Eurac Geoframe Presentation

Cristiano Lanni, Ilaria Pretto University of Trento

My experiences after Master SIGEO and LARAM School

PRESENTATION OUTLINES:

1st step: Landslide risk assessment

2nd step: evaluation of shallow landslide Susceptibility and Hazard map using advanced methods

3th step: tutorial on quantitative risk assessment

PRESENTATION OUTLINES

1) Which type of landslide?

TYPE OF MOVEMENT

TYPE OF MATERIAL

VARNES, 1978

SOME EXAMPLES


δ

SOME EXAMPLES

Earth flow Earth Slide

SOME EXAMPLES

LANDSLIDE IDENTIFICATION IS NOT ALWAYS AN EVIDENT TASK

Understanding maps (topographic, geomorphological,..)

Morphological evidences, deposits, vegetation, indirect evidence

Understanding maps

Aerial photo-interpretation


MORPHOLOGICAL EVIDENCES


DEPOSITION AREA

Type of material

kinematic reconstruction


Type of material



DEPOSITION AREA

DEPOSITION AREA



NEW STREAM PATH

VEGETATION – INDIRECT EVIDENCE


Landslide Susceptibility (S): Areas prone to slope failure or that where a landslide may travel onto or retrogress into it.

Landslide Hazard (H): The probability of occurrence within a specified period of time and within a given area of a potentially damaging phenomenon.

Elements at Risk (Er): Means the population, properties, economic and social activity, etc., at risk in a given area.

Vulnerability (V): Means the degree of loss of a given element or set of elements at risk of a given magnitude. It is expressed on a scale from 0 (no damage) to 1 (total loss).

Specific Risk (Rs): Means the expected degree of loss due to a particular natural phenomenon. It may be expressed by the product of Hazard times Vulnerability

Total Risk (R): Means the expected number of lives lost, person injured, damage to property, or destruction of economy activity due to a particular natural phenomenon, and is therefore the product of specific risk and element at risk

I S H R

Landslide Inventory (I): This is an essential part of any landslide zoning. It involves the location, classification, volume, travel distance, state of activity and date of occurrence of landsliding in an area.

I S H R

€

R = H ×V( ) × Er = Rs× Er

I S H STEFAN

Type of zoning I S H R

The SCALE

SMALL SCALE

MEDIUM SCALE

< 1:100,000

1:100,000 – 1:25,000

LARGE SCALE

DETAILED SCALE

1:25,000 – 1:5,000

> 1:5,000

LOCAL SCALE

BASIN SCALE

REGIONAL SCALE

SMALL SCALE

MEDIUM SCALE

LARGE SCALE

DETAILED SCALE

< 1:100,000

1:100,000 – 1:25,000

1:25,000 – 1:5,000

> 1:5,000

Indicative Range

Typical Area of zoning

> 10,000 km2

1,000 – 10,000 km2

100 – 1,000 km2

Several Hectares to 1,000 km2

Example of Zoning Application

LANDSLIDE INVENTORY AND SUSCEPTIBILITY TO INFORM POLICY MAKERS AND THE GENERAL PUBLIC

LANDSLIDE INVENTORY AND SUSCEPTIBILITY ZONING FOR REGIONAL DEVELOPMENT, OR VERY LARGE SCALE ENGINEERING PROJECTS. PRELIMINARY LEVEL HAZARD MAPPING FOR LOCAL AREAS

LANDSLIDE INVENTORY, SUSCEPTIBILITY AND HAZARD ZONING FOR LOCAL AREAS. INTERMEDIATE TO ADVANCED LEVEL HAZARD ZONING FOR REGIONAL DEVELOPMENT. PRELIMINARY TO INTERMEDIATE LEVEL RISK ZONING FOR LOCAL AREAS AND THE ADVANCED STAGES OF PLANNING FOR LARGE ENGINEERING STRUCTURES, ROADS AND RAILWAYS INTERMEDIATE AND ADVANCED LEVEL HAZARD AND RISK ZONING FOR LOCAL AND SITE SPECIFIC AREAS AND FOR DESIGN PHASE OF LARGE ENGINEERING STRUCTURES, ROADS AND RAILWAYS

INTERNATIONAL GUIDELINES (FELL ET AL., 2008)

The PURPOSE

Frequency Assessment (of landslide or triggering factor) I S H

I

I S

WHERE, WHEN, WHAT, HOW MUCH

- Elements at risk - Temporal-spatial probability - Vulnerability

I S H R

Recapping:

