Contributorsgilles-arnaud-fassetta.org/wp-content/uploads/2018/... · 7.1 Catastrophic landslides: definition, modes of emplacement and geomorphic significance 7.1.1 Definition

C:/ITOOLS/WMS/CUP/556791/WORKINGFOLDER/ALA/9780521769259LOC.3D x [10–12] 26.9.2009 6:13PM

Contributors

Irasema Alcántara-AyalaInstituto de Geografía, UNAMCircuito Exterior, Ciudad Universitaria04510, Coyoacán, México, D.F.México

Gilles Arnaud-FassettaUniversité Paris-DiderotCase Postale 700175205 Paris Cedex 13France

Perry BarteltAvalanches, Debris Flows and Rockfall Research UnitWSL Institute for Snow and Avalanche Research SLFFlüelastrasse 11CH-7260 Davos DorfSwitzerland

Gerardo BenitoLaboratorio de Geomorfología e HidrologíaCentro de Ciencias Medioambientales, CSICSerrano 115 dup.28006, MadridSpain

John BoardmanEnvironmental Change Institute,SoGE, South Parks RoadOxford, OX1 3QYUK

Lisa BorgattiDipartimento di Ingegneria delle Strutture, dei Trasporti,delle Acque, del Rilevamento, del TerritorioAlma Mater Studiorum Università di BolognaViale Risorgimento, 241036 BolognaItaly

William B. Bull6550 N. Camino KatrinaTucson, AZ, 85718-2022USA

Michael BründlWarning and Prevention Research UnitWSL Institute for Snow and Avalanche Research SLFFlüelastrasse 11CH-7260 Davos DorfSwitzerland

Etienne CossartUMR Prodig 8586 – CNRSUniversité Panthéon-Sorbonne (Paris 1)2 rue Valette,75005 ParisFrance

Michael CrozierVictoria University of WellingtonSchool of Geography, Environment and Earth SciencesPO Box 600WellingtonNew Zealand

Monique FortUniversité Paris Diderot (Paris7)Département de GéographieUFR GHSS, Case 700175205 Paris Cedex 13France

Thomas GladeDepartment of Geography and Regional ResearchUniversity of ViennaUniversitaetsstr. 7A-1010 ViennaAustria

Andrew S. GoudieSt Cross CollegeSt GilesOxford, OX1 3LZUK

x

C:/ITOOLS/WMS/CUP/556791/WORKINGFOLDER/ALA/9780521769259LOC.3D xi [10–12] 26.9.2009 6:13PM

Avijit GuptaSchool of GeographyUniversity of LeedsLeeds, LS2 9JTUK

Francisco GutiérrezUniversidad de ZaragozaDepartamento de Ciencias de la TierraEdificio GeológicasC/. Pedro Cerbuna, 1250009 ZaragozaSpain

David HiggittNational University of SingaporeDepartment of GeographyAS2, #03-011 Arts Link, Kent Ridge117570 Singapore

Paul F. HudsonDepartment of Geography and the EnvironmentUniversity of Texas at AustinAustin, TX 78712USA

Gabi HufschmidtSchool of GeographyVictoria University of WellingtonP.O. Box 600WellingtonNew Zealand

Margreth KeilerDepartment of Geography and Regional ResearchUniversity of ViennaUniversitaetsstr. 7A-1010 ViennaAustria

Molly McGrawSoutheastern Louisiana UniversitySLU-10686Hammond, LA 70402USA

David PetleyDurham UniversityDepartment of Geography Science LaboratoriesSouth RoadDurham, DH1 3LEUK

Jürg SchweizerWarning and Prevention Research UnitWSL Institute for Snow and Avalanche Research SLFFlüelastrasse 11CH-7260 Davos DorfSwitzerland

Olav SlaymakerUniversity of British ColumbiaDepartment of Geography1984 West MallVancouver, V6T 1Z2Canada

Mauro SoldatiDipartimento di Scienze della TerraUniversità di Modena e Reggio EmiliaLargo S. Eufemia, 1941000 ModenaItaly

Jean-Claude ThouretLaboratoire Magmas et Volcans UMR 6524 CNRS etOPGCUniversité Blaise Pascal Clermont II5 rue Kessler63038 Clermont-Ferrand cedexFrance

Cees J. van WestenInternational Institute for Geo-Information Science andEarth Observation (ITC)Hengelosestraat 99PO Box 67500 AA EnschedeThe Netherlands

Heather VilesSchool of Geography and the EnvironmentSouth Parks RoadOxford, OX1 3QYUK

Harley J. WalkerDepartment of Geography and AnthropologyLouisiana State UniversityBaton Rouge, LA 70803USA

