landslide current availability and methodology for natural risk map production armonia project...

ARMONIA PROJECT (Contract n 511208) Deliverable 2.1

B.III-1

B.III Landslides

Authors: Giuseppe Delmonaco, Daniele Spizzichino, T6

1 Definition of Landslide..................................................... 4

2 Landslides classification .................................................. 4

2.1 Material type ............................................................................5

2.2 Landslide activity ......................................................................6

2.2.1 State of activity.........................................................................6

2.2.2 Distribution of activity ................................................................8

2.2.3 Style of activity .........................................................................8

2.3 Landslide causes .......................................................................9

3 Intensity or Magnitude (I)............................................. 10

3.1 Velocity.................................................................................. 10

3.2 Dimension.............................................................................. 12

3.3 Energy................................................................................... 13

3.4 Consequences......................................................................... 13

3.5 Hybrid ................................................................................... 14

4 Return period of landslides............................................ 15

4.1 Geomorphological criteria or qualitative analysis......................... 15

4.2 Analysis of temporal series related to effects.............................. 15

4.3 Analysis of temporal series related to causes ............................. 16

4.3.1 Precipitation............................................................................16

4.3.2 Earthquakes............................................................................17

4.4 Monitoring.............................................................................. 18

4.4.1 Mechanical approach ................................................................18

4.4.2 Kinematics approach ................................................................18

5 Hazard assessment (H) ................................................. 19

5.1 Basic concepts ........................................................................ 19

5.2 Prediction of landslide types ..................................................... 20

5.3 Prediction of landslide intensity................................................. 20

5.4 Prediction on landslide affected area ......................................... 21

5.4.1 Methods of relative hazard analysis ............................................21

5.4.1.1 Qualitative methodologies...................................................21

5.4.1.1.1 Field geomorphologic analysis .........................................21

5.4.1.1.2 Combination of index maps or heuristic approach...............22

5.4.1.2 Quantitative methodologies.................................................23

5.4.1.2.1 Statistical analysis .........................................................23

5.4.1.2.2 Geotechnical models ......................................................25

5.5 Temporal prediction ................................................................ 27

5.5.1 Analysis of return time .............................................................27

5.5.2 Analysis of intensity/magnitude .................................................29

5.6 Prediction of evolution ............................................................. 29


B.III-2

5.6.1 Prediction of run-out ................................................................30

5.6.2 Prediction of retrogression limits ................................................30

5.6.3 Prediction of lateral expansion ...................................................30

6 Landslide hazard mapping............................................. 30

6.1 Introduction ........................................................................... 30

6.2 Susceptibility and hazard mapping............................................ 31

6.3 Hazard zoning scales ............................................................... 33

6.4 Landslide Hazard map types..................................................... 33

6.5 Approaches to landslide mapping.............................................. 34

6.5.1 The Inferential Approach...........................................................34

6.5.2 The Statistical Approach ...........................................................34

6.5.3 The Process-Based Approach.....................................................35

6.6 The mapping unit for GIS Technique ......................................... 35

7 Element at Risk (E) or exposure .................................... 36

7.1 Definition ............................................................................... 36

7.2 Cartography related to the element at risk - Map of Exposure -.... 37

7.3 Economic value of element at risk............................................. 37

7.3.1 Human life Value .....................................................................38

7.3.2 Goods and activities value.........................................................38

7.3.3 Global Value ...........................................................................39

8 Vulnerability (V) ............................................................ 39

8.1 Vulnerability: some web definitions ........................................... 40

8.2 Conceptual approach: vulnerability assessment.......................... 40

8.2.1 Definition ...............................................................................40

8.2.2 Vulnerability components..........................................................41

8.3 Human life vulnerability ........................................................... 42

8.4 Vulnerability of good and activities............................................ 42

9 Analysis of Risk (R) ....................................................... 45

9.1 Definition ............................................................................... 45

9.2 Qualitative analysis ................................................................. 45

9.2.1 Damage propensity..................................................................45

9.3 Quantitative analysis ............................................................... 46

9.3.1 Total risk................................................................................46

9.3.2 Potential damage (WL)..............................................................47

9.3.2.1 Rigorous assessment .........................................................48

9.3.2.2 Simplified assessment. .......................................................49

9.3.3 Specific Risk (Rs) .....................................................................50

9.4 Probability of acceptable rupture............................................... 50

10 Risk management.......................................................... 52

10.1 Introduction ........................................................................... 52

10.2 Framework for landslide risk management ................................ 53

10.2.1 General framework ..................................................................53

10.2.2 Increasing of social acceptable threshold risk...............................54


B.III-3

10.3 Acceptable and tolerable risk from landsliding............................ 55

10.3.1 General issues.........................................................................55

10.4 Treatment.............................................................................. 57

10.4.1 Risk mitigation through structural action and measures.................58

10.4.2 Risk mitigation with non-structural action and measures ...............58

10.5 Public Awareness, Education and Capacity Building .................... 59

10.6 Emergency Preparedness Plan.................................................. 59

10.6.1 Before the Landslide ................................................................59

10.6.2 When it Rains..........................................................................60

10.6.3 Key Considerations ..................................................................61

10.6.4 What Can You Do If You Live Near Steep Hills ..............................61

10.6.4.1 Prior to Intense Storms: .....................................................61

10.6.4.2 During Intense Storms:......................................................61

10.6.5 After the Disaster ....................................................................62

10.7 The Phases of emergency Plan (planning, response, recovery) .... 62

10.7.1 Emergency Phases...................................................................62

10.7.2 Planning Phase........................................................................62

10.7.3 Response Phase ......................................................................63

10.7.3.1 Initial Response.................................................................63

10.7.3.2 Extended Response............................................................64

10.8 Recovery Phase ...................................................................... 64

11 Glossary of all keywords................................................ 64

12 Bibliography .................................................................. 70

13 Appendix: Operational Standards for Risk Assessment

aimed at Spatial Planning................................................... 92

13.1 Minimum standard (simplified model) for hazard mapping

aimed at a legal directive................................................................. 92

13.1.1 Various methodologies related to the 3 assumed scales of analysis

in the light of a potential harmonisation of hazard maps, based on a multi-

hazard perspective...............................................................................92

13.2 Minimum standard (simplified model) for risk mapping aimed at

spatial planning .............................................................................. 98

13.2.1 Multi-risk assessment perspective as element of the Strategic

Environmental Assessment....................................................................98

13.2.2 Methodologies, functions and outputs .........................................99

13.2.2.1 ........................................................................................ 101

13.3 Minimum Standard for Landslide Risk Maps ............................. 106

13.3.1 Local Scale mapping ( 1: 10.000) .......................................... 106


B.III-4

1 Definition of LandslideThe term "landslide" describes a wide variety of processes that result in the

downward and outward movement of slope-forming materials including

rock, soil, artificial fill, or a combination of these (UNESCO, 1984).

A landslide event is defined as "the movement of a mass of rock, debris or

earth down a slope" (Cruden 1991). The word 'landslide' also refers to the

geomorphic feature that results from the event. Other terms used to refer

to landslide events include 'mass movements', 'slope failures', 'slope

instability' and 'terrain instability'.

Limits and uncertainties

There is a conceptual confusion in the term landslide since it is referred

both to landslide deposit (displaced mass) and the movement of material

along a slope.

2 Landslides classificationIn spite of the simple definition, landslide events are complex

geological/geomorphological processes and are therefore difficult to classify.

An adequate classification system should be based on parameters and/or

features that can be measured and observed in the field. At the same time,

a classification system should satisfy requisites of uniqueness, rationality,

homogeneity and readiness of application, though the detection of classes

mutually exclusive.

The classification system, most commonly used worldwide, and proposed

for this report, is modified from Varnes (1978). The classification is based

upon material type and type of movement, and is similar to the updated

classification of slope movements suggested by Cruden and Varnes(1996).

This is based on the movement, with a special attention to the spatial

distribution of displacements and their velocity as well as to the shape of

the failure and landslide body.

The Cruden & Varnes classification can be resumed in the following landslide

types:

1. Falls: take place rapidly by free-fall, bouncing, or rolling, and may

develop into either slides or flows.

