04 Run Out Zones Field Suveys Modelling

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Example of Cairns, Australia

Regional risk analysis

Quantitative approach

Landslide risk as basis for planning and emergency

management purposes

Input data:

Historical landslide information

Geological information

Geomorphological information

Run out of landslide (empirical model)

Information on buildings, roads and demography

Michael-Leiba, Baynes, Scott, 1999

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Example of Cairns,

Australia

Michael-Leiba, Baynes, Scott,

1999

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Risk assessment

from magnitude

recurrence graph



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Example of

Cairns, Australia

Landslide data map


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Example of

Cairns, Australia

Landslide risk map

Number of houses and

blocks of flats expected to

be destroyed per square km

per 100 years



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Major findings of the study

Greatest total risk for buildings on hill slopes

Total of 13 buildings could be destroyed in 100 years,

if no mitigation measure is taken

Highest total risk for people is in proximal parts of

debris flows

Total of 16 people could be killed in 100 years

Main access to Cairns, north and south, pass to steep

slopes and can be blocked by landslides

=> Makes Cairns vulnerable to isolation


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Example of Cairns, Australia Drawbacks:

Paucity of data from which landslide magnitude-

recurrence were derived

Regional study

Site-specific assessments should be checked by

geotechnical experts

Lack of discrimination between the effects of shorter,higher intensity rainfall events, of antecedent rainfall and

of longer, lower intensity rainfall events

Assumptions:

shadow angles are uniform for all debris flows in the

study area

Vulnerability is independent of landslide magnitude

Landslide intensity is uniform across a landslide


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Summary: Qualitative & quantitative assessments

Qualitative assessment (Rheinhessen) Regional assessment

Coarse data sets (information from flood research)

First approximation of landslide risk

Quantitative assessments (Iceland / Cairns)

Regional assessments

Detailed spatial information

Differentiate: Specific Risk / Total Risk (MultiRISK)

Be aware that independent on your method ......... these analysis are PURELY approximations

... there are high uncertainties

... local assessments are always needed for critical locations

... analysis can only support - and not enforce - decisions

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Determination of runout zones of landslides

- From field surveys to modelling -

PD Dr. Thomas [email protected]


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Risk assessment & management (1/3)

P robability oflandsliding

Triggeringfactors

Landslideinventory

P reparatoryfactors

Hazardassessment

Runoutbehavior

Land use

E lements atrisk

Vulnerabilityassessment

Riskassessment

Riskmanagement

C ost-benefitanalysis Dai et al. (2002)

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Lecture Overview Repetition:

Landslide activity & rate of movement

Temporal & spatial occurrence of landslides

Soil mechanical basics

Preparatory, triggering and controlling factors

General considerations to spatial modeling

Calculation of runout zones

Parameters defining & contributing to runout behavior

Methods for predicting runout distance

Empirical models

Analytical models

Numerical models

Coupling local and regional assessments


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Active

Suspended

Reactivated

Inactive

Dormant

Abandoned

Stabilized

Relict

Ac ti ve Suspended

Reactivated Inactive

Landslide activity State of activity

Adopted from Cruden & Varnes, 1996

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Active

Suspended

Reactivated

Inactive

Dormant

Abandoned

Stabilized

RelictRelict

Dormant Abandoned

Stabilized

Landslide activity State of activity



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Advancing Retrogressive

Enlarging Diminishing

Confined Moving Widening

Landslide activity - Distribution of activity


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Complex

Composite

Successive

Single

Multiple

Landslide activity Styles of activity

Cruden & Varnes, 1996


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Rate of movement

Cruden & Varnes, 1996

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Rate of movement

http://walrus.wr.usgs.gov/elnino/landslides-

sfbay/photos.html

USGS


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Temporal and spatial occurrence of natural hazards

Earthquake

Tsuna

mi

Snow

avalancheLandslide

Volcanism

Storm

tide

Storm

Flooding

Drought

Desertification

Space

Space

LocalLocal

PunctualPunctual

Second

Second

Day

Day

Year

Year

Decade

Decade

TimeTime

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Space

Time

Topple/Rockfall

Slide/ Sackung

Debrisflow

short

small

large

long

Ahrtal, Germany (PhotoT. Glade)

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Canada(Photo: M. Crozier)

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Mattertal, Switzerland (Photo: H. Grtner)