Yang XiaopingInstitute of Geology and GeophysicsChinese Academy of SciencesP.O. Box 9825Beijing 100029China

xiContributors

C:/ITOOLS/WMS/CUP/556791/WORKINGFOLDER/ALA/9780521769259C07.3D 75 [75–86] 26.9.2009 5:35PM

7 Catastrophic landslides and sedimentarybudgets

Monique Fort, Etienne Cossart, and Gilles Arnaud-Fassetta

Landslides are a dominant geomorphic process affectingmountain slopes worldwide (see also Chapters 6 and 8).They represent a major sediment source that can supply alarge amount of unstable debris to river channels and mayaffect the fluvial sediment yield. A distinctive part of thegeomorphic evolution of active mountain belts, catastrophiclandslides generally develop very rapidly so that they areamong the most powerful natural hazards on Earth. By tem-porarily or persistently impounding river channels, they delayor block the delivery of sediments, affect the (dis)continuity ofthe cachment-scale sediment cascade, and exert a control overthe fluvial valley systems (Hewitt, 2002; Korup et al., 2004).As a consequence, landslides may generate indirect hazardsalong fluvial systems, so that they represent a major threat tosettlements, infrastructures and catchment management thatmay be felt over long distances from the unstable area (Plafkerand Eriksen, 1978; Li et al., 1986; Costa, 1991; Korup, 2005;Hewitt et al., 2008). Their magnitudes, together with theircausative factors, suggest the difficulty, if not theimpossibility, of preventing and/or to predicting them.In this brief overview, we shall firstly define catastrophic

landslides and their geomorphic impacts, with specialattention given to landslide-induced dams; we shall moveon to their influence on sediment budgets, at local andbasin-wide scales, before eventually considering a fewactions that may help to minimize the vulnerability andrisks for the potentially affected populations.

7.1 Catastrophic landslides: definition,modes of emplacement andgeomorphic significance

7.1.1 Definition

The term landslide covers a large array of features.Catastrophic landslides (also referred to as super-large or

giant landslides in the literature) are considered here as lowfrequency, massive rock slope failures characterized bytheir magnitude, the rapidity of their emplacement and bytheir spatial and temporal impacts on geomorphic systems.More specifically, the magnitude of catastrophic landslidesrefers either to the depth of the sliding place, to the scar areaor the total disturbed area, including the deposition zone, orto their volume (Table 7.1). The displaced material gener-ally covers at least five orders of magnitude between 105

and 1010 m3 (Evans et al., 2006; Hewitt et al., 2008).Determination of landslide volumes can be assessed

directly from field data. Alternatively, at a basin-widescale, digital elevation models (DEMs) are useful tools butthey require good positional accuracy (Korup, 2005). Aerialphotomeasurements (basedmostly on the total affected area)allow the production of an exhaustive inventory, and timeseries analysis, yet their use necessitates photogrammetricmodelling and ground truthing; also, the fast recovery by thevegetation in some (tropical) mountains may lead to anunderestimate of the final area/volume (Brardinoni andChurch, 2004; Koi et al., 2008) and introduce biases inmagnitude/frequency estimation. Despite this limitation, anumber of analyses of landslide magnitude/frequency rela-tions have shown they are scale invariant and they obey apower law of the general form N(A) ≈ A−b, where A islandslide area, N(A) the number of events of greater than agiven volume, and b is a constant (Hovius et al., 2007). Thisequation is used to quantify the distribution of landslides inspace and time, and hence their density and recurrence.

7.1.2 Occurrence and modes of emplacementof catastrophic landslides

Landslide occurrence depends on various controls: climate,slope steepness, relief amplitude, bedrock geology, failureplane orientation, vegetation and landuse cover, etc.

Geomorphological Hazards and Disaster Prevention, eds. Irasema Alcántara-Ayala and Andrew S. Goudie. Published byCambridge University Press. © Cambridge University Press 2010. 75


TABL

E7.1Selected

catastrophiclandslidesofthetwentiethcenturyandtheirimpacts

Year

Location

Landslid

evo

lume(m

3)Trigg

erIm

pacts

Com

ments

1911

Usoirockslid

e(Tadjik

istan)

2.2×10

9M

7.4Pam

irearthq

uake

54killed;

LakeSarez,the

largestn

atural

land

slidedam

intheworld

(>60

kmlong

,>50

0m

depth,

volume17

×10

9m

3)across

theMurgabRiver

Con

cernsabou

tapo

ssiblefailu

reof

thedam,

thatwou

ldaffect>5millionpeop

lealon

gthe

Bartang

–Pyanj–AmuDariawatershed

1933

Diexi,S

ichu

anprov

ince

(China)

150×10

6M

7.5earthq

uake

>2,40

0killed;

dam

oftheMin

River

for45

days

(lakevo

lume:40

0×10

6m

3)

Drainingof

thelake

byov

erspilling;

thefloo

dprop

agated

>25

0km

downstream

(average

velocity

5.5–

7m/s)

1959

Malpasset,F

réjus

(France)

30×10

6Heavy

precipitatio

n42

3killed;

railw

aydamaged

alon

g2.5km

;50

farm

sdestroyed

Inadequateartificialdam,builtagainstw

eaken

bedrock(foliatedandtecton

ized

schists+

non-identified

faults)

1963

Vaion

trockslid

e(Italy)

270×10

6Precipitatio

n+high

water

levelfollowed

byrapidlake

draw

down

2000

killed;

rockslidefailedin

the15

0×10

6

m3Vaion

treservo

ir,resultin

gin

acatastroph

icfloo

d(city

ofLangarone

badly

damaged,tog

etherwith

4othervillages)

A10

0m

high

waveof

water

overtopp

edthe

doub

lycurved

arch

ofhy

droelectricpo

wer

dam;p

ossiblereactiv

ationof

arelict

land

slide;brittlefailu

reof

claysin

depth

1967

Tangg

udon

gdebrisslide,

Sichu

an(China)

68×10

6Dam

oftheYalon

gRiver;lakevo

lume60

0×

106m

3Catastrop

hicfailu

reof

thedam

1970

NevadoHuascaran

rock

debris/avalanche

(Peru)