WHERE, WHEN

WHERE

WHERE

I S H R

I S H Type of Zoning : our work at University of Trento

I

• LOCATION

• CLASSIFICATION

• AREAL EXTENT AND VOLUME

• CREEPING ZONES

• STATE OF ACTIVITY

Individuation of the area where first - failure phenomenon or landslide reactivation can occur

I

Mapping of the creeping zone allows the identification of the possible landslide evolution zones

I

SHALLOW LANDSLIDES

I

I. VELOCITY II. VOLUME III. ENERGY

DEBRIS-FLOW, SHALLOW LANDSLIDE WITH COARSE-GRAINED SOILS

DEEP LANDSLIDE WITH FINE-GRAINED SOIL

I

Landslides Inventory

I S

Source Area

Deposition Area

Intensity Duration

Return Time

I S H

Type of Landslide

Magnitude/Intensity

Frequency Triggering

Propagation

PROBABILITY THAT A PARTICULAR DANGER (THREAT) OCCURS WITHIN A GIVEN PERIOD OF TIME IN A GIVEN LOCATION

I S H

TYPE OF LANDSLIDE Intensity


Propagation

I S H

I S H

3 different approaches for landslides Intensity assessment

1) HISTORICAL INFO, GEOLOGY AND TOPOGRAPHY Assess the relative intensity from the estimated Landslide volume and the expected landslide velocity (qualitative)

2) SIMPLE MODELS Estimate expected or observing landslide velocity

3) NUMERICAL MODELS Calculate the kinetic energy (velocity) by means of numerical models

BASIC LEVEL

INTERMEDIATE LEVEL

ADVANCED LEVEL

TYPE OF LANDSLIDE

Intensity


Propagation

1) HISTORICAL INFO, GEOLOGY AND TOPOGRAPHY Assess the relative intensity from the estimated Landslide volume and the expected landslide velocity (qualitative)

BASIC LEVEL

I S H

TYPE OF LANDSLIDE

Intensity


Propagation

• Frequency of landslide can be determined from:

BASIC LEVEL:

• HISTORICAL DATA

INTERMEDIATE LEVEL:

• RELATIONS WITH TRIGGERING EVENT FREQUENCY (I.E. RAINFALL, EARTHQUAKE)

ADVANCED LEVEL:

• RELATING THE INDICATORS OR REVEALING FACTORS OF SLOPE STABILITY CONDITION (I.E. WATER CONTENT, GROUNDWATER LEVEL, PORE-WATER PRESSURE) TO TRIGGERING FACTORS (RAINFALL)

I S H

TYPE OF LANDSLIDE

Intensity


Propagation

AN EXAMPLE OF APPROACH AT REGIONAL SCALE

Definition of an INDEX OF SLIDE PRONE AREA IN

Given by the ratio between the number of towns affected by landslides during an event NI and the total number of threatened towns in the study area NT

IN = NI / NT

Relative to a given return Time T [year] in order to define an annual frequency f [year -1]

I S H

TYPE OF LANDSLIDE

Intensity


Propagation

For example by the definition of Rainfall Intensity-Duration Thresholds, considering the

rainfall that caused landslides in the past and – eventually – the cumulative antecedent rainfall

that predisposed to failure condition

I S H

TYPE OF LANDSLIDE

Intensity


Propagation

Care should be taken in the evaluation of landslide frequencies

Type of Landslide

Intensity

FREQUENCY Triggering

Propagation

&

- BECAUSE THE HISTORICAL DATA COULD BE AFFECTED BY ERRORS - BECAUSE THE CONDITIONS RESPONSIBLE FOR A GIVEN LANDSLIDE FREQUENCY IN THE PAST MAY NO LONGER EXIST

I S H

(BASIC LEVEL) (INTERMEDIATE LEVEL)

INITIAL CONDITION (ANTECEDENT SOIL MOISTURE)

RAINFALL (INTENSITY, DURATION, RETURN TIME)

WATER TABLE LEVEL

WATER CONTENT AND PORE-WATER PRESSURE PROFILE

Type of Landslide

Intensity


Propagation

I S H

ADVENCED LEVEL

SAFETY FACTORS FS<1

Type of Landslide

Intensity


Propagation

WATER TABLE LEVEL

WATER CONTENT PROFILE

I S H

ADVENCED LEVEL

You can assess these relations considering:

- different wet initial conditions (on the base of cumulative rainfall dropped before of the event)

