4 Mountain hazards
Olav Slaymaker
4.1 Introduction to mountain
geomorphic hazards
4.1.1 Mountain geomorphic hazards deﬁned
A geomorphic hazard results from any landform or
landscape change that adversely affects the geomorphic
stability of a site or drainage basin (Schumm, 1988)
and that intersects the human use system with adverse
socio-economic impacts (White, 1974). If there are no
people affected, there is no hazard and if the landform
or landscape is unchanged there is no geomorphic
hazard. Barsch and Caine (1984) have described the
distinctive relief typologies of major mountain systems.
Mountain geosystems are not exceptionally fragile but
they show a greater range of vulnerability to disturbance
than many landscapes (Körner and Ohsawa, 2005)
and their recovery rate after disturbance is often slow.
During the past three decades, the world’s population
has doubled, the mountain regions’ population has
more than tripled and stresses on the physical and biological
systems of mountain regions have intensiﬁed
many fold. The combination of extreme geophysical
events with exceptional population growth and land
use modiﬁcations underlines the urgency of better
understanding of these interactions and working out
the implications for adaptation to and mitigation of
the effects of drivers of change on landforms and landscapes.
Geomorphic hazards intensify and risks multiply
accordingly.
4.1.2 The major drivers of change and ‘key’
vulnerability
The three drivers of environmental change in mountains are
relief, as a proxy for tectonics (Tucker and Slingerland,
1994), hydroclimate and runoff (Vandenberghe, 2002)
and human activity (Coulthard and Macklin, 2001). Not
only are they important in themselves but they are commonly
so closely interrelated that it becomes difﬁcult to
rank their relative importance and, indeed, their status,
whether dependent or independent. One of the greatest
challenges facing mountain scientists is to separate environmental
change caused by human activities from change
that would have occurred without human interference
(Marston, 2008).
Unfortunately, there has been inadequate representation
of the interactive coupling between relief, land use and
climate in the climate change discussions to date
(Osmond et al., 2004). Based on a number of criteria in
the literature such as magnitude, timing, persistence/reversibility,
potential for adaptation, distributional aspects of
the impacts, and likelihood and importance of the impacts,
some of these vulnerabilities have been identiﬁed as ‘key’
(Füssel and Klein, 2006). It is the point of exceedance of
thresholds, where non-linear processes cause a system to
shift from one major state to another, that expresses this key
vulnerability. From the perspective of geomorphologists,
these key vulnerabilities are often expressed as increasing
magnitude and/or frequency of geomorphic hazards.
4.1.3 The scale question
To enhance the complexity of the puzzle, the three drivers
of change (relief, hydroclimate and human activity) operate
variably at different spatial scales
For the purpose of the following discussion we have
adopted a scale typology as follows:
a. Site scale hazards (<10−1
km2
)
b. Drainage basin scale hazards (10−1
to 103
km2
) and
c. Global scale hazards (>103
km2
)
Geomorphological Hazards and Disaster Prevention, eds. Irasema Alcántara-Ayala and Andrew S. Goudie. Published by
Cambridge University Press. © Cambridge University Press 2010. 33
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We do not know where the breaks in the hierarchy should
come. An example of the difﬁculty of establishing a scale
typology for the study of landscapes under global environmental
change is introduced as Figure 4.1. Motivation for
deﬁning preferred spatial and temporal scales of study can
be found in Slaymaker et al. (2009), but in the present
context it is sufﬁcient to note that we have no theoretical
justiﬁcation for the separation of these scales of enquiry.
4.2 Site scale
4.2.1 Relief
The tradition of process geomorphology has focused most
of its attention at smaller scales. In part, this is because site
scale analysis is most amenable to quantitative treatment
and, in part, because the geomorphic response to site scale
disturbance is rapid. In mountains, relief is a major driving
force, alongside of hydroclimate and human activity.
The driving forces associated with relief include:
absolute elevation (which controls temperature and
precipitation); gradient (which controls the erosional force
g sin α); concave slopes (which tend to concentrate water)
and convex slopes (which tend to shed water). Aspect
controls the amount of radiant energy received at the surface
and leads to highly contrasted slope climates. The
resisting forces associated with strength of surﬁcial materials
can be deﬁned in numerous ways, such as cohesive and
frictional strength, erodibility and modes of deformation
(rheological properties deﬁned by strain–time and stress–
strain rate graphs).
4.2.2 Hydroclimate and runoff
Site scale hydroclimate is inﬂuenced by relief (see above)
and also by the nature of the regolith. The regolith controls
the preferred pathways of movement of water and sediment.
The relative importance of weathering, sediment
transport and depositional processes depends on the hydroclimate.