30–50

×10

6M

7.7earthq

uake

>18

,000

killed;

city

ofYun

gaydestroyed

Average

velocity28

0km

/hr;samepeak

already

affected

bythesametype

offailu

rein

1962

(4,000

–5,00

0killed)

1980

Mou

ntStH

elensdebris

avalanche(U

SA)

2.8×10

9Volcanicerup

tion

5–10

killed,

mostp

eopleevacuated;

destructionof

infrastructure;SpiritL

ake

(258

×10

9m

3)dammed

Rotationalrockslide,follo

wed

by23

kmlong

debrisavalanche;averagevelocity

125km

/hr;d

amartificially

stabilizedby

anou

tlet

tunn

el19

83Thistledebrisslide

(USA)

21×10

6Sno

wmeltand

heavyrain

Destructio

nof

infrastructure;d

amof

the

Spanish

ForkRiver;lakevo

lume78

×10

6

m3

Heavy

econ

omiclosses

($US60

0million);

lake

perm

anently

drainedby

human

interventio

n19

83SaleMou

ntainland

slide,

Gansu

Province

(China)

30×10

6Noob

viou

strigger

237killed,

3villagesdestroyed;

toeof

the

displacedmassacross

the80

0mwidevalley

oftheBaxieriver

Mud

ston

eandloessmaterial;compo

site

land

slide,with

prog

ressivefailu

reand

increasing

velocity

upto

20m/s

1984

Mou

ntOntake(Japan)

36×10

6M

6.8earthq

uake

Traveld

istanceof

13km

downto

adjacent

valleys

Failure

ofthesoutheastern

flankof

MtO

ntake

volcano,

transformed

into

alahar(estim

ated

volume56

×10

6m

3);velocity

22–36

m/s


1985

Bairaman

debris

avalanche(Papua

New

Guinea)

180×10

6M

7.1earthq

uake

210m

high

land

slidedam;lake(volum

e50

×10

6m

3)artificially

drained;

villagers

evacuated

Debrisflow

volume12

0×10

6m

3;average

velocity

20km

/hr;affected

39km

Bairaman

River

downto

Salom

onSea

1985

Catastrop

hiclahar,

NevadodelR

uiz

(Colum

bia)

90×10

6Sno

wmeltlahar

triggered

byvo

lcanicerup

tion

>23

,000

killed(buriedcity

ofArm

ero,

40km

distant);smalleruption(<5×10

6m

3magma

ejected),butlargeam

ountof

meltwater(38–

44×10

6m

3)released

in20–90

min

Lahar

velocity

intheLagun

illas

valley,50

to80

km/hr;peak

flow

velocities5–15

m/s;

laharreached10

4km

in4ho

urs;econ

omic

losses

($US1billion

);form

erdestructive

laharin

1845

,with

sameareasaffected

1987

ValPolaland

slide(Italy)

40×10

6Cum

ulativeand

unusually

severe

rainfall

27killedanddestructionof

manybu

ildings

($US40

0million);d

ammingof

theAdd

aRiver

valley

The

rock-valancheproduced

awaveof

muddy

water

2.7km

upstream

;velocity

248–31

0km

/h;spillw

aytunn

elconstructed

1991

Randa

rockfall

(Switzerland

)20

×10

6Meltin

gof

perm

afrost?

Dam

oftheVispa

River,fl

ooding

oftheRanda

village

Con

tinuo

usrockfalllastingseveralh

ours,w

ithexpo

nentialaccelerationbefore

final

mov

ement

1993

LaJoseph

inaland

slide

(Ecuador)

20–44

×10

6Rainfall

Dam

ofthePauteRiver;lakevo

lume17

7×10

6

m3;d

rained

outcatastrop

hically

;71killed,

mostp

eopleevacuatedbefore

lake

breako

ut

Flood

prop

agated

>60

kmdo

wnstream;p

eak

discharge:9,00

0–14

,000

m3/s;1

3×10

9m

3

solid

load;v

elocity

5–20

m/s;h

eavy

econ

omiclosses

($US14

7millions)

1999

Malpa

rockfallanddebris

flow

(Ind

ia)

1×10

6Highintensity

rainfall

221killed;

Malpa

River

dammed,followed

byan

outburstfloo

dFormerland

slides

atthesamesite;proximity

oftheMainHim

alayan

ThrustF

ault

1999

MtA

dams,Sou

thWestland

(New

Zealand

)

10–15

×10

6Noob

viou

strigger

Dam

form

edin

thelowerPoeruaRivergo

rges;

massive

fanh

eadaggradationatthe

mou

ntainrang

efron

t

Outbu

rstfl

oodwith

inlessthan

oneweek(peak

discharge1,00

0–3,00

0m

3/s);persistent

postfailu

reaggradation

2000

Yigon

gdebris

slideanddebrisflow

,TibetProvince(China)

300×10

6Excessive

rainfalland

snow

-meltw

aters

Landslid

edam

oftheYigon

gRiver;n

atural

breach

caused

catastroph

icfloo

d(peak

discharge12

0,00

0m

3/s);loss

ofprop

erty

andinfrastructure

downstream

toIndia

Failure

very

similarto

the1900

Yigong,

large

(5.1×10

8)land

slideandresulting

dam

and

lake

2007

Guinsaugo

nrockslide–

debrisavalanche,

LeyteIsland

,Philip

pines

15×10

9Nospecifictriggerbu

theavyrainfallin

the

precedingdays

Disintegrationof

sliddenmass;average

thickn

essof

debris10

m;run

outd

istance

3,80

0m;>

1,00

0casualtieswhen

Guinsaugo

nvillage,located

onflatvalley

floo

r,was

buried

under4–7m

deep

debris.

