Geoecology and mass movement in the Manaslu … et al...Geoecology and mass movement in the Manaslu-Ganesh and Langtang-Jugal Himals, Nepal R.A. Marston a,), M.M. Miller b, L.P. Devkota

Ž .Geomorphology 26 1998 139–150

Geoecology and mass movement in the Manaslu-Ganesh andLangtang-Jugal Himals, Nepal

R.A. Marston a,), M.M. Miller b, L.P. Devkota c

a UniÕersity of Wyoming, Laramie, WY 82071, USAb Foundation for Glacier and EnÕironmental Research and UniÕersity of Idaho, Moscow, ID 83843, USA

c Central Department of Meteorology, P.O. Box 127, Lalitpur, Nepal

Received 15 May 1996; revised 10 December 1997; accepted 1 March 1998

Abstract

This study describes and explains the spatial distribution of mass movement in the central Nepal Himalaya. Judgmentswere formulated on the origin and rates of mass movement using field evidence, topographic maps, geologic maps, andSPOT imagery. Mass movement scars were mapped in the field during a 240-km traverse of the Langtang-Jugal Himal and a300-km traverse of the Manaslu-Ganesh Himal. Chi-square analyses revealed that the frequency of slope failures varies with

Ž .slope aspect, and position aboverbelow the Main Central Thrust MCT . Human disturbance did not account for astatistically significant increase in mass movement, except in sites occupied by mid-slope roads and where excessively steepslopes, marginal for agriculture or grazing, have been deforested. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: geoecology; mass movement; Nepal; Main Central Thrust

1. Introduction

The geomorphic development of hillslopes in themiddle mountain, high mountain, and high Himalayaphysiographic regions of central Nepal has beendominated by ancient and modern mass movementcoupled with dramatic incision by major rivers.Travel through these regions is difficult without be-ing impressed by the extent of mass movement.Considerable debate rages among Himalayan scien-tists over the relative effect of human activities onthe magnitude and frequency of mass movement.

) Corresponding author.

Excellent summaries of this debate are provided byŽ . Ž .Carson 1985 , Ives and Messerli 1989 , Bruijnzeel

Ž .and Bremmer 1989 and in papers presented at the1995 Workshop on Landslide Hazard Managementand Control in the Hindu Kush Himalaya in Kath-

Ž .mandu e.g., Mool, 1995 . Very few engineeringstudies of slope stability have been reported, primar-ily because of the difficulty in acquiring adequatefield data of the detail needed. In any case, engineer-ing slope stability analyses often do not lead to ageneral understanding of controls on mass movementbecause of the difficulty in extrapolating from onefield site to the next without an equivalent amount ofdetailed field data. Thus, we remain at a stage where

0169-555Xr98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-555X 98 00055-5

( )R.A. Marston et al.rGeomorphology 26 1998 139–150140

Žinventories of mass movement are useful Mool,.1995 .

The consequences of mass movement in the Hi-malaya are well documented. One set of conse-quences involves the loss of productive land forforestry, cultivation, or range use. This dimension ofthe hazard from mass movement is acknowledged bymountain villagers as a serious problem, but in manycases, supernatural causes are blamed. Johnson et al.Ž .1982 have described the range of responses bymountain villagers to mass movement events, includ-ing preventive maintenance and repair for reuse at alower land use intensity. A second consequence ofmass movement involves the sedimentation impact

Ž .on stream channels Fig. 1 . Aggradation promotesstream undercutting of slopes which triggers yetmore mass movement and aggradation—a positivefeedback not easily remedied by engineering works.In some cases, temporary dams are created acrosschannels that cause catastrophic flooding upon fail-ure in the manner described by Costa and Schuster

Ž .1988 . Damage to hydropower and irrigation pro-Žjects is a major impact of this sedimentation HMGr

.WESC, 1987 . Mass movement, and the channelshifting and flooding related to it, are considered bysome to be another manifestation of the destructionof life support systems on the Ganges River Plain bythe actions of subsistence farmers in the mountainsŽ .Ives and Messerli, 1989 .