Precipitation, snow storage, glacier storage, available
soil moisture, groundwater storage, actual
evapotranspiration and surface runoff are the components
FIGURE 4.1. Spatial and temporal
scales in geomorphology. On the
x axis, the area of the surface of
the Earth in km2
is expressed as
8.7 logarithmic units; on the y
axis, time since the origin of the
Earth in years is expressed as 9.7
logarithmic units. (Slaymaker
et al., 2009.)
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of the hydrological cycle that inﬂuence and respond to
environmental change. The magnitude, frequency and
duration of storm events is vital information that often
does not exist because the usual presentation of precipitation
data is in the form of daily totals (Barry, 1992).
4.2.3 Human activity
Timber harvesting is a major land management practice
whose precise inﬂuence on slope stability depends on the
method of harvesting, density of residual trees and understory
vegetation, rate and type of regeneration, site characteristics
and patterns of water inﬂow after harvesting
(Sidle et al., 2002). Roads can be the focus of the highest
rates of denudation in the landscape. Humans do accelerate
slope failures through road building, especially when
roads are situated in mid-slope locations instead of along
ridge tops (Marston et al., 1998). Roads in eastern Sikkim
and western Garhwal have caused an average of two
major landslides for every kilometer constructed. Road
building in Nepal has produced up to 9,000 cubic meters
of landslide per kilometer, and it has been estimated that,
on average, each kilometer of road constructed will eventually
trigger 1,000 tons of land lost from slope failures
(Zurick and Karan, 1999).
4.2.4 Site scale hazards
Sites of initiation of rilling and gullying, the uppermost
ﬁnger-tip tributaries of river networks, zero order basins
(or colluvial bedrock depressions) and sites of mass movement
initiation are all sites at which a threshold safety factor
has been exceeded. The initiation of snow avalanches and
debris avalanches are speciﬁc cases of such hazards.
Steep slopes and heavy snowfall at high elevations are
the main factors affecting avalanche incidence. In countries
such as Canada, Switzerland, Austria, France, Italy and
Norway expenditures involve tens of millions of dollars
annually. No clear trends have been identiﬁed in the frequency
and number of avalanches in the Alps in the past
century (Agrawala, 2007), but in many avalanche-prone
areas the density of buildings and other investments has
increased.
Permafrost degradation is likely to contribute to rockfall
activity (Behm et al., 2006) and may well trigger higher
debris ﬂow activity, but research into this link so far
remains inconclusive. A recent study showed that frequency
of debris ﬂows originating from permafrost areas
in Ritigraben (Swiss Alps) has been decreasing, although a
lower frequency may also be associated with more intense
events due to larger accumulation of materials between
events (Stoffel and Beniston, 2006). Fischer et al. (2006)
have identiﬁed permafrost degradation as a cause of slope
instabilities on Monte Rosa; and Gruber and Haeberli
(2007) provide a comprehensive review of permafrost and
slope instability.
Road systems are often critical in hazard generation;
removal of vegetation from forest to agriculture to urban;
removal of soil for urban and mining purposes; and gravel
extraction all have potential for generating site scale
hazards.
4.3 Drainage basin scale
4.3.1 The sediment cascade in mountains
The greater complexity of the drainage basin scale is
expressed through emergent properties that do not exist at
site scale. Some of these emergent properties are linear
erosion, slope and channel coupling, and preferred surface
and sub-surface pathways for movement of sediment and
water. The literature has traditionally used such variables as
basin area and drainage density to act as surrogates for these
integrated effects.
The sediment cascade that results can be characterized in
terms of four environments (after Caine, 1974) which are
differentiated by dominant processes and forms as:
a. the mountain cryosphere system;
b. the coarse debris system;
c. the ﬁne-grained sediment system; and
d. the geochemical system.
Note that the categories overlap and they are identiﬁed
only in terms of their dominant characteristics.
Each of these components of the mountain geosystem is
sensitive to environmental change, whether in response to
relief, temperature, precipitation, runoff, sediment transport
or land use changes. The cryosphere stores water and
changes the timing and magnitude of runoff which erodes
and transports sediment. Snow responds to environmental
changes on a daily time scale; lake and river ice on an
annual time scale; permafrost and glaciers on annual to
century time scales; associated ecosystem responses are
measured in decades to centuries; and sediment systems
may take decades to millennia to respond (Figure 4.2)
4.3.2 Basin area
At the drainage basin scale, runoff intensity reaches a maximum
within basins whose dimensions approximate those
of the extreme event producing storm cells. Partly for this
reason and also because, at smaller basin scales, slopes and
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channels are often coupled, debris torrent basins tend to
average between 0.5 and 5 km². When basins reach 5 km2
or greater, most slopes and channels are uncoupled and the
sediment response time after extreme hydroclimatic events
increases substantially. Systematic analysis of the relative
magnitude of change in sediment ﬂux in basins of different
size shows that relative changes diminish with increasing
basin size (Figure 4.3). This is because smaller basins are
more sensitive and vulnerable to environmental change
than the larger basins.