Failure

of45

0m

high

,forestedrock

slop

elocatedwith

inactiv

ePhilip

pine

faultzon

e(sheared

andbrecciated

rocks);run

out

distance

enhanced

byfrictio

nredu

ction

whendebrisranov

erfloo

dedpadd

yfields;

estim

ated

andsimulated

meanvelocity

27–

38m/sand35

m/srespectiv

ely.


(Cruden and Varnes, 1996; Dikau et al., 1996). The world-wide distribution of catastrophic landslides shows a goodcorrelation with mean local relief greater than 1000m(which makes up over 5% of Earth’s land surface; Korupet al., 2007), with seismo-tectonically active mountains(e.g. Central and High Asia, New Zealand Alps, CoastalRanges) or volcanic belts (Pacific Rim), and with recentlydeglaciated mountain slopes (Evans and Clague, 1994).A large number of catastrophic rock failures consist of

rock avalanches (or sturzstroms; Hsü 1975) and/or rock-slides, even if other processes such as translational rock-slides, rotational slumps or spreads cannot be excluded.The mode of emplacement of rock avalanches can be verycomplex: it generally involves an initial failure from steepmountain walls of cohesive rock mass that descends severalhundreds or thousands of metres and is disintegrated andcrushed, a process that gives great momentum to the result-ing debris (Hewitt, 2002). The nature and extent of theresulting deposits depend on the geological setting andvalley morphology (Costa and Schuster, 1988). In topo-graphically confined settings, the deposits may pile up onand run up the opposing slope, or split into separate lobesboth upstream and downstream of the impacted slope, allsituations that favour the blockage of the valley and theformation of a lake. Alternatively, wherever there is nospatial confinement, the crushed rocks may travel longdistances and evolve, in the presence of water (or snow,or ice), into giant debris flows (Plafker and Eriksen, 1978;Fort, 1987; Shang et al., 2003). These failures may alsoincorporate a variety of earth materials entrained in theirpath, and the runout process may affect, or be affected by,the type of substrate (Hewitt, 2002; Schneider et al., 2004).Earthquakes and/or high intensity precipitation are the

most efficient triggering factors for rapid collapse. Thefailure site may be controlled by major tectonic lines(Figure 7.1; Fort, 2000), or be considered as a direct adjust-ment to high rates of rock uplift and correlated river inci-sion (Burbank et al., 1996). The failure may be preceded bya phase during which slow, deep-seated deformation leadsto the opening of tension cracks, weakening of the shearstrength of the rockmass, and eventually to high pore-waterpressures. Other unusual mechanisms such as acoustic flu-idization during sliding, or internal, self-accelerating rockfracture (Kilburn and Petley, 2003) may also favour large-scale rock slope failures (Hewitt et al., 2008). Additionally,anthropogenic actions may be the cause of large hillslopedestabilization (e.g. Vaiont reservoir; Table 7.1).These very large rock mass failures are generally con-

sidered as a major denudational process of active orogens,and as formative events influencing landscape development(Brunsden and Jones, 1984; Fort, 1988, 2000; Hewitt,

1988, 1998, 2002; Fort and Peulvast, 1995; Burbanket al., 1996; Korup et al., 2004, 2007). More specifically,they give rise to complex sedimentary assemblages and tospecific constructional and erosional landforms that create‘interrupted valley landsystems’ distinctive of theselarge-scale landslides (Hewitt, 2006).

7.2 Geomorphic impacts of catastrophiclandslides

Catastrophic landslides have direct geomorphic impacts atthe local, basin-wide and mountain-belt scales, as synthe-sized by Costa and Schuster (1988), Hewitt (2002, 2006)and Korup (2005). We want to stress here local and regionalimpacts, which are the most meaningful in terms of hazardsand risks for the population. Geomorphic impacts and theirconsequences on water and sediment fluxes and budgetswill be appreciated in considering different interactionsbetween landslides and fluvial systems.

7.2.1 Interactions with river systems

Three different situations are illustrated: partial blockage ofthe valley by the landslide, complete damming andupstream water ponding, and catastrophic collapse of thelandslide dam (Figure 7.2).In the case of partial blockage (Figure 7.2, case 1), the

landslide mass forces the river to divert its course to theopposite bank; this in turn leads to a change in transverseand longitudinal channel geometry, channel pattern andmorphology. Wherever the opposite bank consists of softmaterial (slope or alluvial deposits), this diversion triggersbank erosion and further destabilizes the entire hillslope(Figure 7.2B). Upstream of the landslide mass, which actsas the local base level, braiding of the river and aggradationpredominate. These may indirectly increase overflow andchannel avulsion-shifting frequencies, and flood hazardsfor the adjacent settlements (Figure 7.2A). Across the land-slide mass, the narrower cross section of the river favourserosion of the landslide debris and increases sedimentfluxes downstream, whereas the larger blocks exceedingthe competence of the stream power form a debris lag,which armours the channel bed and prevents further erosionduring regular high flows.When the volume of the landslide is sufficient to block

the valley entirely (Figure 7.2, case 2 and Figure 7.2D), alake will instantaneously start filling up, whereas the down-stream part is starved of water. Lake depth depends onvalley geometry and landslide height; the latter may reachhundreds of metres (e.g. Lake Sarez; Table 7.1). Landslidedams may be ephemeral (a few minutes to a few hours), or