2. Topples: consist of the rapid rotation of a unit of rock or soil about

some pivot point. Toppling may not lead to either falls, slides or flows.

3. Slides: involve the movement along one or more distinct surfaces.

Slides are subdivided into 'rotational slides' and 'translational slides',

depending upon the shape of the failure plane.

a . Rotational slides: also referred to as slumps, involve movement

along a curved failure plane. Often the failure plane did not exist


B.III-5

before movement occurred. Rotational slides usually involve relatively

few distinct rock or soil units.

b. Translational slides: involve the movement of many rock or soil

units along a plane. If few distinct units are involved, the movement

is referred to as a 'translational block slide'. Often the failure plane

existed before movement occurred.

4. Lateral spreads: are dominated by lateral extension of the ground,

accompanied by shear or tensile forces, and a general subsidence of the

ground surface. They generally occur relatively slowly.

5. Flows describe movement that resembles a viscous fluid. Some flows

occur slowly, others occur rapidly. Velocity within the flowing mass is

usually decreases with depth and laterally. In most cases, water is an

integral component. Creep is a type of flow that occurs very slowly.

6. Complex landslides involve the combination of two or more types of

movement. Commonly one type of movement starts the material

moving, such as a debris slide, and once underway the material takes on

the character of another type of movement, such as a debris flow. The

name of the complex movement is a combination of the types of

movement, in order of occurrence, such as a debris slide-debris flow.

The rate of movement depends on the types of movements and material

types involved.

According to WP/WPLI (1990-1994), complex landslides should be classified

following the types of movement involved related to the genetical and

temporal sequence of the single types that compose the phenomenon.

2.1 Material type

The material involved in a landslide should be classified according to its

state before the initiation of the movement or, if the movement changes in

the time, according to the state that characterizes the material before the

movement where such a change occurs

The material involved in a landslide is classified into two groups, 'bedrock'

and 'soil'. Soil, which is generally unconsolidated superficial material, is

further subdivided into 'debris' and 'earth' depending upon its texture.

Rock refers to earth materials that have lithified by some rock-forming

process. Its strength depends not only on the rock type but also on the

degree of weathering and the density and orientation of the discontinuities,

which are generally the planes of weakness in the rock mass.

Debris is composed of predominantly coarse grained soil, or as mentioned

above, can also include highly fractured bedrock. The strength of coarse

grained soil is generally derived from friction between the grains.

Earth refers to predominantly fine grained soil (primarily of silt and clay

sized materials). The strength of fine grained soil is generally derived from

cohesion, the chemical and electrical bonding between the small particles.


B.III-6

2.2 Landslide activity

The activity defines the temporal evolution of a landslide, through the

analysis of movement, displaced materials and areas involved. The activity

can be regrouped under three headings:

1. State of activity, that is related to the timing of a landslide

2. Distribution of activity, that describes the area where a landslide is

moving

3. Stile of activity, that indicates the contribution of single movements to

the landslide

2.2.1 State of activity

The state of activity is defined as follows Cruden & Varnes, 1996):

active: landslide currently moving; suspended: landslide moved within the last annual cycle of seasons, but

not presently moving;

reactivated: landslide that is again active after being inactive; inactive: landslide that moved more than one annual cycle of seasons

ago.

Inactive landslides can be subsequently divided in:

dormant: if the causes of movement remain apparent; naturally stabilised: inactive landslide protected by its original causes

without human interventions;

artificially stabilised: inactive landslide protected by its original causeswith artificial remedial measures;

relict: inactive landslides developed under geomorphologic and climaticsetting, considerably different from present conditions.

The geomorphological information of the state of activity are generally

related to the type and state of preservation of some diagnostic elements,

as reported in Table 2.1.

Active Inactive

Scarps, terraces and crevasses with sharp bordersScarps, terraces and crevasses with roundedborders

Crevasses and depressions without secondaryfilling

Crevasses and depressions with secondary filling

Secondary mass movements on scarpsAbsence of secondary mass movements onscarps

Fresh striae over the failure surface and marginalshear planes

Absent or weathered striae over the failuresurface and marginal shear planes

Fresh crack surfaces on blocks Weathered crack surfaces on blocks

Irregular drainage system, diffuse ponding anddepressions with internal drainage

Well preserved drainage system

Pressure crests at the contact with sliding margins Abandoned marginal cracks and banks

Absence of soil on the exposed portion of the failuresurface

Growth of soil on the exposed portion of thefailure surface

Presence of rapid-growing vegetation Presence of slow-growing vegetation

Different vegetation between internal and externallandslide areas

No difference of vegetation between internal andexternal landslide areas

Tilting trees without vertical growthTilting trees with vertical growth in the portions

succeeding the tilted part

Table 2.1. Geomorphic criteria for field survey of landslide state of

activity (Crozier, 1984, modified)


B.III-7

The state of activity of a landslide and its geomorphologic characteristics

are strictly depending on climate conditions, as reported in Figure 1.

Fig. 1. Block diagrams of morphologic changes with time of idealized

landslide (a) in humid climate: A, active or recently active (dormant

historic) landslide features are sharply defined and distinct; B,

dormant young landslide features remains clear but are not

sharply defined owing to slope wash and shallow mass movements

on steep scarps; C, dormant mature landslide feature are

modified by surface drainage, internal erosion and deposition, and

vegetation; D, dormant old landslide features are weak and often

subtle. (Turner & Schuster, 1996).


B.III-8

2.2.2 Distribution of activity

The distribution of activity describes where the landslide is moving and

permits to predict the type of landslide evolution in the space. Following the

distribution of activity a landslide is:

advancing: if the surface of rupture is extending in the direction ofmovement;

retrogressive: if the surface of rupture is extending in the directionopposite the movement of the displaced material;

progressive or multi-directional: if the surface of rupture is enlargingin two or mode directions;

diminishing: if the volume of displaced material is decreasing withtime;

confined: landslide with a scarp but no visible surface of rupture inthe foot of the displaced mass;

constant: if the displaced mass continues to move withoutconsiderable change of the rupture surface and volume of the

displaced mass;

enlarging: if the rupture surface is developed on one or both lateralmargins of the landslide.

2.2.3 Style of activity

The style of activity is the way in which different movements contribute to

the landslide. Following the style of activity a landslide can be defined as:

complex: characterised by the combination, with time, of at least twotypes of movement (fall, topple, slide, expansion, flow);

composite: characterised by the combination, with time, of at leasttwo types of movement (fall, topple, slide, expansion, flow)

simultaneously in different areas of the displaced mass;

successive: characterised by a movement of the same type to anearlier and adjacent landslide, where displaced masses and rupture

surfaces are clearly distinct;

single: characterised by a single movement of the displaced mass; multiple: characterised by repeated movements of the same type,

often following enlargement of the failure surface.

In order to characterise the type of evolution of a landslide it can be very

useful to refer to the frequency of reactivations instead of the state of

activity detected during field survey. On this subject the distinction

proposed by Del Prete et al. (1992) among continuous, seasonal and

intermittent (pluriannual or pluridecennial return time) landslides or what

proposed by Bisci & Dramis (1991) and Flageollet (1994) reported in Table

2.2.

State of activity Recurrence Return time Last activation

ACTIVE Continuous - Presently moving

Seasonal > 1 year Recent

Short-term recurrence 1 - 10 years Recent history

QUIESCENT Medium-term recurrence 10 - 100 years Recent history

Long-term recurrence 100 - 1000 years Recent or ancient history

STABILISED Very long-term recurrence > 1000 years Ancient history and prehistory

Tab. 2.2 Landslide activity (BISCI & DRAMIS, 1991 and

FLAGEOLLET, 1994, modified).


B.III-9

2.3 Landslide causes

Landslide causes analysis consists in examining the various factors that

contribute to slope instability, in their parameterisation and subdivision in

classes and, finally, in their representation as thematic maps.