4.3.3 Drainage density
Drainage density (Dd) is inﬂuenced by mean annual
precipitation, precipitation intensity, lithology, soil
characteristics, relief, vegetation, human activity and
stage of drainage network development. Although there
are many ambiguities in the calculation of Dd (Schumm,
1997), data available suggest that values of Dd between 1
and 10 km−1
are representative for maturely dissected ﬂuvial
landscapes.
There is a wide range of values of drainage density
reported in the literature. They range from the very high
drainage densities recorded in the badlands of Perth
Amboy, New Jersey (c. 600 km km−2
) (Schumm, 1956)
to less than 1 on undissected plateau or plains surfaces. The
inﬂuence of lithology and stage of network development
has been clearly demonstrated in many studies but, as
Chorley et al. (1984) point out, contemporary hydroclimate
and surface properties seem to be dominant in controlling
FIGURE 4.2. Components of the mountain cryosphere, ecosphere and sediment cascade and their response times following disturbance
by hydroclimate or by human activity. (Slaymaker and Embleton-Hamann, 2009.)
FIGURE 4.3. Relative magnitude of change between low or baseline sediment ﬂux and maximum sediment ﬂux (dimensionless ratio Smax/
Smin) plotted against lake-catchment area (km2
). (Dearing and Jones, 2003.)
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differences of drainage density. There is a range of drainage
density values from the American Southwest (Melton,
1957), Sri Lanka (Madduma Bandara 1971), the eastern
Caribbean mountainous islands (Walsh 1985); the
Seychelles (Walsh, 1996) and the continental USA
(Hadley and Schumm, 1961; Chorley and Morgan, 1962).
These variations reﬂect the direct and indirect inﬂuence of
climatic variables. The direct inﬂuence is that of large rainstorm
magnitude–frequency, as it is only the large runoff
events associated with such rainstorms that are capable of
eroding ﬁnger-tip sections of the drainage net. The indirect
inﬂuence of climate is exerted via the inﬂuence of annual
rainfall, rainfall seasonality and vegetation on soil
characteristics.
Walsh (1996) suggests that there are three major questions
outstanding: (1) what return period of rainstorm/runoff
event is responsible for controlling the drainage
network? (2) what are the processes responsible? and (3)
how quickly do networks develop or adjust to changes in
climate? Various drainage density models appear to be
applicable. Different slope runoff processes and their spatial
patterns, inheritance from collapsed pipe networks,
channel development in landslide scars (Chorley et al.,
1984) or even the simultaneous operation of uncoupled
pipe collapse and erosion by overland ﬂow may be the
dominant process. The essential point in this present discussion
is that there would appear to be strong evidence for
the relation between drainage density and hydrogeomorphological
hazards, especially the incidence of large rainstorm
and runoff events, whatever the precise mechanism
involved.
4.3.4 The ruggedness number
The ruggedness number, R, deﬁned as the product of drainage
density (km−1
) and basin relief (m), (Dd * Ht) is one of the
three dimensionless numbers that describe the dynamics of
drainage basin evolution (Church and Mark, 1980). It
seems reasonable to suppose that the incidence and intensity
of geomorphic hazards would also be related to the
ruggedness number. Originally deﬁned by Strahler (1952),
the ruggedness number has been used in various guises by
many students of morphometry, such as Schumm (1956)
and Melton (1957) as an indicator of the relative dynamism
of the basin, but rarely in the context of hazard studies.
Kovanen and Slaymaker (2008) showed a strong relation
between the Melton ruggedness number (Ht * A−0.5
) and
debris fan slope; an inverse relation between debris fan
slope and basin area, and a modest relation between basin
area and debris fan area in the Nooksack basin, Cascade
Ranges, USA. The fans, which are active at present, have a
documented history of >7000 years of debris ﬂow activity.
The drainage basin ruggedness number is then a potential
indicator of the hazardousness of a basin, especially of
those hazards that are related to water movement and sediment
mobilization, erosion on slopes and ﬂuvial erosion.
In Table 4.1 we have postulated a direct relation between
ruggedness number and extreme geomorphic events and an
inverse relation between basin area and extreme geomorphic
events.
4.3.5 Climate change
One of the most signiﬁcant impacts of climate change (see
also Chapters 5, 8 and 20) in glacierized basins may be the
changing pattern of glacier melt runoff (Walsh et al., 2005).
Glaciers will provide extra runoff as the ice disappears. In
most mountain regions, this will happen for a few decades
and then cease. For those regions with very large glaciers
the effect may last for a century or more. Kotlyakov et al.