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Hawke Bay, New Zealand (Photo: N. Trustrum)

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Isle of Wight, UK (Photo: T. Glade)

Otago, New Zealand(Photo: M. Crozier)

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Temporal and spatial occurrence

of landslides


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fall, slide

flow, slide,

slump, creep

creep

deposition,

creep

Summerf

ield1991

Spatial occurrence of landslides - local

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Gisborne 2002 ( Michael Crozier))

Earth flow / Debris flow

HawkeBay 1998 ( Noel Trustrum)

Wairarapa1972 ( Noel Trustrum)

HawkeBay 1988 ( Noel Trustrum)

Spatial occurrence of landslides - regional


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Soil mechanical basics of landslides

The shear strength of soils is mainly determined by

Cohesion (c)

Internal friction angle ()

and is expressed in the Coulomb equation

f= c + n . tan

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Selby 1993

Cohesion (c)


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Cohesion (c)

Selby 1993

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Press & Siever, 1997

Cohesion (c) and internal friction angle ()


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Internal friction angle ()

Block of rock lies upon a horizontal surface Weight of the block (N) generates an equal and opposite

reaction (R) -> compressive stress, block is immobile

Addition of horizontal stress

Reaction (R) adjusts from normal to the horizontal plane to

the resultant N and H. The relationship between N, H, R and

the angle is shown by the triangle of forces.

Horizontal stress at failure conditions

Increase in H causes increases in R and. When slidingbegins the frictional contact will be broken and will haveattained its maximum possible value. That maximum value is

the internal friction angle ().

tan = N/H = / = shear stress, = normal stress

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shear stress

normal stress

shear plane

Stress acting at a slope


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f= c + n . tan

f = shear stress at failurec = cohesion

n = normal stress = angle of internal friction

f= c + (n - u) . tan

for saturated soils

taking into account the effect ofpore water pressure (u)

negative pore water pressure -> stabilizing effect

positive pore water pressure -> destabilizing effect

Coulomb equation

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Selby 1993


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Methods of slope stability analysis

Field investigations

Define Purpose: Regional - / Spatial assessment

Slope stability modelling

translational slides (infinite slope model)

Rotational slides

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Crozier1986

Factors indicating slope stability (1/2)


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32 Crozier 1986

Factors indicating slope stability (2/2)

Crozier 1986

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cossin

tan)(cos 2

+==

z

mzcsFS w

FS = Factor of Safety (


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sin

]tan)cos([

+==

A

B

A

B

W

uLWcLs

FS

The slide is divided into a number of slices of length L and the forces acting on each

of these slices are aggregated.

FS = Factor of Safety (


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Preparatory factors - Disposition

Weathering

Change in slope geometry

Change in soil hydrology

Melting permafrost

Change in vegetation

Land use change

...

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Triggering factors

Heavy or long lasting rainfall

Snow melting

Earthquake, volcanic eruption

Undercutting of slope

...


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Controlling factors

Slope

Curvature (convex, concave,...)

Vegetation

Channel roughness

...

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Spatial modeling - Advantages

Abstraction to key-issues

Subjectivity by model development and choice

Objectivity: Repetition of similar analysis gives

identical results

Unambiguous rules - Concepts and structures

Uniformity based on objective criteria

Transparency is inherent

Transferability is possible

Potential for scenarios


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Spatial modeling - Disadvantages

Reduction to single parameter indispensable

Commonly statistical relation (if - when)

Danger: Essential, important process-determining

parameter will not be considered

Quality has to be ensured

Assumptions have to be reflected for interpretations

Transferability has to be critically questioned

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Scientific challenges in spatial modeling

Development of process-specific methods

Scale dependent choice of methods is important

Spatial models have to be improved, or further

developed

Validation of results is essential for the judgement of

the quality

Scenarios of events


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Modeling of runout zones

Delimiting extent of endangered areas is fundamental

to landslide risk assessment

=> Prediction of runout behavior of landslides

How far and how fast?