This study evaluates the spatial distribution ofmass movement in the central Nepal Himalaya. Anattempt was made to formulate some judgments onthe origin and rates of mass movement. On this laterpoint, it is dangerous to try isolating triggeringmechanisms. The processes that trigger mass move-ment operate on different time scales. For instance,contrast the long-term effect of progressive weather-ing along the soil–bedrock contact with the seasonaleffect of fluctuating water tables and the sporadiceffect of earthquakes. Moreover, it is difficult toidentify the triggering mechanism of mass movementevents from hundreds or thousands of years ago. As

Fig. 1. Streamside mass movement and associated channel aggradation common to the central Nepal Himalaya. Located along unnamedtributary to the Malemchi Khola near Talamarang.

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Fig. 2. Route through the Langtang-Jugal Himal and Manaslu-Ganesh Himal. All scars from mass movement within the viewshed along this route were surveyed.


an alternative, it is more useful to focus on theintrinsic and more static elements of those land-scapes prone to mass movement.

2. Methods

Scars from mass movements were mapped in thefield during a 240-km traverse of the Langtang-JugalHimal and a 300-km traverse of the 1987 Manaslu-

Ž .Ganesh Himal Fig. 2 . Each scar from mass move-ment was mapped within the ‘viewshed’ as long asthe location and degree of vegetation disturbancecould be evaluated. The surveys included portions of

the middle mountain, high mountain and high Hi-malaya physiographic regions as described in

Ž .HMGrWESC 1987 and in the work of Ives andŽ .Messerli 1989 . The middle mountain and high

mountain physiographic regions are divided by theŽ .Main Central Thrust MCT . The MCT is a major

lithologic, metamorphic, and structural discontinuity.Below the MCT occur low grade, argillaceous andcalcareous metasediments. Above the MCT occurhigh grade arenaceous metasediments, but with aninverse metamorphic gradient. The MCT is the onlyindisputable thrust fault within the eastern HimalayaŽ .Schelling, 1987 . In the field, kyanite is often foundimmediately above the MCT, and serves as an aid tomapping.

Table 1Frequency of 272 recorded slope failures by slope aspect, position aboverbelow the MCT, and whether natural or human-caused

Aspect Aboverbelow Cause Observed no. Expected no.Ž . Ž .MCT for chi-square calculations for chi-square calculations

N 14.9% below 4.1% natural 3.0% 5 8human 1.1% 1 3

above 10.8% natural 7.0% 3 19human 3.8% 1 10

NE 10.5% below 6.8% natural 5.4% 8 14human 4.1% 2 4


E 10.2% below 3.0% natural 1.5% 16 5human 1.5% 3 4


SE 15.4% below 5.6% natural 1.1% 17 3human 4.5% 3 12


S 13.9% below 3.6% natural 1.8% 31 5human 1.8% 11 5


SW 10.5% below 6.8% natural 4.8% 15 4human 2.0% 4 5


W 9.2% below 2.5% natural 1.6% 13 4human 0.9% 2 3


NW 15.4% below 5.6% natural 2.4% 17 6human 3.2% 5 9


( )R.A. Marston et al.rGeomorphology 26 1998 139–150 143

Fig. 3. Debris slide of 75,000 m3 in undisturbed forest of the Middle Mountains below the Ganesh Himal. For scale, the waterfall at topright is 80 m high.