A list of major predisposing factors, derived by literature (Terzaghi, 1950;

Carrara & Merenda, 1974; Cotecchia, 1978; Varnes, 1978; Hansen, 1984;

Esu, 1984; Crozier, 1986; Canuti et al., 1992; Amanti et al., 1992; Cruden

& Varnes, 1994; Hutchinson, 1995) is reported in the following table:

LITHO-STRATIGRAPHYa) lithological characteristics (i.e. weak materials, presence of clayey

particles);b) stratigraphical characteristics (i.e. rheological contrast: rigid

material over ductile material;c) textural characteristics (i.e. porosity, materials with metastable

texture);d) primary structures (layers, schistosity, fissility);e) mineralogical, petrographical, geochemical characteristics (i.e.

typology of clayey minerals, weathering).

TECTONICa) secondary structures (joints, faults, folds, shear bands, bending);b) tectonic setting of the area;c) neo-tectonic uplift and/or e/o tilting;

QUATERNARY GEOLOGYa) weathering (physical and/or chemical);b) depth of soil, regolith and degradation covers;c) climatic fluctuations;d) eustatic fluctuations;e) melting of glaciers (i.e. unloading of pressures);f) melting of permafrost;g) glacio-eustasy and glacio-isostasy;h) ancient or relict landslides.

HYDROGEOLOGYa) hydrography and springs;b) permeability, contrast of permeability;c) drainage conditions;d) characteristics of groundwater (i.e. free, suspended, confined,

semi-confined aquifers);e) pore pressure in soils and discontinuities;f) filtration;g) capillarity and negative pore pressures;h) evapotranspiration, runoff and infiltration;i) groundwater geochemistry;

GEOTECHNICS AND GEOMECHANICSa) granulometry;b) index properties (i.e. Atterberg limits, point load);c) cementation (i.e. content of CaCO3);d) density (with natural water content, dry, saturated);e) shear strength of material (peak, critical condition, residual;

drained or undrained);f) tensile strength;g) geometrical characteristics of discontinuities (i.e. orientation,

spacing, persistence, roughness);h) mechanical characteristics of discontinuities (i.e. shear strength);i) presence of pre-existing shear surfaces;j) presence of organic matter;k) sensitivity (change in strength with remoulding);l) fragility and progressive rupture;m) cracking and softening;n) swelling and consolidation;o) stress history (i.e. pressure and overconsolidation ratio);p) in situ tensional state;

SEISMICITYa) neotectonics, regional geodynamic model;b) historical and instrumental seismicity, seismotectonic model;c) characteristics of the source (recurrence, maximum expected

magnitude, hypocentral depth, geometry of the source);d) attenuation law;e) seismic macrozoning: ground shaking (i.e. intensity, peak

acceleration, response spectrum)f) seismic microzoning: local seismic response (i.e. amplification),

dynamical effects on terrain properties (liquefaction; dynamicaleffects on pore pressure and shear strength).

VOLCANIC ACTIVITYa) steam emission and subsequent condensation;b) tephra accumulation;c) hydrothermal alteration;d) ice and snow melting;e) volcanic uplift;f) diversion of hydrographic network.

GEOMORFOLOGYa) slope morphometry (i.e. slope angle, height, length, shape,

orientation);b) morphometry of catchments and channels;c) relief energy;d) erosion (i.e. fluvial, marine, glacial) on the foot of the slope;e) superficial erosion (diffuse or concentrated runoff);f) underground erosion (i.e. solution, karst, piping);g) slope loading;h) deforestation (natural causes);

CLIMATOLOGY, METEOROLOGY E HYDROLOGYa) rainfall pattern;b) frequency, intensity and duration of extreme events (i.e. intense

rainfall, extremely prolonged rainfall);c) thermal pattern (ice/snow melting, freezing of spring waters);d) fluctuation of hydrographical level (lakes or rivers);e) fluctuation of sea level;f) thermal excursion (freeze-thaw cycles);g) fluctuation of soil humidity (imbibition / desiccation).h) pre-existing mass movements and frequency of reactivations.

VEGETATION AND LAND USEa) pedologic characteristics (i.e. soil type, texture, structure, depth,

organic matter, content in carbonates);b) agricultural land use (i.e. crop, tree crop, specialised cultures,

grassland, bush, forest)c) type and state of vegetation (i.e. leave cover, depth and strength

of roots, weight of vegetation);d) agricultural techniques (i.e. superficial tillage, strip, terrace).

HUMAN FACTORSa) excavation on the slope or slope toe;b) overloading on slope or crest;c) fluctuation of piezometric levels of artificial basins (i.e. critical

height; rapid emptying);d) deforestation and forestation;e) irrigation;f) mining activity;g) artificial vibrations;h) water leakage (reservoirs, aqueducts, sewage systems).

Tab.2.3 Causative factors of landslides.


B.III-10

3 Intensity or Magnitude (I)Geometrical and mechanical severity of a landslide. It may be expressed as

a relative scale or as one or more landslide parameters (i.e. velocity,

volume, energy).

A conceptual distinction between magnitude and intensity is reported in

Hungr (2001) in the dynamic analysis of fast-moving landslides (flows and

slides). The magnitude is a parameter that describes the scale of an event.

As a general rule, the volume of the involved material can express the

magnitude of a landslide. The initial volume (i.e. the volume of a rock mass

before the detachment) should be distinguished from the volume of a

landslide deposit, that can be larger due to swelling or erosion phenomena,

or smaller due to partial deposition or diversion of material during the

travel.

Therefore, the magnitude defines the total volume of the displaced mass, as

a sequence of connected episodes, considered as a single event. The

intensity, as for earthquakes, is not a single parameter but a spatial

distribution of different characteristics that describe, qualitatively or

quantitatively, the impact of a landslide in different sites. Velocity, duration

of movement, height of displaced mass, depth of deposits are some of

quantitative parameters of intensity. These parameters are a function of the

areas involved by a landslide and tend to annul at lateral and distal margins

of the path, defining in such a way the extension of the impact zone. With

respect to other field such as seismicity and flood where the intensity may

be clearly defined (i.e. magnitude of an earthquake, height or discharge of

water flow) the definition of the severity of a landslide (in terms of intensity

or magnitude) is a very difficult task for experts due to the objective

difficulty in the assessment of the various parameters. In addition, the

expression of intensity related to potential damage or losses should be

avoided being depending on vulnerability of potential exposed element.

3.1 Velocity

Hungr (1981) proposed a scale of landslide intensity based on classes of

velocity related to a scale of damage. This scale has been partially modified

by Cruden & Varnes (1996) as shown in Table 3.1.

Class Description Potential damage Velocity (mm/s)

7 Extremely rapidCatastrophe of major violence; buildings destroyed by impact ofdisplaced material; many deaths; escape unlikely.

5 m/s 5 x 103

6 Very rapidSome lives lost; velocity too great to permit all persons toescape

3m/min 5 x 101

5 RapidEscape evacuation possible; structures, possessions andequipment destroyed

1.8 m/h 5 x 10-1

4 ModerateSome temporary and insensitive structures can be temporarilymaintained

13m/month

5 x 10-3

3 Slow

Remedial construction can be undertaken during movement;insensitive structures can be maintained with frequentmaintenance work if total movement is not large during aparticular acceleration phase

1.6m/year

5 x 10-5

2 Very slow Some permanent structures undamaged by movement15

mm/year5 x 10

-7

1 Extremely slowImperceptible without instruments; construction possible withprecautions

Table 3.1. Scale of landslide intensity based on velocity and induced

damage (Cruden & Varnes, 1996)


B.III-11

The assessment of velocity of a landslide is generally a difficult task; it may

be assessed approximately considering the typology and activity of a

landslide (Varnes, 1978) (Tab 3.2).

CLASSES OF VELOCITY

1 2 3 4 5 6 7

Fall

Topple

Rock slide (F)

Rock slide (R)

Debris slide

Earth slide (F)

Earth slide (R)

Lateral expansion of rock mass

Lateral expansion of rock mass overlaying clay bedrock

Lateral expansion due to liquefaction

Rock flow

Debris flow

Cohesive earth flow (F)

Cohesive earth flow (R)

Tab. 3.2 - Landslide velocity (referred to the classes proposed by

Cruden & Varnes, 1996) based on typology, material and state of

activity; F =first-triggered landslide; R = reactivation

A similar approach has been proposed by Canuti & Casagli (1996), based on

landslide typology, involved material and state of activity (first-triggered or

reactivated landslides) (Tab. 3.3).