Calculation/Modeling is often simply ignored

Modeling is complex and data demanding

Runout behavior is a set of quantitative and

qualitative spatially distributedparameters thatdefine destructive potential of a landslideDai et al., 2002

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Parameters defining runout behavior

Runout distance

Damage corridor width

Velocity

Depth of the moving mass

Depth of deposits

Wong et al, 1997 &

Hungr et al., 1999 in Dai et al., 2002


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Factors contributing to runout behavior

Factors that control travel:

Slope characteristics

Mechanisms of failure & modes of debris movement

Downhill path

Residual strength behavior sheared zones

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Slope characteristics

Slope geometry

Redistribution of potential energy at failure into:

Friction energy

Disaggregating energy

Kinetic energy

Slope-forming material

Convergence of hydrologic pathways

Upslope influence zone


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Mechanisms of failure & modes of debris movement

Velocity and travel distance is influenced by:

Modes of debris movement

Disintegration of the failure debris

Convergence of surface runoff

Contractive soils often evolve into debris flows that

may travel great distances

Dilatant soils tend to be slow-moving landslides

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Downhill path

Gradient of downslope path

Possibility of channelization

Characteristics of ground surface

Susceptibility to depletion

Response to rapid loading

Type of vegetation

Extent of catchment


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Residual strength behavior of sheared zones

Presence or absence of pre-existing shears

Degree of brittleness

Three types of residual strength

Neutral rate effect

Constant residual strength

Positive rate effect

Soils showing an increase in residual strength

above the slowly drained residual value at

increasing rates of displacement

Negative rate effect

Soils showing a significant drop in strength when

sheared at rates higher than a critical value

Increasingrateofdisplacement

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Rate effect and landslide velocity/runout distance

After initial failure => landslide ceases equilibrium =>

movement to new position

Positive rate effect:

Strength increases with velocity => Landslide deceleration

Negative strength effect:

Landslide acceleration => development of fast movement

=> possibility of long runout distance


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Methods for predicting runout distance

Empirical models

practical tools for predicting runout and distribution

Analytical models

Physical behavior of movement

Numerical simulations

Dynamic motion of debris and/or

Rheological model to describe material behavior

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Empirical models

1. Mass change model

Volume of mobilized material/length of trail

2. Angle of reach

Angle of the line connecting the crest of the landslide

source to the distal margin of the displaced mass

Corominas (1996): linear correlation between

volume and angle of reach for all types of failures

Decrease of angle of reach with increase in volume

Scatter of data is large => preliminary predictions of

travel distance => incorporation of judgement

But: required information can be generated easily

with historic landslide database


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Empirical models: Corominas 1996

Types of topographic constraints considered

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Empirical models: Corominas 1996

Landslide volume vs. tangent of

the reach angle for 204 landslide

events

Regression equations for

considered individual groups


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Analytical models

Based on lump mass approaches in which the debris

mass is assumed as a single point

Cannot account for lateral confinement and spreading

=> suitable only for comparing similar paths

(geometry, material)

Required parameters:

Pore pressure parameters

Debris thickness

Relation of residual strength with shear rate

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Numerical Models

1. Fluid mechanics models

Conservation equations of mass, momentum and

energy => dynamic motion

Rheological model => material behavior

Rheological properties are difficult to determine

2. Distinct element method

Model of large strain particle movement

Important for understanding failure mechanics of

landslides through back analysis

Sophisticated & Time-consuming


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References

Bundesamt fr Umwelt, Wald und Landschaft 1999:Risikoanalyse beigravitativen Naturgefahren - Fallbeispiele und Daten. In: Umwelt-Materialien

Nr. 107/II Naturgefahren. Bern, 129 pp.

Corominas, J. 1996: The angle of reach as a mobility index for small and large

landslides. Canadian Geotechnical Journal. Vol. 33, pp 260-271.

Crozier, M.J. 1986:Landslides: causes, consequences and environment. London.

Cruden, D.M. and Varnes, D.J. 1996: Landslide types and processes. In Turner,

A.K. and Schuster, R.L., editors,Landslides: investigation and mitigation,

Washington, D.C.: National Academey Press, 36-75.

Dai, F.C., Lee, C.F. and Ngai, Y.Y. 2002:Landslide risk assessment and

management: an overview. Engineering Geology 64, 65-87.

Press, F. & Siever, R. 1997: Understanding Earth. New York.

Summerfield, M.A. 1991: Global geomorphologyan introduction to the study

of landforms.New York. pp 537.

Turner, A.K. and Schuster, R.L. (editors) 1996:Landslides: investigation and

mitigation, Washington. pp. 673.

Landslide movie: http://walrus.wr.usgs.gov/elnino/landslides-sfbay/photos.html

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04 Run Out Zones Field Suveys Modelling