Several characteristics at each site were recordedŽfor a total of 272 scars from mass movement Table

.1 . Slope aspect was recorded, but slope gradientcould not be measured accurately from a distance orfrom topographic maps. The lithologic–structuralsetting of each scar was noted, using direct observa-tion where the scar could be inspected. Otherwise,reference was made to the 1:200,000 scale French

Ž .geologic map Colchen et al., 1980 of the Anna-purna–Manaslu-Ganesh region or the 1:1,000,000scale map produced by the Nepal Department of

Ž .Mines and Geology HMGrDMG, 1980 . More de-tailed geologic maps of this region are not available.In terms of the origin of the scar, a simple classifica-tion as ‘natural’ or ‘human-caused’ was used, fol-

Ž .lowing the simple criteria of Laban 1979 . If thescar was located in a forest or was undercut by a

Ž .river, it was placed in the natural class Fig. 3 . If thescar was located in cleared or cultivated land, or wasdirectly attributed to road construction, it was classedas human-caused. This classification has some inher-ent ambiguity because natural mass movement canoccur in disturbed areas, thereby possibly overstatingthe extent of mass movement triggered by humans.

In the final analysis, however, results were not af-fected by this procedure. Meteorological variables

Ž .were ignored in this study; Caine and Mool 1982have noted that this is not a limiting factor. Poorseismicity records prevented analysis of earthquakeactivity as a control on the spatial distribution ofmass movement, although in some portions of theHimalaya, this factor can be an important driving

Ž .force for mass movement Sarkar et al., 1995 .

3. Results

3.1. The form and origin of scars from mass moÕe-ment by physiographic region

The middle mountain physiographic region of theManaslu-Ganesh Himal is situated below the MCT,so the bedrock is dominated by phyllites, quartzites,and garnet mica schist. The combination of theselithologic units with the hot–wet climate and densevegetation has led to deep weathering and a roundingof slope breaks by soil creep. Slope gradients range


Fig. 4. Debris slide of 225,000 m3 on terraced hillslopes of Middle Mountains near the village of Linju. Note the breached dam andlacustrine deposits behind the former dam.

Fig. 5. Debris slide of 900,000 m3 in the Middle Mountains of the Helambu District, Langtang-Jugal Himal. The scale of this massmovement scar prohibits any rehabilitation.


up to 308. Stream undercutting is locally important asa trigger to debris slides along major rivers such asthe Marsyandi and Buri Gandaki and their tributariesŽ .Figs. 4 and 5 .

The high mountain physiographic region of theManaslu-Ganesh Himal is underlain by a medium- tocoarse-textured augen gneiss. This lithology is struc-turally more competent, providing the framework forthe prevailing cuestaform topography. The cuestasdip to the north, leading some to speculate that thetopography is the surface expression of thrust sheets.Slope breaks are sharp between the dip slope and

scarp slopes. Irrigation drainage from the dip slopes,sometimes discharged onto the scarp slope, triggers

Ž .debris slides Fig. 6 . The angle of dip increasesfrom 108 to more than 308 as one moves from southto north. At higher elevations, the dip slopes becomeexcessively steep, dipping up to 458, and soils aremore shallow, with abundant evidence of ancient andmodern debris slides. Deforestation does acceleratemass movement on steep slopes which are marginal

Ž .for agriculture Bishop, 1990 .The middle mountain and high mountain physio-

graphic regions of the Langtang-Jugal Himal are

Fig. 6. Debris slide of 6000 m3 in High Mountains of Manaslu-Ganesh Himal. This slide was caused by discharge of excess irrigationdrainage from terraced fields onto a scarp slope in the loess mantled, cuestaform slopes.


Fig. 7. Debris slide of 5000 m3 in loess mantled slopes on ridgeabove Trisuli River. Note Hindu chortens along ridgetop trail inthis Middle Mountain region of the Langtang-Jugal Himal.

mantled with loess probably derived from the Ti-betan Plateau. Deep-seated slides and slumps are thedominant form of mass movement in undisturbedsituations. Deforestation and poor control of terracedrainage are more important here in triggering mass

Žmovement than in the Manaslu-Ganesh Himal Fig..7 .