Typology Fall Slide Flow

Material Rock Rock Debris Earth Rock Debris Earth

State of activity - F R - F R - - -

Class of velocity 6-7 5-6 1-5 1-6 5-6 1-5 1-2 1-7 1-4

Table 3.3. Landslide velocity proposed by Canuti & Casagli (1996)

based on typology, material and state of activity. F =first-triggered

landslide; R = reactivation

The relationship between movement and velocity is quite evident: a debris

flow or a rock fall are generally very rapid or extremely rapid, while an

earth flow is usually slow or very slow. Generally a first-triggered landslide

exhibits a higher velocity than a reactivation, except for some cases

discussed by Hutchinson (1987). The former is characterised by a fragile

rupture mechanism while the latter by a ductile rupture mechanism where

the shear strength is proximate or equal to residual values.

Another way to assess landslide intensity, based on velocity, can be done

comparing the surface area of present or potential landslides with their

estimated velocity (Tab.3.4 and 3.5). The potential unstable areas are

assessed taking into account landslide phenomena that are developing over

slopes with similar geological and morphological setting.

VELOCITY

Class V0 V1 V2 V3

Values - < 10-6 m/s

(< m/month)10

-6 - 10

-4m/s

(m/month - m/h)> 10

-4 m/s

(>m/h)

Class Values Description NEGLIGIBLE SLOW MODERATE RAPID

A0 - NEGLIGIBLE I0 I0 I0 I0

A1 < 103 m

2SMALL I0 I1 I2 I3

A2 103 - 10

5 m

2MEDIUM I0 I1 I2 I3

AREA A3 > 10

5 m

2LARGE I0 I2 I3 I3

Tab. 3.4 Diagram for simplified assessment of intensity


B.III-12

Intensity Description

I0 NEGLIGIBLE Stable or potentially stable areas

I1 SMALL Areas with small or potentially small landslides

I2 MEDIUM Areas with intermediate or potentially intermediate landslides

I3 LARGE Areas with large or potentially large landslides

Tab. 3.5 Classes of intensity referring table 3.4

3.2 Dimension

Landslide intensity can be also estimated through the dimension of the

displaced mass (Fell, 1994) (Tab. 3.6).

Intensity (I) Description Volume (m3)

7 Extremely large > 5 x 106

6 Very large 1 x 106 5 x 10

6

5 Large 2.5 x 105 1 x 10

6

4 Medium 5 x 104 2.5 x 10

5

3 Small 5 x 103 5 x 10

4

2.5 Very small 5 x 102 5 x 10

3

2 Extremely small < 5 x 102

Table 3.6 - Scale of intensity of landslides based on the volume of

the displaced mass (Fell, 1994)

The estimation of the displaced mass is often difficult to calculate;

therefore, the intensity may be preferably expressed as landslide area.

The assessment of intensity, based on dimension, should take into account

the different landslide typologies. DRM (1990) has proposed to associate

the volume of displaced mass following landslide types (Tab. 3.7) compared

with intensity levels and consequent damage on human life and economy.

The intensity of slides is evaluated through the depth of phenomena since

the estimation of volumes, for this type of landslides, is rather difficult.

FALLS AND TOPPLES

Volume (m3) Description

H1 E1 < 102

Fall of isolated blocks

H2 E2 102 - 10

4Fall, topple or sliding of blocks

H3 E3 104 - 10

6Large fall of blocks

H3 E4 > 106

Catastrophic fall or sliding

FLOWS

Volume (m3) Description

H1 E1 < 5 x 102

Mud flow or mud slide

H2 E2 5 x 102 - 10

4Mud or debris flow

H3 E3 104 - 10

6Rapid debris flow

H3 E4 > 106

Exceptional mass movement

SLIDES

Depth (m) Description

H0 E1 < 2 Superficial slide or solifluxion

H0 E2 2 10 Localised slide

H0 E3 10 50 Slide of a slope

H0 E4 > 50 Exceptional slide

Table 3.7 - Relationship between intensity and physical

characteristics of landslides (DRM, 1990)

In the table 10 the distinction between intensity and consequences on

human setting (H0 H3) and economic setting (E1 E4) has been kept. With

respect to the velocity of movements, the scales of intensity referred to falls

and slides (rapid movements) are the same; as regarding slides, that are

generally slow movements, the intensity, referred to human setting, is


B.III-13

equal to zero (H0) independently from landslide dimension. This definition

includes an analysis of social and economic vulnerability. Finally, the

symbols H and E are referred to the French terminology humaine and

conomique (human and economic respectively) and not to the UNESCO

terminology H (Hazard) and E (Element at risk).

3.3 Energy

As reported by Morgenstern (1985) and Cruden & Varnes (1996), small and

very rapid landslides may be more disastrous than large slow-moving

landslides due to their high kinetic energy.

Intensity may be considered as equal, or proportional, to the kinetic energy

developed by landslides. The energy is variable with the time: equal to zero

at the initial conditions, increasing up to the highest value after the

landslide triggering, then decreasing up to zero again. Landslide intensity

can be measures as the maximum kinetic energy or the average value

developed by a landslide.

An assessment of the energy balance may be done following the sled

model, proposed by Heim (1932) and after by Scheidegger (1973), Hs

(1975) and Sassa (1988), based on the assumption that all the energy of

the movement is lost for friction.

3.4 Consequences

Another way of assessing landslide intensity is to relate the event with the

potential damage. This approach has been proposed by the French project

PER (Plan dExposition aux Risques) (DRM, 1988, 1990) where the intensity

levels are defined with respect to the potential damage for human lives

(Tab. 3.8) and economy (Tab. 3.9).

Degree Intensity Potential consequences Landslide types

H0 Very lowImprobable damage (except induceddamage)

Slow-moving landslides

H1 Medium Isolated damage Isolated falls

H2 High Some victims Falls, slides or mud flows

H3 Very high Catastrophe (some tens victims)Catastrophic falls and slides,rapid mud/debris flows

Table 3.8 - Scale of landslide intensity with respect to potential

consequences on human life (DRM, 1990)

Degree Intensity Potential consequences Example

E0 Low10% of worth of an individual singlehouse

Detachment of unstable blocks

E1 MediumTechnical intervention on few houses orsmall lots

Rock detachment or fallingrock barriers; drainage ofsmall unstable areas

E2 HighHigh qualified technical intervention of alarge area, with relevant costs

Stabilisation of a largelandslide; consolidation of arock slope

E3 Very highAny technica l intervention hasunacceptable costs for populations

Catastrophic fall or slide

Table 3.9 - Scale of landslide intensity with respect to potential

economic losses (DRM, 1990)


B.III-14

3.5 Hybrid

Landslide intensity can be assessed using different parameters as velocity,

kinetic energy, depths and heights of landslide deposits, applied, following

their suitability, for different landslide types. This approach is adopted in the

guidelines for landslide hazard and risk assessment in Switzerland (Raetzo

et al., 2002). Indicative values of these parameters are used to subdivide

potential unstable areas or risk areas in three different intensity classes:

high, medium, low (Tab. 3.10). The criterion for assessing landslide

intensity is applied on the most diffuse landslide types, distinguishing for

each one those parameters that properly characterise the intensity.

Landslide types Low intensity Average intensity High intensity

Rock falls E < 30 kj 30 < E < 300 kj E > 300 kj

Rock avalanches E > 300 kj

Slides v 2 cm/year v: dm/year (> 2 cm/year) Large differential movements: v >0.1 m/day for superficial slides;displacement > 1 m per event

Earth/debris flow

Potential e < 0.5 m 0.5 m < e < 2 m e > 2 m

Real - h < 1 m h > 1 m

Table 3.10 - Criteria for intensity assessment. E = kinetic energy; v

= long-term average velocity; e = depth of unstable mass; h =

height of deposit (Raetzo et al., 2002)

The BUWAL method (1998), used for the assessment of hazard in

Switzerland, provides an intensity analysis by defining the magnitude as

product of velocity vs. geometrical severity. The velocity is compared with

velocity classes provided by Cruden & Varnes (1996) (see table 3.11).

