moraine in relation to the lake surface level was -* mat the left. Thus, were the left side of the end moraine or dead ice to melt or collapse, a GLOF would
occur. And if the left side of the moraine and deadice area were to completely collapse, the lake water up
to -* m depth would ﬂow away. The water volume
reached ,*.0῍+*0 m-. It is necessary to continue toobserve carefully the topographical change of themoraine and dead ice and the change in the Imja
Glacial Lake level.

Acknowledgments
We would like to express our thanks to Mr. A. P.
Pokhrel, Mr. O. Bajracharya, Dr. B. Rana, Dr. A. Shres-Fig. 0. Topographical map of dead ice and end moraine of Imja Glacier. Shaded area
indicates altitude is less than /*+* m a.s.l. Hatched area indicates spillway in +33.
and star mark shows BM-B, with reference to Fig. 2 in Watanabe et al. (+33/).

36

Bulletin of Glaciological Researchtha and other sta# members in the Department ofHydrology and Meteorology, Ministry of Science andTechnology, Government of Nepal, for their generous
assistance and cooperation in our research activity in
the Khumbu region. We also are grateful for thekind assistance of all the project members in the ﬁeld
work. The ﬁeld research and data analyses were
ﬁnancially supported by a Grant-in-Aid for ScientiﬁcResearch (Project Nos. +--1-**0 and +-/1-**.) from the
Ministry of Education, Culture, Sports, Science andTechnology of Japan (MEXT). The analysis was supported by Dynamics of the Sun-Earth-Life Interactive
System G-., the ,+th Century COE Program from
MEXT.

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