In the high Himalaya physiographic region of theManaslu-Ganesh and Langtang-Jugal Himals, frostaction generates huge talus cones and felsenmeerŽ .see Watanabe et al., 1998, this issue , especiallyalong fractures. Slopes at elevations above 3000 m,oversteepened by glaciation, may have near verticalslopes with local relief in excess of 2000 m. Asheeting structure because of multiple joint sets wasidentified in gneissic and granitic bedrock whichmay contribute to massive block slides. What may be

the largest slide in the world in crystalline rock hasŽ .been reported by Heuberger et al. 1984 and

Schramm et al., 1998, this issue. An estimated massof 10 km3 was displaced along a fault plane. Thesliding surface generated fused crystals. Quaternaryage glaciers in Langtang have removed or buried 60to 75% of the deposits from this event.

3.2. Statistical analyses of scars from mass moÕe-ment

The chi-square statistical procedure was used totest several hypotheses regarding the spatial distribu-tion of the 272 scars from mass movement. In eachtest, the division between classes was normalized bythe percent of the study area sampled that occurredin each class, a key procedure that was not followedin all past studies. The first hypothesis could bestated as follows.

H : No difference in the frequency of mass move-o

ment exists among slope aspects.

Fig. 8 illustrates the difference between the ob-served frequency of mass movement and the ‘ex-

Žpected’ frequency of mass movement i.e., the fre-quency if mass movement was distributed between

Fig. 8. Expected and observed frequency of mass movement byslope aspect.


slopes of different aspects proportional to area sam-.pled in each aspect . The data in Table 1 reveal that

mass movement on south-facing aspects was morefrequent than expected. This aspect is on the wind-ward side of summer monsoon storms and receivesthe most direct solar insolation. Therefore, soils maybe subject to numerous wet–dry cycles which cancontribute to mass movement. In addition, aban-doned land on south-facing slopes is not as quick torevegetate. The calculated chi-square value was71.78, greater than the critical chi-square value of

Ž .24.32 for dfs7 i.e., eight different slope aspects atp-0.001. Therefore, we rejected the first hypothe-sis and concluded that mass movement did vary withslope aspect.

The second hypothesis regarding the spatial distri-bution of mass movement can be stated as follows.


ment exists above and below the MCT.

Fig. 9 illustrates that mass movement was morefrequent than expected below the MCT and lessfrequent than expected above the MCT. The deeplyweathered gneiss above the MCT appears to be moresusceptible to piping and gullying than to massmovement, confirming the findings of Brunsden et

Ž .al. 1981 from studies in eastern Nepal. The calcu-lated chi-square value was 39.06, greater than thecritical chi-square value of 10.83 for dfs1 at p-

0.001. Therefore, we rejected the second hypothesisand concluded that mass wasting did vary with posi-tion above and below the MCT. No significant dif-ference in mass movements could be discerned be-tween the phyllites, shales, and schists below theMCT.

Fig. 9. Expected and observed frequency of mass movement byposition aboverbelow the MCT.

Fig. 10. Expected and observed frequency of mass movement inlandscapes in a ‘natural’ condition and in landscapes disturbed byhuman activities.

The third hypothesis can be stated as follows.


ments exists between disturbed and undisturbedlandscapes.

Fig. 10 illustrates that mass movement was moreŽ .frequent than expected in undisturbed ‘natural’ ar-

eas and less frequent than expected in disturbedŽ .‘human-caused’ areas. The calculated chi-squarevalue was 10.99, greater than the critical chi-squarevalue of 10.83 for dfs1 at p-0.001. Therefore,we rejected the third hypothesis and concluded thatmass movement did vary with the degree of distur-bance, but opposite to the trend often reported forother regions of the world. Does this finding meanthe vegetation is unimportant? It is necessary todistinguish between shallow and deep-seated formsof mass movement. On unvegetated slopes, massmovements are smaller and more shallow. Larger,deeper slides occur independent of vegetation coverŽ .Fig. 11 . Moreover, human activities do account fora disproportionate share of mass movement in somesettings, poor road construction, and trail disruption

Ž .of slopes being the most notable Fig. 12 . Clearcut-ting or poor drainage from terraced fields ontoloess-derived soils or steep slopes with shallow soilsŽespecially in the cuestaform topography of the high

.Himalaya also leads to accelerated mass movement.Nevertheless, these data help refute the broad as-sumption that human activities greatly increase sedi-ment production from mountain regions of Nepal, a

Ž .conclusion also reached by Ives and Messerli 1989Ž .and Stevens 1993 . The notion that deforestation

accelerates mass movement can be exemplified by


Fig. 11. Deep-seated slump of 2,000,000 m3 west of Trisuli Bazar in the Middle Mountains.