Class Description Velocity Potential damage for peopleVelocityBUWAL

7 Extremely rapid 5 m/s

6 Very rapid 3m/minSevere injury and/or death 3

5 Rapid 1.8 m/h

4 Moderate 13m/monthModerate injury 2

3 Slow 1.6 m/year

2 Very slow 16 mm/year

1 Extremely slow

Minor/no injury 1

Table 3.11 - Landslide intensity scale following BUWAL (1998)

compared with intensity scale from Cruden & Varnes, 1996

The geometrical severity is defined through the following table.

Falls Slides and flows Geometrical severity

Diameter of blocks > 2 m Depth > 10 m 3

Diameter of blocks 0.5 - 2 m Depth 2 - 10 m 2

Diameter of blocks < 0.5 m Depth < 2 m 1

Table 3.12 - Landslide intensity scale following BUWAL (1998) based

on geometrical severity compared with different landslide types

The magnitude is the product of geometrical severity by velocity (Tab 3.13)

Potential damage to structures Magnitude

Severe (disruption) 6-9

Functional 3-4

Minor 1-2

Table 3.13 - Landslide intensity expressed as total magnitude

(BUWAL, 1998)


B.III-15

4 Return period of landslidesThe most common criteria for the assessment of landslide return times are

the following:

a) Geomorphological criteria or qualitative analysis;

b) Analysis of temporal series related to effects: the analysis of temporal

series of mass movements enables to define directly the return times of

landslides;

c) Analysis of temporal series related to causes: the correlation between

landslides and long-time records of possible triggering elements (i.e.

rainfall, seismicity) permits to define critical thresholds and associated

return times;

d) Monitoring: the instrumental observation of piezometric levels or slope

displacement for single landslides allows the prediction of slope

movements through the comparison with thresholds or models calibrated

on the observed unstable or potentially unstable areas.

4.1 Geomorphological criteria or qualitative analysis

The recurrence of landslides can be assessed subjectively from general or

qualitative information such as historical, geomorphological and

geotechnical analyses. The direct analysis, using geomorphological and

geotechnical information, in case of lack of historical data, can provide a

probabilistic estimation on landslide occurrence. This approach, although

subjective, is an effective way for assigning relative probability of return

times of landslides in a given area. For instance, a lower probability of

landslide occur in slope that exhibit rounded and vegetated crown areas

while higher probability can be associated to steep slopes with tension

cracks.

4.2 Analysis of temporal series related to effects

Historical analysis is the main source for assessing landslide return times.

The main sources are:

a) Multi-temporal analysis of maps (i.e. geomorphological maps, landslide

inventory maps);

b) Multi-temporal analysis of aerial photos or satellite images;

c) Newspapers;

d) Direct observations;

e) Scientific publications;

f) Technical reports.

Long-term activity of landslides can be analysed with datation methods,

commonly used in Earths Sciences, such as radiocarbon, lichenometry and

dendrocronology methods (Starkel, 1966; Schoeneich, 1991; Corominas et

al., 1994).

The annual frequency f(N) of landslides in a period of N years, is the ratio

between the number of events n and the number N of observed years. If N

is quite long, f(N) is an estimation of the annual probability of occurrence P.

Landslide hazard can be calculated as follows:

H(N) = 1 - (1 - P)N = 1 - (1 - 1/T)N (4.1)


B.III-16

When landslide events are very low compared to the time under analysis,

namely N


B.III-17

movements is analysed without implementing physical laws that rule the

transformation rainfall-infiltration-piezometric response;

b) deterministic models: where hydrological models are use for analysis of

various parameters (rainfall, run-off, effective infiltration) and

hydrogeological models for analysis of piezometric height and aquifer

recharge;

c) hybrid models: where the above approaches are usually coupled (e.g.

aquifer recharge through a hydrological model and piezometric response

by means of a statistical analysis).

4.3.2 Earthquakes

The influence of earthquakes on slope stability is still quite controversial

and, often, overestimated.

The effects of an earthquake on a slope can be direct or indirect. The former

may cause slope movements during the seismic event; the former may

occur from some hours to some days after an earthquake (Hutchinson,

1993).

The main effect results in increasing of destabilizing stresses through the

application of a transient horizontal inertial stress (F = KW), where W is the

weight of the potential unstable mass and K is the coefficient of seismic

acceleration. Such a stress is generally assumed in the calculation of limit

equilibrium for the assessment of the safety factor under pseudo-static

conditions. Since the wavelengths of strong earthquakes, in most of

terrains, are some tens meters, the effects of direct destabilization may

occur only along short slopes, with length generally < 30m (Ambraseys,

1977; Hutchinson, 1987). For longer slopes, in fact, the destabilizing

acceleration produced toward the external part of slopes is balanced by a

stabilizing acceleration, with opposite direction, caused by the succeeding

waves. The highest synchronous acceleration, associated to the maximum

peak of K, results for landslides that have a dimension equal to half

wavelength.

Rock falls, debris flows and earth flows are the most common landslide

types triggered by earthquakes, especially under saturated conditions of

terrains. Re-activation of slides and flows may occur also in cohesive

materials (Hutchinson & Bhandari, 1971).

Another important effect of earthquakes, affecting saturated loose granular

soils, is the dynamic liquefaction (Seed & Idriss, 1967; Seed, 1968; Valera

& Donovan, 1977; Crespellani et al., 1988). An earthquake can cause a

consolidation of soils due to a structural collapse and, consequently,

promote critical pore pressure values that result in liquefaction. In this case,

the shear strength of soils can decrease abruptly causing sliding also along

gentle slopes. Many landslide-induced earthquakes are generally associated

to liquefaction (Seed, 1975). Moreover, since ground failure is fragile,

landslides generated by liquefaction are characterised by high velocity and

long run-out, constituting very hazardous phenomena. The assessment of

potential liquefaction of a soil is based on seismic parameters (e.g.

magnitude, duration, number of cycles, distance from epicentre, maximum

site acceleration) as well as on geotechnical parameters (e.g. grain-size

distribution, uniformity, relative density, SPT test, initial stress state).


B.III-18

The indirect effects, generally, cause the re-activation of pre-existing

landslides, also large landslides in cohesive soils, due to the cyclic load on

pore pressure regime. Some investigations (Lemos et al., 1985; Sassa,

1992) have shown that the application of rapid cyclic loads in special shear

ring apparatus can cause, in some cohesive materials, a progressive

decreasing of residual shear strength after an initial peak. Such a behaviour

can explain the delay between the seismic shake and landslide trigger.

The triggering thresholds of landslides can be assessed as local seismic

response parameters (i.e. intensity, peak acceleration) or as earthquake

source parameters (i.e. magnitude). A common adopted methodology is to

implement a deterministic analysis (i.e. pseudo-static slope stability

analysis) to calculate the critical acceleration that produce a safety factor

equal to 1. Another method is to compare empirically landslides occurrence

with site intensity of magnitude at the source.

4.4 Monitoring

The assessment of landslides occurrence based on monitoring provides the

most detailed and reliable information, especially in landslide hazard and

risk analysis at local/site scale.

Two distinct approaches can be adopted in landslide hazard analysis

through monitoring: mechanical and kinematical approaches.

4.4.1 Mechanical approach

Theoretically, the prediction of landslide movements or potentially unstable

slopes can be done through the monitoring of all parameters, variable in

time, that define the safety factors such as pore pressure, slope

morphology, geotechnical parameters of terrains, loads.

In practical terms, pore pressure is the parameter that exhibits a large

uncertainty as well as a wide variability in time. Therefore, the mechanical

approach is essentially based on piezometric measures.

Generally, 1-2 years of piezometric observations are used to deduce a

possible correlation between rainfall and groundwater fluctuation. The

prediction of hazard can be done through a statistical analysis of

precipitation. Estimated pore pressures can be used as input in slope

stability models for assessing the safety factor; in addition, their

probabilistic assessment through the analysis of annual extremes enables to

associate a return time to each safety factor value (hazard analysis).

A wide scientific literature is available on the prediction of piezometric

fluctuation related with rainfall and other meteorological parameters, based

on empirical, deterministic and hybrid approaches applied to slope stability

assessment.