(1991) have provided estimates of change for Central Asia,
which give a threefold increase of runoff from glaciers by
TABLE 4.1. Theoretical characteristics of basins with variable ruggedness number
Dd (km * km−2
) Ht (m) Ra
Coupling Geomorphic process Incidence of hazards
Mountains
>10 >1000 >10 High Debris ﬂows High
1–10 300–1000 c. 1 Intermediate Fluvial Intermediate
<<1 >>1000 <1 Low Mass movement High
Non-mountains
>>10 <300 >3 High Badlands High
<1 <300 <0.3 Low Desert Low
a
R is ruggedness number, deﬁned as Dd × Ht, a dimensionless number.
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2050 and a reduction to two-thirds of present runoff by the
year 2100. In the short term, a signiﬁcant increase in the
number of ﬂood events in Norway is projected (Bogen,
2006). Bogen found that suspended sediment concentrations
and volumes were dependent on the availability of
sediments, the type and character of the erosion processes
and the temporal development of the ﬂood. In the
Norwegian case, it appears that the glacier-controlled rivers
are unlikely to respond dramatically in terms of sediment
transport because of limited sediment availability.
Nevertheless, in global sediment yield terms, Hallet et al.
(1996) have conclusively demonstrated the importance of
glacier melt waters. The global data and the regional
Norwegian data demonstrate the difﬁculty of making generalizations
about the probable effects of climate change on
sediment transport in glacierized basins. Changing patterns
of snowmelt resulting from climate change are also complex
(Woo, 1996) and are sensitive both to elevation and to
the seasonal and event distribution of temperature change
and precipitation.
4.3.6 Human activity
There is growing evidence since the Third Assessment
Report (IPCC, 2001b.) that adaptations that deal with
non-climatic drivers are being implemented in both developed
and developing countries. Examples of adaptations to
land use change, such as construction, decommissioning
and management of reservoirs, and adaptations to extreme
sediment cascades and relief, as well as their interactions
with climate include the following:
a. partial drainage of the Tsho Rolpa glacial lake in Nepal
was designed to relieve the threat of GLOFs (glacial lake
outburst ﬂoods) (Shrestha and Shrestha, 2004);
b. Sarez Lake in Tajikistan and the so-called ‘quake lakes’
in Szechwan Province, China are commanding national
and international funding;
c. increased use of artiﬁcial snow making by the alpine ski
industry in Europe, Australasia and North America.
Glacial retreat favors the formation of glacial lakes and ice
avalanches, and disastrous events such as glacial lake outburst
ﬂoods (GLOFs). GLOFs are the most destructive
hazards originating from glaciers due to the large water
volume and large areas covered. Luckily, glacial lakes,
from which GLOFs originate, usually form slowly and
can be monitored. McKillop and Clague (2007) have estimated
the probability of the occurrence of GLOFs in
southern British Columbia.
In mountain landscapes, extreme geophysical events
interact with social systems in dramatic ways. The May
12, 2008 earthquake in Szechwan, China, which generated
hundreds of large landslides and which blocked more than
30 large lakes, killing 75,000 people and causing millions
of dollars in damage, is a case in point. Earthquakes and
ﬂoods, which are not exclusively mountain hazards,
account for more than 50% of the damage caused by natural
hazards globally; and the highest damage in recent years in
Switzerland, Austria and France, for example, was due to
ﬂoods and windstorms (Agrawala, 2007).
The country of Tajikistan is part of the Pamir-Alai mountain
system. Ninety-two percent of its land area of
c.140,000 km2
is mountainous; nearly 50% lies above
3,000 m and is classiﬁed as dry, cold desert. It is drained
by the headwaters of the Amu Darya and Syr Darya rivers,
the major feeders of the Aral Sea. This is a zone of intensive
seismic activity and steeplands that are geomorphically
highly active. Earthquakes with magnitudes exceeding 5
on the Richter scale have a recurrence interval of 75 days
and the region is well known for the earthquake-triggered
natural dams that have blocked large lakes, holding as
much as 17 km3
of water in the case of Sarez Lake, and
presenting a permanent risk of catastrophic draining
(Alford et al., 2000). Rock falls and massive rock slides
have accounted for the deaths of more than 100,000 victims
during the twentieth century, and this in a country of barely
6 million people. Thirty-one percent of the country is said
to be agricultural land and 13% is under forest, but overgrazing
of the rangelands by sheep and rapid deforestation
has encouraged widespread erosion on slopes and more
frequent occurrence of mudﬂows and landslides.
4.3.7 Drainage basin scale hazards
High drainage densities have implications for slope proﬁles.
A logical implication of ﬁne dissection by the stream
network is that slope angles will be steeper (and slope
lengths shorter), even in areas of moderate altitudinal relief,
than in other humid environments with lower drainage
densities. This makes landsliding more likely and rainsplash
erosion (heavily dependent on slope angle) more
effective.
Japan is well known for its debris ﬂow hazards
(Figure 4.4) associated with the volcanic mountain landscapes
and high drainage densities.