- the seasonal mean humidity conditions

Type of Landslide

Intensity


Propagation

I S H

ADVENCED LEVEL

GEOLOGICAL, GEOMORPHOLOGICAL AND HIDROGEOLOGICAL INVESTIGATION AND STUDIES - thickness of soil cover - hydrogeological features

GEOTECHNICAL AND HYDROLOGICAL INVESTIGATION AND STUDIES - In situ investigation on the stratigraphic condition of soil cover - Laboratory investigation for mechanical and hydraulic characterization of soil - - In situ measurements

Type of Landslide

Intensity

Frequency TRIGGERING Propagation

I S H

Type of Landslide

Intensity


PROPAGATION

EMPIRICAL METHODS

GEOMETRICAL APPROACH

GEOMORPHOLOGICAL APPROACH

ANALYTICAL METHODS

SINGLE-BLOCK MODEL

FLUID DYNAMICS MODEL

Based on statistical relationship and/or expert opinion Modelling the physical process

I S H

Type of Landslide

Intensity


PROPAGATION

EMPIRICAL METHODS

GEOMETRICAL APPROACH

Based on statistical relationship and/or expert opinion

I S H

Type of Landslide

Intensity


PROPAGATION

ANALYTICAL METHODS

FLUID DYNAMICS MODEL

Modelling the physical process

I S H

THE TYPE AND LEVEL OF DETAIL OF THE ZONING AND THE SCALE OF THE MAPS DEPENDS ON THE PURPOSE TO WHICH THE LANDSLIDE ZONING IS TO BE APPLIED AND A NUMBER OF OTHER FACTORS:

1) The stage of development of the land use zoning plan or engineering project 2) The classification, activity, volume or intensity of landsliding. Risk zoning is more

likely to be required where the landslides are likely to travel rapidly and or have a high intensity. For these situations life loss is more likely

3) The founding available

4) The amount and quality of available information. Quantitative (Advanced) hazard and risk zoning cannot be performed where data are not available

Key words:

RECOMMENDED TYPES AND LEVELS OF ZONING AND ZONING MAP SCALES RELATED TO LANDSLIDE ZONING PURPOSE

I S H R INTERNATIONAL GUIDELINES (FELL ET AL., 2008)

BUT YOU MUST ALSO THINK THAT SOMETIMES ADVANCED METHOD COULD NOT BE BETTER THAN SIMPLIFIED METHOD WHEN A BIG NUMBER OF INFORMATION ARE AVAILABLE, BECAUSE OF:

1) The big variability of the soil, bedrock and so on (heterogeneity) 2) Uncertainties in the processes understanding 3) Uncertainties in the measurements 4) Representativity of site-measurements and laboratory tests at the

investigated area (Hydraulic Retention function, Hydraulic Conductivity function, thickness of the cover, bedrock permeability, etc)

5) DEM resolution 6) The dominant role of some factors respect to the others

ANYWAY, ALSO THE OPPOSITE COULD BE TRUE……..

• Shallow landslide can occurr only for saturation from below.

• Does not account for unsaturated flow dynamics and for the suction contribute on the shear-strength resistance

• Considers only the steady-state condition for the subsurface water flow

• Overland flow is neglected, but It should modify the boundary conditions, (e.g. the water level in a channel located at the toe of the hillslope)

• Does not account the effects due to boundary conditions.

AN EXAMPLE: THE SIMPLE SHALSTAB MODEL (MONTGOMERY AND DIETRICH, 1994)

€

Qsub

= Ksat

Z cosβ b sinβ hZ

=T b sinβ hZ

€

Qsup

= ν ⋅ A

€

Qsup

+Qsub

= I ⋅ A

€

Ksat= saturated hydraulic conductivity

€

Z = soil thick

€

h = soil-water thick in steady-state condition

€

β = slope gradient

€

ν ⋅ A+Ksat

b Z cosβ sinβ hZ

= I ⋅ AWATER BALANCE IN STEADY-STATE CONDITION

€

T b sinβ hZ

= I −ν( )A

€

T b sinβ hZ

= q A

€

hZ

=qT

Ab sinβ

€

T = Ksat

Z cosβ€

A = Upslope contributing area

Steady-state subsurface flow

Steady-state overland flow

€

FS =tanφ'tanβ

1− γw

γ s

hZ

€

q = I − v Effective rainfall in steady-state condition

• PARALLEL

• CONVERGENT

• DIVERGENT

ACTUAL INTENSITY RAINFALL

Ip reale Ip efficace GEOtop

Ip reale Ip efficace GEOtop

EFFECTIVE INTENSITY RAINFALL

GEOTOP

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