4.4.2 Kinematics approach

A direct approach for predicting the time of failure of a slope is based on

displacement monitoring, through topographic, extensometric or

inclinometric measures.


B.III-19

5 Hazard assessment (H)Hazard is defined as the probability that a landslide with a given intensity

may occur in a specific time and area. It can be expressed as annual

probability or return time of a landslide. Landslide hazard is to be referred

to a well defined intensity:

H = H (I)

5.1 Basic concepts

As a basis for landslide hazard assessment and zoning any methodology

should be referred to the following four fundamental assumptions, widely

accepted by landslide experts (Varnes et al., 1984; Carrara et al., 1991;

Hutchinson & Chandler, 1991; Hutchinson, 1995; Turner & Schuster, 1995):

Landslides will always occur in the same geological, geomorphological,

hydrogeological and climatic conditions as in the past;

The main conditions that cause landsliding are controlled by identifiable

physical factors and laws that can be empirically, statistically and

deterministically defined;

The degree of hazard can be evaluated;

All types of slope failures can be identified and classified.

On this basis a complete assessment of landslide considers prediction of the

following parameters (Hartln & Viberg, 1988):

Typology: prediction of the landslide type that may occur in the

considered area;

Affected area: prediction of where a landslide may occur;

Return time: prediction of when a landslide may occur;

Intensity: prediction of dimension (area and/or volume), velocity or

energy of a landslide;

Evolution: prediction of run-out, retrogression limits and/or lateral

expansion of a landslide.


A rigorous landslide hazard assessment needs a large amount of

information on the various parameters. In addition, considering that reliable

scenarios should be done at local/site scale, this implies a time and money-

consuming activities for analysis and mapping.

The basic principle for which past and present unstable areas will be

affected by landslides in the future can be valid for factors that are constant

in time, such as geological, structural and geotechnical setting. A correct

landslide hazard assessment should take into account the variability of

those factors that may play a role in slope stability like climate and land use

modifications.

Due to the objective conceptual and operational limits, most of landslide

hazard maps should be defined as landslide susceptibility maps.

Limitations of landslide hazard analysis mainly include:

o Spatial and temporal discontinuity of landslides,


B.III-20

o Objective difficulty in the prediction of causes, triggering

factors and cause-effect relations,

o Long-term and exhaustive historical records on landslides and

potential triggering factors (i.e. long-term climatic records).

5.2 Prediction of landslide types

The prediction of landslide types that may occur in an area can be done

starting from an accurate landslide inventory map. In this case, the spatial

and temporal prediction of landslides can be differentiated according to

Cruden & Varnes classification.

As concerning potentially unstable areas, the expected landslide type can be

assessed on the basis of the typologies occurring in areas with similar

geological, geomorphological, geotechnical and land use characteristics.

5.3 Prediction of landslide intensity

The prediction of the landslide intensity is mainly depending on the amount

and quality of information from landslide inventory. Landslide hazard, or the

probability of occurrence, should be differentiated according to the

intensity, in order to have an estimation of consequences (risk analysis).

On this issue, some authors (i.e. Fell, 1994) provide a different definition of

landslide hazard that can be expressed as the product of intensity (I) and

probability of occurrence (P).

The methodology proposed by Fell (1994) considers the product of an index

of intensity by an index of probability associated to intensity and hazard

classes (Tab. 5.1).

INTENSITY PROBABILITY HAZARD

I Description Volume (m3) P Description P (annual) H = I P Description

7 Extremely high > 5 x 106

12 Extremely high 1 30 Extremely high6 Very high

1 x 106 5 x

106

8Very high 0.2

20-29Very high

5 High2.5 x 10

5 1 x

106

5High 0.05

10-19High

4 Medium5 x 10

4 2.5 x

105

3Medium 0.01

7-9Medium

3 Small5 x 10

3 5 x

104

2Small 0.001

3-6Small

2.5 Very small5 x 10

2 5 x

103

1Very small 0.0001

2Very small

2Extremely

small< 5 x 10

2

Table 5.1 - Assessment of landslide hazard with prediction of

intensity (Fell, 1994)

The approach proposed by DRM (1990) for PER implementation is more

correct since it provides the definition of the probability of occurrence

depending on different levels of intensities. Anyway, in the implementation

of PER sometimes the local authorities have used operational criteria where

landslide hazard is not properly expressed as combination of probability by

intensity (Perrot, 1988).


B.III-21


The expression of hazard as I x P is not coherent with the UNESCO

terminology and can present some problems. In fact, an event of small

dimensions and very frequent can have the same hazard of a very large and

less frequent landslide event. In addition, the vulnerability is already related

to landslide intensity. This implies that intensity is taken into account two

times in the risk assessment. To overcome this problem Fell defines the

specific risk as the product of probability by vulnerability.

5.4 Prediction on landslide affected area

The spatial prediction of hazard consists in the assessment of relative

hazard. This is the degree of hazard of a slope compared with other slopes,

without indicating the probability of occurrence of landslides (temporal

occurrence), generally based on historical and present instability.

5.4.1 Methods of relative hazard analysis

The methods for the assignment of the different hazard levels can be

qualitative or quantitative, direct or indirect:

Qualitative methods are subjective and describe the hazard zoning in

terms substantially descriptive;

Quantitative methods provide numerical estimations, in terms of

probability, on landslide occurrences for each hazard class;

Direct methods produce essentially geomorphological mapping of

landslide hazard (Verstappen, 1983);

Indirect methods are essentially carried out through distinct stages. First

of all, these require a field analysis and a landslide inventory map in the

study area or in a subset area (training area). The second step is the

detection and mapping of a set of physical factors that are directly or

indirectly related to slope instability (predisposing factors). Finally, an

assessment of the relative contribution of the various predisposing

factors for landsliding and a hierarchisation of landslide hazard in areas

with distinct hazard degree (hazard zoning).

5.4.1.1 Qualitative methodologies

In general qualitative approaches are based entirely on expert judgement.

The input data are usually derived from assessment during field trips,

possibly integrated by aerial photo interpretation. These methodologies can

be divided into two types: field geomorphological analysis and combination

or overlaying of index maps (heuristic approach).

5.4.1.1.1 Field geomorphologic analysis

This approach is a direct and qualitative method based on the experience of

the earth scientist in assessing present and potential landsliding without any

specific indication of rules that have led to the assessment and/or zoning. In

this case the stability maps are directly evolved from detailed

geomorphological maps. The assessment of slope stability takes into

consideration a very large number of factors. They can be used successfully

at any scale and adapted to specific local requirements. The field

geomorphological analysis does not require the use of a Geographical


B.III-22

Information System which, in this case, is simply a drawing tool. Examples

of geomorphological-based hazard analysis are very frequent in scientific

literature of the 70s and 80s.

Perhaps one of the most comprehensive projects reported in the literature is

the French ZERMOS (Zones Exposes des Risques aux MOuvements du

Sol et du sou-sol) procedure which involves two main phases: analysis and

extrapolation. In the first phase, all the predisposing factors are examined,

both permanent (e.g. topography, geology, hydrogeology) and temporary

(e.g. climate, land use and other man-made factors). Active and/or inactive

landslides may be analysed. In the following phase all the factors are

extrapolated by the author to areas with similar physical conditions, thus

enabling zoantion of the area into three sections with varying degrees of

hazard (defined as risque):

null or low hazard, areas in which no instability should occur;

potential or uncertain hazard, areas with potential instability of uncertain

nature and extent;

ascertained hazard, areas with declared instability and certain threat of

failure.

The hazard calculated is of a relative nature and the authors acknowledge

that the various hazard categories cannot be compared from one area to

another. The choice of three classes is inspired to the necessity to let the

hazard maps comprehensive by stakeholders and end users (Public

Administration)


The main disadvantages of such approaches are:

The subjectivity in the selection of both the data and the rules that

govern the stability of slopes or the hazard of instability; this fact makes

it difficult to compare landslide hazard maps produced by different

investigators or experts;

Use of implicit rather than explicit rules hinders the critical analysis of

results and makes it difficult to update the assessment as new data

become available;

Lengthy field surveys are required.