Monique Fort et al.78


FIGURE 7.1. The large, prehistoric Dhumpu–Kalopani rock avalanche and its morpho-sedimentary impacts (Nepal Himalayas). Thecollapsedmass failed along theNorth HimalayanDetachment Fault (NHDF) and blocked durably the Kali Gandaki River, which had carvedthe deepest gorges in the world across the Annapurna (8091m) and Dhaulagiri (8172m) ranges. The 23 km long, >200m deep Marphalake developed and filled in with sediments brought from upstream and from glaciated tributary valleys. The braided pattern of the rivercourse reflects the still persisting role of the rock-avalanche barrier in the denudational evolution of the mountain.

79Catastrophic landslides and sedimentary budgets


short-term (a few weeks or months), or persistent (severalthousand years), a duration that directly influences theirpotentially catastrophic nature and their control on sedi-ment fluxes and budgets (Figure 7.1 and Figure 7.2E). Damlongevity is a function of its stability, which depends on thesize and shape of the dam, the characteristics of the geo-logical material composing the dam (material properties,grain size distribution), the volume and rate of water andsediment inflow to the newly formed lake, and the rate ofseepage through the dam (Costa and Schuster, 1988).Water ponding occurs up to the dam height, resulting inupstream backwater flooding that can be damaging forsettlements and infrastructures (Figure 7.2C). The rapidityof the water level rise depends on the river inflow versusthe size and shape of the inundated valley; it generallyleaves enough time for evacuation of the threatenedpopulations to take place.In contrast, catastrophic downstream flooding

(Figure 7.2, case 3) will occur following a rapid failure ofthe landslide dam, caused either by overtopping and imme-diate retrogressive incision across the dam (Figure 7.2F), orby increasing shear stress because of seepage at the base ofthe dam (that can be reinforced by the weight of theimpounded water mass upstream), or by displacementwaves triggered by landslide failures directly into thelake. The catastrophic nature of the (flash)flood, involvingthe release of a large amount of sediments and water, isrecorded by extreme peak discharges (usually back-calculated from field surveys and hydraulic equations)and high velocities that permit the transport of sediments

derived from the dam and from the upstream lacustrinereservoir. The geometry and extent of the coarse debriswedge formed immediately downstream of the dam(Figure 7.2G) suggest both the competence of the debrisladen waters (often in the form of debris flows) and theirrapid evolution in both time and space into hyper-saturatedflows and then into ‘normal’ high flows, in relation toprogressive deposition of the coarser material in the floodplain (Figure 7.2H).The catastrophic draining of a landslide-dammed lake

and associated flooding may create secondary impacts.Firstly, new landslides may develop, both upstream of thelake, following the rapid drawdown of the water, and acrossand downstream of the dam, where the propagation of theflood contributes over very large distances to a sudden riseof water, bank undercutting and hillslope instabilities(e.g. Yigong debris flow; Table 7.1), which may in turngive rise to new dams and flooded areas. Secondly, specificaggradational landforms may develop, such as ‘barrier-defended’ terraces upstream of the dam, with the sedimen-tary facies reflecting the architecture and sequences ofdebris inputs from the trunk river and adjacent tributariesand slopes (Hewitt, 2002). Thirdly, a distinctive morphol-ogy of ‘interrupted valleys’ appears as a legacy of pastcatastrophic events, consisting of partial obstructions bylandslide barriers (e.g. boulder accumulations resting inthe middle of river channels) or in persistent impoundmentsthat ‘create a chronically fragmented drainage system’

(Hewitt, 2006). This is the case for the Dhampu–Chooya–Kalapani rock avalanche that dammed the upper Kali

FIGURE 7.2. Types of geomorphic impacts of landslide dams on hydrosystems.



Gandaki valley (Nepal Himalaya) some 70,000 years agoand is still influencing the bed profile and planform pattern(braiding) of the river channel (Fort, 2000) (Figure 7.1).This influence is related not only to the stability of the dam,but also depends on the stream power of the river beingsufficient to breach the dam. This is a function of thelongitudinal profile, and of the liquid and solid dischargeof the river (parameters controlled by climate, autocyclicprocesses, slope and tectonics).

7.2.2 Impacts on sedimentary fluxesand budgets

In the frame of sedimentary budgets, landslide masses forma particular type of sediment store, characterized by theirvolume and calibre of materials, and by their distance fromthe channel (Slaymaker, 2006). As already mentioned,large landslides impact adjacent channels and valleys,either by accelerating, or slowing down, or even interrupt-ing, the conveyance of sediment to the downstream reser-voirs. These different impacts may occur successively intime, depending on the magnitude of the landslides andtheir capacity to act or not as an efficient barrier; they mayrandomly affect the functioning of the sediment fluxes, andcontribute to the formation of a cascading sequence ofintramontane storage units (Korup, 2005).