Fig. 12. Mass movement triggered by mid-slope road construction, near village of Tarkughat, Middle Mountains, Manaslu Himal.


studies of the effect of clearcutting around the Pa-Ž .cific Rim reviewed by Sidle et al. 1985 . They

found that long-term rates of mass movement inclearcuts were 7.8 times greater than in forestedareas. Also, mass movement from individual stormevents was 17.1 times more frequent in clearcutsthan in forested terrain. The combination of terrainvariables in the Himalaya, however, are not foundelsewhere in the world. Our observations were thatterraces can serve to stabilize slopes, especially with‘kari’ type terraces that include a berm on the out-

Ž .side edge of the terrace a bund to control downs-lope water movement. This finding confirms the

Ž .observations of Kienholz et al. 1984 .

4. Conclusions

This study has identified a few of the key terrainand land use variables that can explain the spatialdistribution of scars from mass movements in thecentral Nepal Himalaya. Some indication has beenprovided of just how dominant mass movement is asa modern geomorphic process in the evolution ofhillslopes in the region. The study demonstrates thathuman activities do not account for a disproportion-ate share of mass movement, contrary to a largenumber of references in the scientific literature andin the popular media linking deforestation with massmovement. Deforestation is occurring, although thestyle and extent varies from one region of Nepal tothe next. At the same time, devastating mass move-ment is occurring, but the great leap in logic linkingthese two phenomena cannot be supported by thedata in this study area.

Acknowledgements

Assisting in the liaison with the Nepal govern-ment and providing other help and advice scientifi-cally and logistically were: Dr. Bidhur Upadhyay,Head of the Department of Meteorology, TribhuvanUniversity; Professor Suresh Chalise, Dean of Sci-ence at Tribhuvan University and currently a consul-tant to ICIMOD; Shailesh Chandra Singh, ExecutiveDirector of the National Council of Science andTechnology in Nepal; Dr. Allen Bassett, geologist;

Dr. David Wilson, Mission Director of US AID inNepal; Lew McFarlane, Charge d’Affairs in theAmerican Embassy in Kathmandu; and Major RobinMarston and Dr. Lute Jerstad of Mountain TravelNepal. Support for the expedition was provided bythe Foundation for Glacier and Environmental Re-search, Pacific Science Center, Seattle, WA; theUniversity of Idaho via the Glaciological Instituteand American–Nepal Education Foundation; and theWyoming Water Resources Center and College ofArts and Sciences at the University of Wyoming.This manuscript was improved by the suggestions of

Ž .John R. Giardino Texas A&M University , Dr.Ž .Jean-Paul Bravard Universite Paris IV—Sorbonne ,´

Dr. John F. Shroder, Jr., and anonymous reviewers.

References

Bishop, B.C., 1990. Karnali under stress: livelihood strategies andseasonal rhythms in a changing Nepal Himalaya. GeographyResearch Paper Nos. 228–229. University of Chicago,Chicago, IL, 460 pp.

Bruijnzeel, L.A., Bremmer, C.N., 1989. Highland–lowland inter-actions in the Ganges-Brahmaputra River basin: a review ofpublished literature. ICIMOD Occasional Paper No. 11, 136pp.

Brunsden, D., Jones, D.K., Martin, R.P., Doornkamp, J.C., 1981.The geomorphological character of part of the low Himalayaof eastern Nepal. Z. Geomorphol. Suppl. Bd. 37, 25–72.