By contrast with the site scale hazards, drainage basin
scale hazards involve coupled debris avalanches and debris
ﬂows and coupled slopes and channels in the headward
parts of the basins, which is one degree of complexity. As
drainage basins grow, slopes and channels are gradually
decoupled and the rate at which this decoupling occurs is
speciﬁc to the hydroclimate and nature of the regolith in
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each basin. The relative importance of hydroclimate, relief,
regolith and land use is difﬁcult to establish.
The debris ﬂow hazard in Austria is instructive in this
regard. Debris loads involved in about 2,000 torrent disasters
from catchments up to 80 km2
in size have been
calculated and mapped (Figure 4.5).
The regional pattern of debris ﬂow loads (declining from
west to east) reﬂects rather closely the inﬂuence of relief
and hydroclimate. However, individual torrent basins with
intensive land use have a higher hazard and risk rating
simply because of the scale and nature of socio-economic
investment in the valley bottoms. Land use has become
more important in determining the magnitude of the hazard
and the dimensions of the risk. It is also important to
observe that land use in the form of afforestation can
mitigate and even enhance the landscape (Figure 4.6)
If climate warming continues, destabilized mountain
walls, increasing frequency of rock fall, incidence of rock
avalanches, increase and enlargement of glacial lakes and
destabilization of moraines damming these lakes are
expected to accompany increased risk of outburst ﬂoods.
Geomorphic hazards associated with glacier retreat
include rock avalanches, deep-seated slope sagging (sackung),
debris ﬂows, debris avalanches, debris slides, rock
FIGURE 4.4. Debris ﬂows as geomorphic hazard, illustrated from the effects of an intense rainstorm in 1966 at Ashiwada in Yamanashi
Prefecture, Japan: (a) pre-1966; (b) post-Typhoon 26, 1966. (From Akagi, 1973.)
FIGURE 4.5. Regional distribution of torrent and debris ﬂow hazard in Austria, based on observed maximum debris load. (From
Kronfellner-Kraus, 1989; Embleton-Hamann, 2007.)
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fall, moraine dam failures and glacier outburst ﬂoods
(Geertsema et al., 2006). The effects of glacier retreat on
sediment transport are controversial. Schiefer et al. (2006)
identiﬁed three periods of accelerated sedimentation in a
montane lake in the Coast Mountains over the past 70
years: a period of intense rainstorms, a year of massive
slope failure and a period of rapid glacier retreat between
1930 and 1946. The present period of rapid glacier retreat
does not seem to be generating exceptional sedimentation
events.
If climate warming is accompanied by increasing storminess
(IPCC, 2007b) and intense rainfall, then peak stream
discharges will increase and erosion, sediment transport
and sediment deposition downstream will presumably
also increase. Hazards will include ﬂooding, possibly
increasing both magnitude and frequency of ﬂoods and
increased lateral instability of stream banks. A prominent
indication of a change in extremes is the observed evidence
of increases in heavy precipitation events over the midlatitudes
in the last 50 years, even in places where mean
precipitation amounts are not increasing. (Kunkel, 2003).
In Central and Southwest Asia the 1998–2003 drought
provides an example of unanticipated effects of extreme
events. Flash ﬂooding occurred over hardened ground
desiccated by prolonged drought in Tajikistan, central and
southern Iran, and northern Afghanistan leading to accelerated
erosion in early 2002 (IPCC, 2007a). In terms of
impact on the landscape it is the extreme events (both high
and low) and the freshet ﬂows that mobilize most of the
sediments and, to a lesser extent, the solutes. More frequent/intense
occurrences of extreme weather events will
exceed the capacity of many developing countries to cope.
The growth of cities in the mountain world places further
stress on mountain stability. In Latin America, the
Caribbean and countries in transition, nearly half of the
mountain population lives in urban areas. In the case of
tropical mountains by contrast, the towns are in the uplands
and roads are often in ridge top locations. In the case of
temperate mountain areas, the only space for urban agglomerations
and transport routes is on the ﬂat ﬂoor of broader
valleys and mountain basins (Bätzing et al., 1996). These
used to be ﬂood-prone wetland areas. Therefore all rivers
needed to be turned into artiﬁcial channels in these areas.
The consequence is that geomorphic hazards have been
enhanced: concreting of the riverbanks and the waterproofing
of urban areas represent aggravating factors for ﬂoods.