Local scaleAn illustration is provided by the sedimentary budget ofthe upper Cerveyrette catchment (Southern French Alps).This was directly influenced by a series of earth-flows thatdeveloped after glacial retreat, and was caused by thecombined effects of post-glacial debuttressing and lique-faction of the serpentinite bedrock, in a context of seismicactivity (Cossart and Fort, 2008) (Figure 7.3A). The larg-est slide-earth flow of the Chenaillet (c. 3 × 107m3; 3.5 kmlong) efficiently dammed the valley, so that the BourgetPlain developed in response to the impoundment.Longitudinally, the trap was infilled with alluvial gravelsand silts fed by alpine periglacial and/or still-glaciatedhillslopes. This debris accumulated as a prograding fandelta encroaching upon the lake that developed in thedistal part of the valley, closest to the dam. Lacustrinedeposits are interfingered with colluvial debris derivedfrom the steep slopes of Mount Lasseron as scree andavalanche cones.The sediment budget was estimated by combined field

surveys, DEM and GIS approaches (Figure 7.3B; Cossartand Fort, 2008). The sediment stored in the Bourget Plainrepresents approximately the same volume (22.3 × 106m3)as the Chenaillet landslide dam (21 × 106m3), whereas

debris still blankets the hillslopes (18.4 × 106m3) withoutreaching the Bourget Plain. This store developed betweenthe Late Pleistocene and c. 5,000 BP; it first interrupted,then considerably reduced the sediment fluxes downstream.As soon as the blockage of the valley occurred, the down-stream part of the Cerveyrette torrential river started adjust-ing its longitudinal profile by retrogressive erosion. Thepresent situation corresponds to a total sediment removaland export of only 106m3 (c. 1/60 of the debris). However,sporadic dissection of the landslide dam triggered byextreme meteorological events is directly threatening theCervières village, as during the 1957 >100-years recurrenceflood that destabilized the entire hydrosystem. This exam-ple shows how, in alpine headwater contexts, landslidedams are persistent features controlling the sediment fluxeslong after their occurrence.

Regional scaleLarge landslides may contribute to a significant increasein sediment yield in a very short period following failure,not only by direct massive input of landslide debris intothe fluvial system, but also by secondary hillslope pro-cesses that rework the landslide material before vegetationregrowth. Such landslide-derived sediment pulses resultin aggradation in the downstream reaches from the land-slide site, and are often accompanied by metamorphosisand/or change in the river course that may induce off-sitehazards and damaging impacts to downstream settlementsand infrastructures. For instance, Korup et al. (2004)calculated that the 1999 Mount Adams rock avalanche(New Zealand) produced a specific sediment yield inexcess of approximately 75,700 ± 4,600 t km−2 a−1, asexpressed by the massive fanhead aggradation and theopening of a major avulsion channel at the mountainrange front. This sediment yield represents a sedimentdischarge of 2.5 × 106 m3 a−1 calculated for the threeyears following failure, an amount that rapidly declinedafter the event. In the Nepal Himalayas, Fort (1987)described a giant (>4 × 109 m3) collapse of the southface of the Annapurna IV peak that occurred about 500years ago, and filled the 35 km distant Pokhara valleyunder a 60 to 100m thick gravel aggradation, whichblocked and caused the flooding of adjacent tributaryvalleys. The calculated annual contribution ofAnnapurna rockslide-derived sediment is in the order of4 × 106 m3 a−1 and represents, for the upper catchment, asediment yield of 22,860m3 a−1 km−2 (Fort, 1987; Fortand Peulvast, 1995). This is a figure averaged over a500-year period, which in fact is exceptionally high ifone considers the fact that the aggradation took place ina very short time, i.e. ‘instantaneously’ after the failure.



7.3 Forecasting and preventing

Catastrophic rock failures are generally preceded by peri-ods of accelerated creep, which may result in observableslope deformation and ‘sackung’ related features: develop-ment and widening of tension cracks, buckling, increasedrockfall activity and toppling, or break-out across bedding.Measured stress drops in crustal rock, and related fracturing

could provide a physical basis for quantitatively forecastingcatastrophic slope failure (Kilburn and Petley, 2003).However, monitoring of all potential threatening slopes isnot realistic, especially in developing countries where otherpriorities (food, shelter, employment) come well ahead ofunpredictable natural hazards.Other forecasting methods are based on statistical

assessment of landslide susceptibility; they rely on a series

FIGURE 7.3. Sedimentary budget assessment in response to the Chenaillet earthflow dam (French Southern Alps). (Modified andcompleted after Cossart and Fort (2008.)



of variables (landslide inventory with the help of GIS,terrain predisposing factors, neo-predictive variableswith a geomorphological meaning) that are tested (simu-lations) and eventually evaluated by expert judgement(Thiery et al., 2007). Another similar approach is basedon geotechnical modelling and 3-D model calibration setup from a known and well-studied event (quantitativedata), as was done after the 1987 Val Pola landslide, oneof the most destructive and costly natural disasters that hasoccurred in Italy during recent decades (Costa, 1991;Crosta et al., 2004). In their study, Crosta et al. (2004)showed how modelling techniques help one to understandthe rheology of such a failure, and to predict its timing inhighlighting the transformation of potential energy tokinematic energy.The power law curve of landslide magnitude/frequency

can also be used for landslide hazard assessment (Evanset al., 2006), and as input into a quantitative risk calcu-lation if combined with vulnerability data, as was done toassess rockfall risk along transportation corridors (Hungret al., 1999).To prevent the failure of landslide dams and resulting