5.4.1.1.2 Combination of index maps or heuristic approach

In this approach, the expert selects and maps the factors that affect the

slope stability and, based on personal experience, assigns to each a

weighted value that is proportionate to its expected relative contribution in

generating failure. The following operations should be carried out:

subdivision of each parameter into a number of relevant classes;

attribution of a weighted value to each class;

attribution of a weighted value to each of the parameters;

overlay mapping of the weighted maps;

development of the final map showing hazard classes.

The advantages of such a methodological approach are that it considerably

reduces the problem of the hidden rules and enables total automation of the

operations listed above through appropriate use of a GIS. Furthermore, it

enables the standardisation of data management techniques, from

acquisition through final analysis. This technique can be applied at any


B.III-23

scale. In order to provide a better coherence to physical processes, the

attribution of weights can be supported by statistical analysis on the

assessment of the contribution of single predisposing parameters to slope

instability.


The major disadvantage is the lengthy operations involved, especially where

large areas are concerned. In addition, the reliability of heuristic methods

depends largely on how well and how much the investigator understands

the geomorphological processes acting upon the terrain. Since this

knowledge can be formalised into rules, the method could take into account

local geomorphological variability or specific conditions leading to slope

failures. Major limitations refer to the fact that in most cases the available

knowledge on the causal relations between environmental factors and

landslides is inadequate and, most importantly, is essentially dependent on

the experience of the investigator. At present, maps obtained by this

method cannot be readily evaluated in terms of reliability or certainty.

Additionally, landslide hazard is not directly expressed in terms of

probability, limiting the use for risk evaluation and economic estimates.

5.4.1.2 Quantitative methodologies

5.4.1.2.1 Statistical analysis

The attribution of weighted values on a subjective basis to the numerous

factors governing slope stability represents the main limitation in all the

methods described above. The solution to this problem could be to adopt a

statistical approach that compares the spatial distribution of landslides with

the parameters that are being considered. The results could then be applied

to areas currently free of landslides but where conditions may exist for

susceptibility to future instability. The major difficulty consists in

establishing the slope failure processes and in systematically identifying and

assessing the different factors related to landsliding. One of the principal

advantages is that the investigator can validate the importance of each

factor can validate the importance of each factor and decide on the final

input maps in an interactive manner. The use of GIS makes these

operations much easier and to a large extent explains the increasing

adoption of the statistical approach which closely parallels the ever-

increasing application of GIS techniques.

Statistical analyses can be either bivariate or multivariate.

Bivariate statistical analysis

In bivariate statistical analysis each individual factor is compared to the

landslide map. The weighted value of the classes used to categorise every

parameter is determined on the basis of landslide density in each individual

class. The following operations are required:

selection and mapping of significant parameters and their categorisation

into a number of relevant classes;

landslide mapping;

overlay mapping of the landslide map with each parameter map;


B.III-24

determination of density of landslides in each parameter class and

definition of weighted values;

assignment of weighting values to the various parameter maps;

final overlay mapping and calculation of the final hazard or susceptibility

value of each identified land unit.

The simplest models consider the determination of threshold value of slope

angle, for each lithological type, that may cause potential slope instability.

The models are based on a series of empirical functions that relate two or

more significant parameters, such as slope height or slope angle of

landslide areas.

The bivariate statistical approach is widely employed by the earth scientists

and numerous parameters may be taken into consideration: lithology, slope

angle, slope height, land use, distance from major structures, drainage

density, relief morphology, closeness of the facet to a river, attitude of

lithotypes. This approach has been successfully employed by researchers

mapping hazard of superficial failures that affect prevalently weathered and

soil covers, triggered by heavy rainfall.

Multivariate statistical analysis

Although the multivariate statistical approach had already been successfully

applied in several areas of applied geology, such as petroleum exploration,

the application of this technique to landslide hazard assessment began since

the second half of 70s. The so-called black-box statistical approaches are

based on the analysis of functional links among the various predisposing

factors and present and past landslide distribution.

The procedure involves several preliminary steps which are undertaken in a

test area. Once the results achieved have been verified, they are extended

to the entire area under examination. The following steps are required.

1. Classification of the study area into land units.

2. Identification of significant factors and creation of input maps.

The input variables include information concerning the landslides (i.e.

typology, degree of activity) and geo-referencing. Several attributes are

automatically derived from statistical operations performed on the basic

parameters (mean, standard deviation, maximum or minimum values). An

important aspect is the conversion of various parameters from nominal to

numeric, such as geological composition or vegetation cover. This can be

done through the creation of dummy variables or by coding and ranking the

classes based on the relative percentage of the area affected by landsliding.

The two methods are similar but the latter is to be preferred.

3. Construction of a landslide map.

4. Identification of the percentage of landslide-affected areas in every land

unit and their classification into unstable and stable units.

The threshold value of this classification is fixed every time on the basis of

two requirements: a) in areas with high landslide density, the threshold

should be based on relatively high percentages in order to achieve two

statistically representative groups; b) however, even a relatively low

landslide density must be taken into consideration as it could represent a


B.III-25

risk for human activities. In general, the decision to carry out the analysis

on two groups only (unstable and stable land units) simplifies the problem

from a statistical point of view but hinders the identification of various

combinations of factors related to different hazard types.

5. Combination of the parameter maps with the land unit map and creation

of an absence/presence matrix of a given class of a given parameter within

each land unit.

6. Multivariate statistical analysis.

The statistical analysis most frequently used are discriminant analysis or

regressive multiple analysis which are often employed in parallel within the

same project. It is preferable to apply discriminant analysis (stepwise or

canonical discriminant analysis) with continuous variables, while the

regressive analysis can be used even with nominal variables.

7. Reclassification of land units based on the results achieved in the

previous phase and their classification into susceptibility classes.

In the discriminant analysis for example, the inspection of the standardised

discriminant coefficient allows the contribution of various parameters in

causing slope instability to be quantified and, as a result, enables objective

reclassification of the study area. By transforming the classification function

scores into probabilities, the susceptibility map can then be converted into a

hazard map.


Black-box are conceptually simple but, due to the great complexity in

identifying the slope failure processes and the difficulty in systematically

collecting the different factors related to landsliding, the task of creating

a geomorphological predictive model enabling actual/potential unstable

slopes to be identified over large areas, is difficult operationally.

Errors in mapping past and present landslides will exert a large and not

readily predictable influence on statistical models, particularly if errors

are systematic in not recognizing specific landslide types.

Additionally, being data-driven, a statistical model built up for one region

cannot readily be extrapolated to the neighbouring areas.

5.4.1.2.2 Geotechnical models

Deterministic analysis

This type of approach involves analysing specific sites or slopes in

engineering terms. The main physical properties are quantified and applied

to specific mathematical models and the safety factor is calculated. These

models (mono, bi- and tri-dimensional) are commonly used in soil

engineering for slope-specific stability studies. The approach is widely

employed in civil engineering and engineering geology and has been applied

to landslide hazard assessment and mapping, especially after the

introduction of GIS. Accuracy and reliability is improved as detailed

knowledge of the area of application increases. A deterministic approach

was traditionally considered to be sufficient for both homogenous and non-

homogenous slopes. The index of stability is the well known safety factor,

based on the appropriate geotechnical model. The calculation of the safety


B.III-26

factor, F, requires geometrical data, shear strength parameters and

information on pore water pressure. Moreover, decisions must be made on

whether to use peak shear strength values or residual shear strength values

(or values in between) for specific parts of the slip surface. For these

reasons such methods are normally applied only in small areas and at

detailed scales. Calculations of the safety factor must be made for each

individual slope or area before a hazard map can be prepared.

The safety factor enables the evaluation on the slope stability degree and

an objective comparison among different slopes. Usually the probability of

occurrence of a landslide is not calculated unless the safety factor is related

to the temporal occurrence of the possible triggering factor (i.e. critical

precipitation return time). A reference for assessing the relative hazard

based on F has been proposed by Ward (1996).

Tab.5.2 - Hazard classes related to safety factor F (Ward, 1976).

Probabilistic analysis

For decades, geotechnical modelling and analysis within a deterministic

framework has facilitated the quantification of safety or reliability. However,

performance indicators such as the factor of safety, F, do not take into

account the variability of geotechnical parameters of terrains such as

cohesion c, angle of internal friction and the undrained shear strength susome of which may also vary in magnitude with time. The spatial and

temporal variability of pore water pressure is again very important but is

not reflected in the calculated values of the conventional factor of safety.