4.4 Global scale
4.4.1 Relief
At the larger mountain system and global scales, elevation
and gradient are the most important relief elements insofar
as they inﬂuence temperature, and precipitation. Elevation
controls the incidence and intensity of freeze–thaw events
as well as orographic precipitation, and many associated
climatic effects. Gradient deﬁnes the gravitational driving
force (g sin α) and inﬂuences radiation and precipitation
receipt, wind regimes and snow. Erosion rates reported for
the Nanga Parbat massif are among the highest measured
(22 ± 11 m per 1,000 years) and reported rates of uplift for
the Himalayas vary from 0.5 to 20 m per 1,000 years
(Owen, 2004). Ahnert (1970) developed an equation relating
denudation and local relief:
FIGURE 4.6. Source area of the Gangbach. In the foreground is Aebnenegg, Canton Uri, near Altdorf, Switzerland. (a) A 33-hour rainfall
event in June 13–14, 1910 caused extensive debris ﬂow activity. (b) The same landscape in 1981 after the completion of afforestation
between 1932 and 1960. (R. Kellerhals, personal communication, 2008.)
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D ¼ 0:1535h (4:1)
where D is denudation in mm/1,000 yrs and h is local relief
in m/km.
Summerﬁeld and Hulton (1994) analyzed 33 basins with
an area >500,000 km2
from every continent except
Antarctica. Total denudation (suspended plus dissolved
load) varied from 4 mm ka−1
(Kolyma in the Russian Far
East) to 688 mm ka−1
(Brahmaputra). They found that more
than 60% of the variance in total denudation was accounted
for by basin relief ratio and runoff.
4.4.2 Disturbance regimes
There is an increasing sense that almost all mountain landscapes
are transitional from one landscape-forming regime
to another. This condition has been described by Hewitt
(2006) as a disturbance regime landscape. By this, he meant
that disturbances occur so frequently that the landscape gets
no chance to equilibrate with contemporary processes.
Mega-landslides in the Himalayas are the regional example
that Hewitt has described. Note contrast with models of
Brunsden (1993).
Mountain societies contain a higher incidence of poverty
than elsewhere, and therefore have a lower adaptive
capacity and a higher vulnerability to environmental
change. Mountain peoples have been made more vulnerable
to natural extreme events by vast numbers being
uprooted and resettled in unfamiliar and more dangerous
settings (Hewitt, 1997).
Many studies suggest that at least until 2050 land use
change will be the dominant driver of change in humandominated
regions (UNEP, 2002). Not only are there geosystem
disturbance regimes, such as those discussed by
Hewitt (above), but land use change, ﬁre and insect outbreaks
can also be analyzed as disturbance regimes, using a
shorter response time scale (Sala et al., 2005). Similarly,
over-grazing, trampling and vegetation destabilization have
been analyzed in this way in the Caucasus and Himalayas
(IPCC, 2001b).
4.4.3 A conditionally unstable landscape
The landscape of British Columbia (B.C.) is in transition
between the Last Glacial Maximum (LGM) and present and
illustrates a speciﬁc kind of geosystem disturbance regime.
The Cordilleran Ice Sheet covered almost the whole of the
province before 14,500 BP and since that time the process
of transition towards a ﬂuvially dominated landscape has
been on-going. Nevertheless, B.C.’s mountains still contain
over 29,000 km2
of glaciers and ice caps, in spite of warming
since the Little Ice Age and marked glacier retreat.
Schiefer et al. (2007) estimate total ice loss of
22.5 km3
a−1
between 1990 and 2005.
The suite of landforms that evolved was characterized by
‘non-glacial processes that are directly conditioned by glaciation’
(Church and Ryder, 1972). A distinctive pattern of
sediment yield (Figure 4.7) is interpreted as evidence of a
transitional landscape with a relaxation time of the order of
10 ka (Slaymaker, 1987; Church and Slaymaker, 1989).
This transitional landscape lasts until the glacially conditioned
sediment stores are either removed or attain stability
(Schumm and Rea, 1995). The landscape of B.C. is a
disturbance regime landscape in so far as the post-glacial
landscape has had insufﬁcient time to recover from the
effects of the last major disturbance, namely the LGM.
In B.C., and indeed in most of Canada, larger geomorphic
systems have not yet achieved a form that is adjusted
to the contemporary ﬂuvial landscape (Church et al., 1999).
The implication of this conclusion is that the B.C. mountain
landscape is conditionally unstable and it can be anticipated
that small changes in hydroclimate and/or land use may
cause landscape change.
The regional climate projection of the Fourth
Assessment Report (IPCC, 2007a) anticipates for B.C.
a mean annual temperature increase and a mean annual
precipitation increase, incorporating a summer decrease
and a winter increase in precipitation. If this larger
FIGURE 4.7. Temporal pattern of paraglacial sediment yield in
formerly glaciated upland and valley sites in coastal British
Columbia and Alaska. Values are for basins of order 100, 1000 and
10 000 km2
based on the contemporary spatial pattern of sediment
yield. (From Church, 1998.)
41Mountain hazards
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volume of moisture comes in the form of high magnitude,
low frequency rainfall events, debris ﬂow hazard
may increase markedly. A decrease in snow season
length and snow depth is also projected. The general
tendency under climate warming will be an upslope
shifting of hazard zones and widespread reduction in
stability of formerly glaciated or perennially frozen
slopes (Barsch, 1993; Ryder, 1998).