catastrophic floods, the most commonly used controlmeasure consists of the construction of spillways eitheracross the landslide crest or across the adjacent bedrock.Alternatively, drainage by siphon pipes, pump systems ordiversion tunnels (Val Pola, Mount St Helens, Bairaman;Table 7.1) are short-term measures to control lake level.More radical methods consist in large-scale blasting, aswas done across the landslide dams that were formed nearBeichuan city after the 18 May 2008 Sichuan earthquakein China. However, the disaster may occur beforeadequate control measures can be completed (more par-ticularly in remote and rugged areas that render transportof heavy equipment very difficult), or because high rapidinflow to the impoundment often exceeds general predic-tions and causes an early dam failure (Shang et al., 2003).In the case of Lake Sarez and the related Usoi landslidedam (Tadjikistan), the highest dam on Earth, and despiteextensive observations and technical studies that suggest asatisfactory stability of the dam against sliding, the possi-bility of a catastrophic outburst flood that would destroythe many villages and infrastructures of the Bartang–Pyang–Amu Darya catchment, situated between the lakeand the Aral Sea, cannot be entirely ruled out (>5 millionpeople would be affected). Combined measures are thusbeing implemented, consisting of (i) the monitoring of thestability of the dam and slopes surrounding the lake, (ii) anearly warning system to alert inhabitants of the upper AmuDarya valley, together with (iii) the modelling of a seriesof flood scenarios to determine the degree of risk and

vulnerability of downstream villages and infrastructures(Alford and Schuster, 2000). Whatever these measures, asimple doubling of the present mean streamflow volumeof 2000m3 s−1 of the Bartang River would readily destroylarge portions of existing roads, low-lying villages andagricultural land for more than 100 km downstream fromthe dam, whereas a lake outburst flood would generate aninstantaneous, catastrophic peak flow of about one millioncubic metres per second (Alford and Schuster, 2000).

7.4 Conclusions

Despite their low frequency, large landslides are naturalhazards that induce geomorphic impacts that can badlydamage human settlements (Table 7.1). They are generallyassociated with episodes of extreme rainfall and/or withearthquakes, which are the natural hazards that cause thehighest human losses on Earth. The rapid failure of a land-slide dam will cause catastrophic downstream floodingwhereas a long-lasting dam and its resulting filling by sedi-ments will mostly affect mountain valley morphology andthe sediment cascade.The development of large-scale, catastrophic landslides

should be put in the broader, long-term perspective of theevolution of a mountain range. In the context of the overallbalance between erosion and rock uplift rates (Burbanket al., 1996; Montgomery, 2001), superficial variablessuch as sediment fluxes and hillslope angle respond toboth climate forcing and sporadic, catastrophic mountainslope collapse. This results in punctuated epicycles ofaggradation and/or river incision that offset the equilibriumstate at a shorter, 104–105 year time-scale (Fort, 1988; Pratt-Sitaula et al., 2004). These high magnitude, low frequencyfailures are the very ‘formative events’ of the present mor-phology of active mountains and accomplish the main partof the denudation process of these orogens.

References

Alford, D. and Schuster, R. L. (2000). Introduction and sum-mary. In Usoi Landslide Dam and Lake Sarez: An Assessmentof Hazard and Risk in the Pamir Mountains, Tajikistan. UnitedNations, ISDR Prevention Series n°1, 1–18.

Brardinoni, F. and Church, M. (2004). Representing the land-slide magnitude-frequency relation: Capilano river basin,British Columbia. Earth Surface Processes and Landforms,29, 115–124.

Brunsden, D. and Jones, D. K. C. (1984). The geomorphology ofhigh magnitude-low frequency events in the Karakorammountains. In K. J. Miller (ed.), The InternationalKarakoram Project, vol. 1, Cambridge: CambridgeUniversity Press, pp.383–388.



Burbank, D.W., Leland, J., Fielding, E. et al. (1996). Bedrockincision, rock uplift and threshold hill slopes in the northwesternHimalayas. Nature, 379, 505–510.

Cossart, E. and Fort, M. (2008). Consequences of landslide damson alpine river valleys: examples and typology from theFrench Southern Alps. Norsk Geografisk Tidsskrift(Norwegian Journal of Geography), 62, 75–88.

Costa, J. E. (1991). Nature, mechanics, and mitigation of the ValPola landslide, Valtellina, Italy 1987–1988. Zeitschrift fürGeomorphologie, 35, 15–38.

Costa, J. E. and Schuster, R. L. (1988). The formation and failureof natural dams. Geological Society of America Bulletin, 100,1054–1068.

Crosta, G. B., Chen, H. and Lee, C. F. (2004). Replay of the 1987Val Pola Landslide, Italian Alps. Geomorphology, 60,127–146.

Cruden, D.M. and Varnes, D. J. (1996). Landslide types andprocesses. In Landslides, Investigation and Mitigation.Washington, DC: National Academy Press, 36–75.

Dikau, R., Brunsden, D., Schrott, L. and Ibsen, M. (eds.) (1996).Landslide Recognition: Identification, Movement and Causes.New York; Chichester: Wiley.

Evans, S.G. and Clague, J. J. (1994). Recent climatic change andcatastrophic geomorphic processes in mountain environments.Geomorphology, 10, 107–128.

Evans, S. G., Scarascia Mugnozza, G., Strom, A. L. et al. (2006).Landslides from massive rock slope failure and associatedphenomena. In S. G. Evans et al. (eds.), LandslidesFrom Massive Rock Slope Failure, Dordrecht: Springer,pp. 3–52.

Fort, M. (1987). Sporadic morphogenesis in a continental sub-duction setting: an example from the Annapurna Range, NepalHimalaya. Zeitschrift für Geomorphologie Supplement-Band,63, 9–36.