The probability of failure is defined as the probability that the performance

function has a value below the threshold value. Considering the factor of

safety, F, as the performance function, the threshold value is 1 and

probability of failure pf may, therefore, defined as:

pf = P[F 1.7


B.III-27

The numerator gives the extent to which the average value is above the

threshold value and the denominator reflects the dispersion from this

average value.

Three commonly used methods of probability calculation are:

First Order Second Moment Method (FOSM);

Point Estimate Method;

Monte Carlo Simulation Method.

The availability of GIS can facilitate the use of a deterministic or a

probabilistic geotechnical approach as part of a methodology of landslide

hazard assessment. For example, after subdivision of an area or region into

elements or small areas, the factor of safety of individual sloping areas may

be computed and then mapped. Alternatively, using a probabilistic

framework, the probability of failure of individual slopes could be computed

and then mapped. Depending on the adopted hazard assessment approach,

this information could be used on its own or combined with other factors or

factor maps to produce susceptibility maps and/or hazard maps.


The deterministic models that provide safety factor calculation usually do

not provide return time of landslide phenomena.

Systematical uncertainties derive from the following considerations:

o a soil mass can only be investigated at a finite number of points;

o the number of field and laboratory tests conducted to determine soil

parameters is limited by financial and time constraints;

o the testing equipment and methods are not perfect.

Other uncertainties are associated with geotechnical models, landslide

mechanisms, their occurrence and impact.

A deterministic analysis of landslide hazard can be done only for single

slopes or limited areas where a large and detailed geotechnical data are

available.

Reliable mechanical models are not yet available for several types of

structurally complex rock units.

5.5 Temporal prediction

The temporal prediction of landslide hazard is essentially based on definition

of the probability of occurrence of landslides. While spatial prediction

provides a relative hazard of different slopes, the temporal prediction

provides an absolute hazard.

For some authors the term absolute hazard is not referred to the probability

of occurrence, but to the definition of the safety factor (F) that, as already

discussed, indicates only a relative zoning of a slope-failure propensity and

not a probabilistic estimate of occurrence.

5.5.1 Analysis of return time

If P is the annual probability of occurrence of a landslide, the return time T

of the event is given by 1/P.


B.III-28

The probability of occurrence of m events in a period of N years is given by:

P m NN

m N mP Pm N m( , )

!

!( )!( )=

1

In case of rare events compared with the available temporal scale (for

N>30 or P


B.III-29

5.5.2 Analysis of intensity/magnitude

The BUWAL method (1998) is used for the analysis of landslide hazard. This

is calculated through a matrix between intensity, expressed as velocity

(Tab. 5.5) and magnitude (Tab. 5.6), vs. return times of potential

landslides.

Velocity Return time (years) Probability (annual) Hazard1 < 1 (active) 1 11 1-30 0.03 31 30-100 0.01 2

1 100-1000 0.001 2

2 < 1 (active) 1 32 1-30 0.03 42 30-100 0.01 32 100-1000 0.001 22 > 1000 0.0001 1

3 < 1 (active) 1 4

3 1-30 0.03 43 30-100 0.01 43 100-1000 0.001 33 > 1000 0.0001 2

Table 5.5 - Potential landslide hazard related to velocity and return

time (BUWAL, 1998)

Magnitude Return time (years) Probability (annual) Hazard1-2 < 1 (active) 1 2

1-2 1-30 0.03 31-2 30-100 0.01 2-31-2 100-1000 0.001 2

3-4 < 1 (active) 1 33-4 1-30 0.03 3-43-4 30-100 0.01 33-4 100-1000 0.001 2-3

3-4 > 1000 0.0001 1

6-9 < 1 (active) 1 46-9 1-30 0.03 46-9 30-100 0.01 46-9 100-1000 0.001 36-9 > 1000 0.0001 1-2

Table 5.6 - Potential landslide hazard related to magnitude and

return time (BUWAL, 1998)

Since vulnerability is proportional to landslide phenomena, the levels of

landslide hazard, when associated to the elements at risk, correspond to

specific risk (Cruden & Varnes, 1996).

5.6 Prediction of evolution

The prediction of landslide evolution consists in the detection of the area

that can be directly or indirectly affected by a landslide, through the

analysis of:

o prediction of run-out;

o prediction of retrogression limits;

o prediction of lateral expansion.


B.III-30

5.6.1 Prediction of run-out

Current an past research into the run-out calculation of a fast-moving

landslide (i.e. rock falls, debris flows, rock avalanches) can generally be

grouped into three categories. The first includes empirical models aimed at

providing practical tools for predicting the run-out distance and distribution

of landslide accumulation. The second category includes simplified analytical

models that describes the physical behaviour of mass movement, based on

lumped mass approaches in which the mass is assumed as a single point.

The third includes numerical simulations of conservation equations of mass,

momentum and energy that describe the dynamic motion of landslide mass,

and/or a rheological model to describe the material behaviour of sliding

mass.

5.6.2 Prediction of retrogression limits

The prediction of retrogression limits is essentially based on the analysis of

geomorphologic evidences on the field that define the distribution of activity

(i.e. tension cracks, counter slopes). Some landslide typologies are

generally characterised by a retrogressive evolution such as rotational and

translational slides, rock topples/falls affecting jointed rocks. In most cases,

the ultimate limit is represented by the catchments divide so that the

analysis, although conservative, can be carried out simply by using

topographical information.

5.6.3 Prediction of lateral expansion

The prediction of lateral expansion is important in the analysis of earth

flows or liquefaction phenomena, when the displaced mass is very fluid and

may expand to the slope toe. The prediction is very complex and mostly

depends on slope morphology, grain-size and water content of soils, shear

strength of materials, pore pressure and lateral stress coefficient.

6 Landslide hazard mapping

6.1 Introduction

The identification and map portrayal of areas highly susceptible to

damaging landslides are first and necessary steps toward loss-reduction

(Zeizel, 1988). There are four general categories of potential users of

landslide hazard information (Wold and Jochim, 1989):

o scientists and engineers who use the information directly;

o planners and decision makers who consider landslide hazards among

other land-use and development criteria;

o developers, builders, and financial and insuring organizations;

o interested citizens, educators, and others with little or no technical

experience.

Members of these groups differ widely in the kinds of information they need

and in their ability to use that information (Wold and Jochim, 1989).

Most local governments do not have landslide hazard maps and do not have

funding available for mapping activities, and such communities usually look

to a higher level of government for mapping. The U.S. Geological Survey

(USGS) has provided maps in some areas (e.g., demonstration


B.III-31

mapping of San Mateo County, California; Brabb et al., 1972), but in

general, landslide hazard mapping by the USGS has had limited geo-

graphic coverage. Although most local communities look to their state as

the primary source of maps, few states have undertaken significant

landslide hazard mapping programs. However, there are important

exceptions. California and Oregon, for example, have undertaken landslide

hazard mapping at standard USGS mapping scales. These maps provide an

excellent starting point for local communities and, importantly, form the

basis for state laws that require a certain level of compliance with the

information they provide.

The considerable variability among state geological agencies, particularly in

terms of their existing mapping capabilities and projected funding

environments, makes it difficult to provide detailed commentary and

suggestions regarding the partnerships between the USGS and states for

landslide hazard mapping and assessment. Historically, there have been

strong ties between the USGS and state geological surveys in the realm of

mapping (e.g., Ellen et al., 1993; Coe et al., 2000b) and, to a lesser extent,

for the identification and mitigation of natural hazards. The suggestion in

the national strategy proposal (Spiker and Gori, 2000) of mapping

partnerships, using a model based on competitive grants and matching

funds (as with the existing National Geologic Cooperative Mapping

Program), would undoubtedly provide resources for a considerable amount

of much-needed mapping. However, such a model raises the possibility that

hazard ma

landslide current availability and methodology for natural risk map production armonia project...

Documents

landslide activity

qualitative analysis

landslide causes

statistical analysis

analysis of temporal

prediction of landslide

t61 definition of landslide

field geomorphologic