4.4.4 Human activity, population and land use
Human activity, in the form of population density and land
use, is a direct driver of environmental change in mountains
(Vorosmarty et al., 2003; Syvitski et al., 2005). It is not,
however, the sheer numbers of people but aspects of population
composition and distribution, especially the level of
urbanization and household size, that exercise the greatest
demands on the land (Lambin et al., 2001). High population
densities in the developing world, for example, may
lead to better management, such as in Kenya and Bolivia,
described by Tiffen et al. (1994) as cases where the presence
of more people has led to less erosion. The creation of
infrastructure, especially roads, is a crucial step in triggering
land use intensiﬁcation. In developing countries, the
largest mountain populations are found in the midelevation
zones. In developed and transitional countries,
by contrast, the lowest mountain zones are most heavily
populated.
4.4.5 A typology of mountain systems sensitive
to relief, hydroclimate and land use
changes
If one is to get a realistic view of the incidence and source of
geomorphic hazards, mountain systems should be differentiated
not only in terms of relief and hydroclimate but in
ways that reﬂect demography and land use. In this simplistic
typology, we incorporate population density as a proxy
for the intensity of the human signature on the landscape.
Higher population densities lead to a higher pressure on
land resources and intensiﬁed land use and therefore the
human signature (though not necessarily a negative one)
will be higher.
Polar mountains (population density <0.1/km²)
Svalbard, for example, has few permanent residents
(around 2,300 as of 2000) and a few isolated mining activities,
but it is estimated that c. 40,000 tourists visit each year
(as of 2007). Sixty-ﬁve percent of the surface of Svalbard
consists of protected areas. Climate and relief are the
dominant drivers of change and hazards associated with
permafrost degradation and glacier retreat are most evident.
Low population density temperate mountains
(population density 0.1–25/km²)
Mountains with a history of less dense settlement retain
more of their traditional agriculture and forestry. Relief and
hydroclimate are the most important drivers of environmental
change. Formerly glaciated mountains are strongly
controlled by the historical legacy of glaciation.
High population density temperate mountains
(population density 25–75/km²)
In western Europe and Japan, mountain regions are experiencing
increasing land use pressures because of competition
between conservation use, mineral extraction and
processing, recreation development and market oriented
agriculture, forestry and livestock grazing. The human
impact (both positive and negative) on these mountains
far exceeds the documented effects of relief and
hydroclimate.
Tropical mountains (population density 50–100/km²)
Many developing countries (deﬁned as having a relatively
low standard of living, an undeveloped industrial base and a
moderate to low human development index) are located in
tropical and semi-arid environments. In those regions, the
mountain areas are usually cooler and/or wetter than the
lowlands and more hospitable for living and commercial
exploitation. They also have deeper soils and fewer diseases.
Human encroachment has reduced vegetation cover,
increasing erosion and siltation, thereby adversely affecting
water quality and other resources. Direct anthropogenic
inﬂuence on these mountain regions appears to greatly
surpass climate effects.
4.4.6 Global scale hazards
Global scale geomorphic hazards necessarily involve a
consideration of tectonic and historical legacy. Hovius
et al. (1998) emphasizes geomorphic hazards controlled
by tectonic activity in earthquake-dominated landscapes.
Scaled for drainage basin area, sediment yields increase
through almost ﬁve orders of magnitude from tectonic
cratons (typical sediment yields of 100 t km−2
a−1
) to
contractional mountains (up to 10,000 t km−2
a−1
).
Tectonically active mountain belts not only provide relief
to drive erosion processes but they also combine high
regolith loss with the rapid uplift of new bedrock into the
weathering zone to continually refresh these erosion processes
and maintain high sediment yields. On the rapidly
Olav Slaymaker42
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uplifting (5–7 mm a−1
) island of Taiwan, erosion appears
driven by interaction of erodible substrates, rapid deformation
in the form of frequent earthquakes and typhoondriven
runoff variability. Earthquakes produce sediment
by rock mass shattering and landsliding, and landslides
and debris ﬂows are also triggered by typhoon-generated
storm runoff which ﬂushes sediments from the mountains
(Dadson et al., 2003). Thus changes in the frequency,
magnitude and track positioning of typhoons in the
Philippine Sea, western North Paciﬁc Ocean, consequent
upon climate change will have consequences for denudation
rates; these processes are more important than simple
relief and average precipitation controls (e.g. Andes: Aalto
et al., 2001; Himalayas: Finlayson et al., 2002).