Fort, M. (1988). Catastrophic sedimentation and morphogenesisalong the High Himalayan front: implications for palaeoenvir-onmental reconstruction. In P. Whyte, J. S. Aigner,N. G. Jablonski et al. (eds.), The Palaeoenvironment of EastAsia fromMid-Tertiary, Centre of Asian Studies, University ofHong Kong, pp.170–194.

Fort, M. (2000). Glaciers and mass wasting processes: theirinfluence on the shaping of the Kali Gandaki valley (higherHimalaya of Nepal). Quaternary International, 65/66,101–119.

Fort, M. and Peulvast, J. -P. (1995). Catastrophic mass-movements and morphogenesis in the Peri-Tibetan Ranges:examples from West Kunlun, East Pamir and Ladakh. InO. Slaymaker (ed.), Steepland Geomorphology. Chichester:Wiley, 171–198.

Hewitt, K. (1988). Catastrophic landslide deposits in theKarakoram Himalaya. Science, 242, 64–67.

Hewitt, K. (1998). Catastrophic landslides and their effects onthe Upper Indus streams, Karakoram Himalaya, northernPakistan. Geomorphology, 26, 47–80.

Hewitt, K. (2002). Postglacial landforms and sediment associa-tions in a landslide-fragmented river system: theTranshimalayan Indus stream, Central Asia. In K. Hewitt,M. L. Byrne, M. English and G. Young (eds.), Landscapes ofTransition: Landform Assemblages and Transformations inCold Regions. Dordrecht: Kluwer, pp. 63–91.

Hewitt, K. (2006). Disturbance regime landscapes: mountaindrainage systems interrupted by large rockslides. Progress inPhysical Geography, 30, 365–393.

Hewitt, K., Clague, J. J. and Orwin, J. F. (2008). Legacies ofcatastrophic rock slope failures in mountain landscapes.Earth Science Reviews, 87, 1–38.

Hovius, N., Stark, C. P. and Allen, P. A. (2007). Sediment fluxesfrom a mountain belt derived by landslide mapping. Geology,25, 231–234.

Hsü, K. J. (1975). Catastrophic debris streams (sturzstroms) gen-erated by rockfalls. Geological Society of America Bulletin,86, 129–140.

Hungr, O., Evans, S. G. and Hazzard, J. (1999). Magnitude andfrequency of rockfalls and rock slides along the main trans-portation corridors of Southern British Columbia. CanadianGeotechnical Journal, 36, 224–238.

Kilburn, C. R. J. and Petley, D. N. (2003). Forecasting giant,catastrophic slope collapse: lessons from Vajont, NorthernItaly. Geomorphology, 54, 21–32.

Koi, T., Hotta, N., Ishigaki, I. et al. (2008). Prolonged impact ofearthquake-induced landslides on sediment yield in a moun-tain watershed: the Tanzawa region, Japan. Geomorphology,101, 692–702.

Korup, O. (2005). Geomorphic imprint of landslides on alpineriver systems, southwest New Zealand. Earth SurfaceProcesses and Landforms, 30, 783–800.

Korup, O., McSaveney, M. and Davies, T. R. H. (2004).Sediment generation and delivery from large historic land-slides in the Southern Alps New Zealand. Geomorphology,61, 189–207.

Korup, O., Clague, J. J., Hermanns, R. L., et al. (2007). Giantlandslides, topography, and erosion. Earth and PlanetaryScience Letters, 261, 578–589.

Li, T., Schuster, R. L. andWu, J. (1986). Landslide dams in south-central China. In R. L. Schuster (ed.), Landslides Dams:Processes, Risk, and Mitigation. American Society of CivilEngineers, Geological Special Publication, 3, pp. 21–41.

Montgomery, D. R. (2001). Slope distributions, threshold hill-slopes, and steady-state topography. American Journal ofScience, 301, 432–452.

Plafker, G. and Ericksen, G. E. (1978). Nevados Huascaranavalanches, Peru. In B. Voight (ed.), Rockslides andAvalanches: I, Natural Phenomena. Amsterdam: Elsevier,pp. 277–314.

Pratt-Sitaula, B., Burbank, D.W., Heimsath, A. and Ohja, T.(2004). Landscape disequilibrium on 1000–10,000 yearscales, Marsyandi River, Nepal, Central Himalaya.Geomorphology, 58, 223–241.



Schneider, J. L., Pollet, N., Chapron, E., Wessels, M. andWassmer, P. (2004). Signature of Rhine Valley sturzstromdam failures in Holocene sediments of Lake Constance,Germany. Sedimentary Geology, 169, 1–2, 75–91.

Shang, Y., Yang, Z., Li, L. et al. (2003). A super-large landslidein Tibet in 2000: background, occurrence, disaster and origin.Geomorphology, 54, 225–243.

Slaymaker, O. (2006). Towards the identification of scaling rela-tions in drainage basin sediment budgets. Geomorphology, 80,8–19.

Thiery, Y., Malet, J. -P., Sterlacchini, S., Puissant, A. andMaquaire, O. (2007). Landslide susceptibility assessment bybivariate methods at large scales: application to a complexmountainous environment. Geomorphology, 92, 38–59.


Documents

Contributorsgilles-arnaud-fassetta.org/wp-content/uploads/2018/... · 7.1 Catastrophic landslides: definition, modes of emplacement and geomorphic significance 7.1.1 Definition