Caine, N., Mool, P.K., 1982. Landslides in the Kolpu Kholadrainage, middle mountains, Nepal. Mountain Res. Dev. 2,157–173.

Carson, B., 1985. Erosion and sedimentation processes in theNepalese Himalaya. ICIMOD Occasional Paper No. 1. Interna-tional Centre for Integrated Mountain Development: Kath-mandu, Nepal, 39 pp.

Colchen, M., Le Fort, P., Pecher, A., 1980. Geologic research inthe Nepal’s Himalaya: Annapurna–Manaslu-Ganesh Himal.CNRS, Anatole, France, 136 pp.

Costa, J.E., Schuster, R.L., 1988. The formation and failure ofnatural dams. Geol. Soc. Am. Bull. 100, 1054–1068.

Heuberger, H., Masch, L., Preuss, E., Schrocker, A., 1984. Qua-ternary landslides and rock fusion in central Nepal and in theTyrolean Alps. Mountain Res. Dev. 4, 345–362.

HMGrDMG, 1980. Geological Map of Nepal. Department ofMines and Geology, Kathmandu, Nepal, 1:1,000,000.

HMGrWESC, 1987. Erosion and sedimentation in the NepalHimalaya. Report No. 4r3r010587r1r1, His Majesty’s Gov-ernment of Nepal, Water and Energy Commission Secretariat,Ministry of Water Resources, Kathmandu, Nepal.

Ives, J.D., Messerli, B., 1989. The Himalayan Dilemma: Reconcil-ing Development and Conservation. The United Nations Uni-versity and Routledge, London, 295 pp.


Johnson, K., Olson, E.A., Manandhar, S., 1982. Environmentalknowledge and response to natural hazards in mountainousNepal. Mountain Res. Dev. 2, 175–188.

Kienholz, H., Hafner, H., Schneider, G., 1984. Stability, instabil-ity, and conditional stability mountain ecosystem conceptsbased on a field survey of the Kakani area in the middle hillsof Nepal. Mountain Res. Dev. 4, 55–62.

Laban, P., 1979. Landslide occurrence in Nepal. Phewa TalProject Report No. SPr13. Integrated Watershed ManagementProject, ICIMOD, Kathmandu, Nepal.

Mool, P.K., 1995. GIS and remote sensing application in slopeinstability study. Paper presented at the Regional Workshop onLandslide Hazard Management and Control in the Hindu KushHimalayas, 12–14 July, Kathmandu, Nepal.

Sarkar, S., Kanungo, D.P., Mehrotra, G.S., 1995. Landslide haz-ard zonation: a case study in Garhwal Himalaya, India. Moun-tain Res. Dev. 15, 301–309.

Schelling, D., 1987. The geology of the Rolwaling-Lapchi KangHimalayas, east-central Nepal: preliminary findings. J. NepalGeol. Soc. 4, 1–19.

Schramm, J.-M., Weidinger, J.T., Ibetsberger, H.J., 1998. Petro-logic and structural controls on geomorphology of prehistoricTsergo Ri slope failure, Langtang Himal, Nepal. Geomorpho-logy 26, 107–121, this issue.

Sidle, R.C., Pearce, A.J., O’Loughlin, C.L., 1985. Hillslope stabil-ity and land use. Water Resour. Monogr. Ser. 11. AmericanGeophysical Union, Washington, DC, 140 pp.

Stevens, S.F. 1993. Claiming the High Ground: Sherpas, Subsis-tence, and Environmental Change in the Highest Himalaya.University of California Press, Berkeley, CA, 537 pp.

Watanabe, T., Dali, L., Shiraiwa, T., 1998. Slope denudation andthe supply of debris to cones in Langtang Himal, CentralNepal Himalaya. Geomorphology 26, 185–197, this issue.

Documents

Geoecology and mass movement in the Manaslu … et al...Geoecology and mass movement in the Manaslu-Ganesh and Langtang-Jugal Himals, Nepal R.A. Marston a,), M.M. Miller b, L.P. Devkota