One of the implications of the fact that many mountain
landscapes are disturbance regime landscapes is that they
are exceptionally sensitive to environmental change and are
in this sense geomorphically vulnerable. Geomorphic vulnerability
is expressed not only by the frequent incidence of
earthquake-triggered landslides, but, for example, by the
fate of Himalayan glaciers, which cover 17% of the mountain
area, and are predicted to shrink from the present
500,000 to 100,000 km2
by 2035 (WWF, 2005). The glaciers
on Mt. Kilimanjaro are likely to disappear by 2020
(Thompson et al., 2002) and Bolivian glaciers are heading
for the same fate (Thompson et al., 1998).
Climate change can be expected to alter the magnitude
and frequency of a wide variety of geomorphic processes
(Holm et al., 2004). Increased triggering of rock falls and
landslides could result from increased groundwater seepage
and pressure. Large landslides are propagated by
increasing long-term rainfall whereas small landslides are
triggered by high intensity rainfall. These tendencies will
probably lead to enhanced sediment transport. Increased
sediment input to glacier-fed rivers may lead to increased
channel instability, erosion and ﬂooding. The hazard zones
related to most of these ﬂuvial processes will extend a long
way beyond the limits of the mountain area (Ashmore
and Church, 2002). Rainfall amounts and intensities are
the most important factors in water erosion and they
affect slope stability, channel change and sediment transport.
Increased precipitation intensity and variability is
projected to increase the risk of ﬂoods and droughts in
many areas. Changes in permafrost will affect river
morphology through destabilizing of banks and slopes,
increased erosion and sediment supply (Vandenberghe,
2002).
Agriculture has been the greatest force of land transformation
on this planet (Lambin and Geist, 2006).
Nearly a third of the Earth’s land surface is currently
being used for growing crops or grazing cattle (FAO,
2007). Much of this agricultural land has been created at
the expense of natural forests, grasslands and wetlands. In
Africa, for example, most of the mountains are under pressure
from commercial and subsistence farming activities. In
unprotected areas, mountain forests are cleared for cultivation
of high altitude adapted cash crops such as tea, pyrethrum
and coffee. Grazing and forestry are the
predominant uses of mountain land in all regions.
Extensive grazing has little impact on slope processes, but
overgrazing can have severe impacts.
4.5 Conclusion in light of accelerating
environmental change
It is possible to learn from ecology, where recent models
have placed environmental change and system collapse as
central to an understanding of contemporary change and
where there are similar complexity problems. Panarchy is a
metaphor designed to describe systems of ecosystems at
varying spatial and temporal scales. The terminology
developed for panarchy (Holling, 2001) is entirely ecological
and needs to be translated for the needs of geomorphology.
Holling suggests that complex systems are driven
through adaptive cycles which exist at a range of spatial
scales. The term adaptive is self-evident in ecological
systems; in geomorphic systems we often speak of selfregulating
systems (Phillips, 2003). Adaptive cycles are
deﬁned as consisting of four phases, namely exploitation
(the environmental disturbance regime), conservation (the
response), collapse (threshold exceedance and unpredictable
behavior of the system) and reorganization (recovery).
A geomorphic analogue would be a disturbance regime
landscape characterized by both orderly evolution and
system collapse. The duration of these phases of adaptation
depends on the intrinsic strength of the system, the
connectivity of the system and the time required for the
recovery of the system (resilience).
The panarchy metaphor has a fascinating ﬂexibility in
dealing with complex systems. Although there are evident
differences between geophysical systems and ecosystems,
there are many parallels in the behavior of self-organizing
systems that can assist in improving attempts to understand
and manage the environment sensitively. The concepts
and terminology of the panarchy model are consistent
with geomorphic concepts such as complex response
(Schumm, 1973), threshold exceedance, landscape sensitivity
and barriers to change.
Many mountain hazards can be viewed in terms of scale,
ruggedness number, lithologic strength and connectivity
(criticality of the adaptive system). The risks and losses
associated with these hazards depend on how close to a
43Mountain hazards
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condition of collapse the adaptive system is allowed to
proceed (Table 4.2).
4.6 Conclusions
There appears to be a hierarchy of mountain hazards and
in Holling’s (2001) terms this is a panarchy of adaptive
systems. Each adaptive system, at its own spatial scale,
evolves towards a critical condition leading either to collapse
(hazard) or to self-reorganization (adaptation or
mitigation in socio-economic context). Climate change
is just one of the drivers that operates on the adaptive
system.
If the IPCC deﬁnition of vulnerability is seriously
engaged, then geomorphologists also have to investigate
adaptive systems. Reliance on reactive, autonomous
adaptation to the cumulative effects of environmental
change is ecologically and socio-economically costly.
Planned and anticipatory adaptation strategies can provide
multiple beneﬁts. But there are limits on their
implementation and effectiveness. Enhancement of adaptive
capacity reduces the vulnerability of landscapes to
environmental change, but adaptive capacity varies considerably
among regions, cultures, and socio-economic
groups. Improved understanding of geomorphic hazards
at many temporal and spatial scales is urgently needed.
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