Tutorial on QRA Rockfall hazard risk assessment. Andorra la Vella, E. Pyrenees Jordi Corominas Department of Geotechnical Engineering and Geosciences Civil Engineering School. Barcelona UPC
What is the problem? Location of Andorra la Vella (AV) Santa Coloma
What is the problem?
For QRA we need answers to the following questions Where the rockfall will take place? How often? How far will it travel? What magnitude (intensity) will it have? What are the potential damages?
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 magnitude P(T:L): probability of a landslide reaching the building level P(S:T): probability of the building being on the landslide path V(D): vulnerability of the building C: cost of the building the analysis must consider all landslide magnitudes and all the potentially affected buildings
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 life P(R): probability of occurrence of a rockfall of a given magnitude P(T:R): probability of a rockfall reaching the house level P(S:T): probability of the house being on the rockfall path P(T:P): probability of the person being in the house P(S:P): probability of the person being in the rockfall path V(D): vulnerability of the person
Identification of rockfall sources
Definition of basin and associated talus slopes Spatial unit: basin
How often? P (R) Couloir of Couloir Couloir of Pica Couloir of of Couloir Coll Luixent of d'eres Passader Cirera Roques Blanques Couloir of Couloir Couloir of Boneta of Alzina Forat Negre Apr-2000 250 m Jan-1997 May-2001 Basora Mateu Dec-1983 Jan-1996 1974 Basera rock wall wedge Jan-1994 1968, 1984, other events between these years About ten events between 1956 and 1985 Several events before 1974 End of 1960-69 decade 1962 Rockfall of known age Rockfall with only an approximate age Plot of tree-ring sampling Ramenada Roc de Apr-1999 Between Jul-1998 and Jan-1999 Sant Vicens rock wall
Frequency (P R ) P R = 1/T R Estimated return Site period from: 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 Frequency obtained at the base of the talus slopes
Preparing magnitude-frequency relation Location Location Percentage of block of different volumes 0,5 1 2,5 5 10 m 3 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 Percentage of blocks of different volumes 1 1-5 >5 m 3 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
Assessment of travel distance (P (T:R) ) Frequency obtained at the talus slope. Not necessary
Impact probabillity (P (S:T) ) and (P (S:P) ) P (S:T)1 = L E /L D P (S:T)2 = L B /L E
Impact probability on buildings Location Length talus (m) L D Impact probability Length buildings (m) L E Width of exposed flat (m) L B 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 L E L B L D Block size (m 3 ) L R Cubic root (m) 0,5 0.794 1 1 2.5 1.357 5 1.710 10 2.154
Vulnerability (V)
Case 2: one line of rockfall fences Rockfall fences: 6 m height 2000 KJ absorbed kinetic energy
Preparing magnitude-frequency relation Location Percentage of block of different volumes 0,5 1 2,5 5 10 m 3 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
Individual risk of lost of life k ( ) P = P P P P P V (ILOL) (R) (T:R) (S:T) (T:P) (S:P) (D:T) i i i=1 Different block sizes will give different values of P (S:P) K is the number of block-size classes P (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
Assessment of travel distance (P (T:R) ) Numerical modelling
Rockfall fence efficiency: new (P (T:R) )