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Tutorial on QRARockfall hazard risk assessment.

Andorra la Vella, E. Pyrenees

Jordi Corominas

Department of Geotechnical Engineering andGeosciences

Civil Engineering School. BarcelonaUPC

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What is the problem?

Location of Andorra la Vella (AV) – Santa Coloma

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What is the problem?

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For QRA we need answers to the following questions

Where the rockfall will takeplace?

How often?

How far will it travel?

What magnitude (intensity) will it have?

What are the potentialdamages?

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Risk for properties (i.e. buildings)

R(P) = P(L) x P(T:L) x P(S:T) x V(D) X C

R(P): expected annual loss due to landsliding (€/yr)P(L): probability of occurrence of a landslide of a given

magnitudeP(T:L): probability of a landslide reaching the building levelP(S:T): probability of the building being on the landslide pathV(D): vulnerability of the building C: cost of the building

the analysis must consider all landslide magnitudes and all the potentially affected buildings

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Risk for persons (i.e. inside a house)

Individual risk

R(ILOL) = P(R) x P(T:R) x P(S:T) x P(T:P) x P(S:P) x V(D)

R(ILOL): annual probability of loss of lifeP(R): probability of occurrence of a rockfall of a given

magnitudeP(T:R): probability of a rockfall reaching the house levelP(S:T): probability of the house being on the rockfall pathP(T:P): probability of the person being in the houseP(S:P): probability of the person being in the rockfall path V(D): vulnerability of the person

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Identification of rockfall sources

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Definition of basin and associated talus slopes

Spatial unit: basin

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How often? P(R)

Couloir

ofLuixent

Passader

Couloir

ofC

irera

Couloir

ofR

oquesB

lanques

Couloir

ofPica

Couloirof

Colld'Eres

Couloir of

Couloirof

Boneta

Forat Negre

Alzina

of

Couloir

Basera

wedge

Basora Mateu

rock wall

Jan-1996

1974

Dec-1983

End of 1960-69 decade

May-2001

Jan-1997

Several events before 1974

Jan-19941968, 1984, other events between

these years

Apr-2000

1962

About ten events between 1956 and

1985

250 m

Rockfall of known age

Rockfall with only an approximate age

Plot of tree-ring sampling

Ramenada

Apr-1999

BetweenJul-1998 and

Jan-1999

Roc de

rock wallSant Vicens

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Frequency (PR)

Estimated return period from:

Site

Historical record

Forat Negre 5 Alzina couloir 15 Boneta and Cirera

couloirs 10

Bassera Mateu > 25 Ramenada couloir 50 Coll d'Eres couloir 25 Pica couloir 25

PR = 1/TR

Frequency obtained at the base of thetalus slopes

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Preparing magnitude-frequency relation

Location Percentage of blocks of different volumes

≤1 1-5 >5 m3 Forat Negre 83,5 15,97 0,51 Alzina couloir 83,5 15,97 0,51 Boneta couloir 83,5 15,97 0,51 Basera Mateu 83,5 15,97 0,51 Ramenada c. 83,5 15,97 0,51 Coll d’Eres c. 83,5 15,97 0,51

Location Percentage of block of different volumes 0,5 1 2,5 5 10 m3 Forat Negre 49,6 33,9 14,5 1,47 0,51 Alzina couloir 49,6 33,9 14,5 1,47 0,51 Boneta couloir 49,6 33,9 14,5 1,47 0,51 Cirera couloir 49,6 33,9 14,5 1,47 0,51 Basera Mateu 49,6 33,9 14,5 1,47 0,51 Ramenada c. 49,6 33,9 14,5 1,47 0,51 Coll d’Eres c. 49,6 33,9 14,5 1,47 0,51 Pica couloir 49,6 33,9 14,5 1,47 0,51 Pica talus 49,6 33,9 14,5 1,47 0,51

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Assessment of travel distance (P(T:R))

Frequency obtained at thetalus slope.

Not necessary

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Impact probabillity (P(S:T)) and (P(S:P))

P(S:T)1 = LE/LD

P(S:T)2 = LB/LE

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Impact probability on buildings

Location Impact probability Length

talus (m) LD

Length buildings

(m) LE

Width of exposed

flat (m) LB Forat Negre 180 180 10 Alzina couloir 150 70 10 Boneta couloir 190 140 10 Basera Mateu 310 28 10 Ramenada c. 130 20 10 Coll d’Eres c. 120 120 10

LE

LD

LB

Block size (m3) LR Cubic root (m)

0,5 0.794 1 1

2.5 1.357 5 1.710

10 2.154

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Vulnerability (V)

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Case 2: one line of rockfall fences

Rockfall fences:

6 m height

2000 KJ absorbed kineticenergy

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Preparing magnitude-frequency relationLocation Percentage of block of different volumes 0,5 1 2,5 5 10 m3 Forat Negre 49,6 33,9 14,5 1,47 0,51 Alzina couloir 49,6 33,9 14,5 1,47 0,51 Boneta couloir 49,6 33,9 14,5 1,47 0,51 Cirera couloir 49,6 33,9 14,5 1,47 0,51 Basera Mateu 49,6 33,9 14,5 1,47 0,51 Ramenada c. 49,6 33,9 14,5 1,47 0,51 Coll d’Eres c. 49,6 33,9 14,5 1,47 0,51 Pica couloir 49,6 33,9 14,5 1,47 0,51 Pica talus 49,6 33,9 14,5 1,47 0,51

Case of Forat Negre basin

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Individual risk of lost of life

( )k

(ILOL) (R) (T:R) (S:T) (T:P) (S:P) (D:T)i = 1 i i

P = P × P × P × P × P × V∑

Different block sizes will give different values of P(S:P)

K is the number of block-size classesP(R), P(T:R) and P(S:P) depend on block size

For P(T:P), two scenarios have been considered:

The most exposed person: 20h/24h = 0.83

Average person: 14h/24h = 0.58

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Assessment of travel distance (P(T:R))

Numerical modelling

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Rockfall fence efficiency: new (P(T:R))