This contain the talk I gave at Cosenza November 7, 2013. It summarises our view of the state of the topic, and present some work we did recently

1. Process -based models of landslide triggering
(especially the hydrological parts)
Riccardo Rigon, Giuseppe Formetta, Giovanna
Capparelli, Fabio Ciervo, Mariolina Papa
Giornata di Studio su: “La modellazione dell’innesco dei movimenti franosi - Rende
7 Novembre 2013

2. We feel clearly that we
are only now beginning to acquire reliable
material for welding together the sum total of all
that is known into a whole; but, on the other
hand, it has become next to impossible for a
single mind fully to command more than a small
specialized portion of it.
!
E. Schroedinger (What is life ?)

3. Introduction
What’s the physics of landslides ?
Hillslope Hydrology
+
Hillslope stability
!3
Rigon et al.

4. Introduction
Hillslope Hydrology
bricks
Darcy hypothesis
Mass conservation
Soil water retention curves (SWRC)
Fluxes Hypothesis
Eventually assume a theory that relates SWRC to fluxes
!4
Rigon et al.

5. Hillslope hydrology
Darcy’s hypothesis
This is the REV
Representative Elementary Volume
!5
Rigon et al.

6. Hillslope hydrology
Richards’ equation core
is that what it is true is this
Mass conservation (no nuclear reactions) !
but actually true if the continuum (a.k.a. Darcy) hypothesis is valid
!6
Rigon et al.

7. Hillslope hydrology
Assume a parametric form
of soil water retention curves
Se = [1 + (
C(⇥) :=
⇥) )]
m
⇤
w ()
⇤⇥
But other forms are possible ...
Rigon et al.
n
Se :=
Parametric
van Genuchten (1981)
w
r
⇥s
r
!7

8. Hillslope hydrology
Flux hypothesis
Buckingham, 1907, Richards, 1931
Darcy-Buckingham Law
~
~
Jv = K(✓w )r h
Hydraulic conductivity times
gradient of the hydraulic head
Volumetric flow
through the surface
of the infinitesimal
volume
!8
Rigon et al.

9. Hillslope hydrology
A theory for getting hydraulic conductivity
from soil water retention curves
K(
w)
= Ks
⇧
Se
⇤
1
(1
But other forms are possible also here...
Rigon et al.
Se )
1/m
⇥ m ⌅2
!9

10. Hillslope hydrology
Now you can simplify it
http://abouthydrology.blogspot.it/2013/06/ezio-todini-70th-symposium-my-talk.html
Rigon et al.
!10

11. Simplifications with more details
An outcome of this project
http://abouthydrology.blogspot.it/2013/07/hillslope-hydrology-from-point-of-view.html
Rigon et al.
!11

12. The medium is the message
Soil depth
Where does water flow ?
Soil
rocks
!12
!12
Rigon et al.

13. The medium is the message
Soil depth is particularly important for landslides
http://abouthydrology.blogspot.it/2012/09/soil-depth-estimation.html
Because is usually believed that it is the perched water
table that can form at soil
discontinuity that causes shallow landslides
Perched water table
Water table
!13
Rigon et al.

14. Geomechanics
Hillslope stability
bricks
Stresses (and strains)
Effective stresses
Soil stresses characteristics curves (SSCC)
Material strength (Coulomb Hypotesis)
Factor of safety
!14
Rigon et al.

15. Geomechanics
I do not enter into
the geotechnical
details
http://www.camilab.unical.it/summer_school/index.html
!15
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16. Geomechanics
Commercial
a good book save a lot of searching
N. Lu, J. Godt,Hillslope Hydrology and Stability, Cambridge University
Press, 2012
!16
Rigon et al.

17. Advancing Knowledge
Geomechanics
Promoting Learning
! Suction stress is the effective stress (skeleton stress) with zero total
stress.
!
However, let me say something
Suction stress is the work done by the inter-particle stress or internal
stress due to water.
Lu and Godt, 2012
dε2
s
s
dε2
s
Dry: no stress
(a)
Dry: no stress
s
dε1
dε1
Dry: equivalent suction stress
(c)
Wet: internal suction stress
(b)
Wet: internal
suction causes , and Dry: equivalent suction stress
! What are the components correspondent
of the inter-particle stress σs?
! s = !! strain ! ! ! S u ! u
= !!
C
6.#Effec(ve#Stress#in#Soil
cap
pc
(
a
w
)
11
!17
Rigon et al.

18. Geomechanics
!18
Ning Lu

19. efine a Learninghydrology
that
moting stresszone accounts for soil saturation an
Geomechanics and vadose
cribing strength and deformation at Representati
Effective stress at REV is the composition of several
V). Area of REV Cross-Section
forces acting at micro scale
Lu and Godt, 2012
Aa1
Fcap
Aw1
ua
Aa2
Aw2
A
Ac1
Fc
Fpc
uw
Ft
Aa3
Aw3
Aa4
Ac2
Ac3
Rigon et al.
=-
s
'=(
!19
ua ) + S ( ua u

20. Geomechanics and vadose zone hydrology
Lu and Godt, 2012
Effective suction stress is then:
Suction
stress
air pressure
Effective stress
Stress in
material
!20
Rigon et al.

21. Geomechanics and vadose zone hydrology
Effective suction stress is then:
interparticle
stresses
capillarity
water pressure
relative water
content
!21
Rigon et al.

22. Geomechanics and vadose zone hydrology
Soil Suction Characteristic Curves
SSCC
As a result of Lu et al. 2010
work, suction stress can be expressed as a
function of the SWRC:
Se = [1 + (
⇥) )]
m
n
Se :=
Parametric
van Genuchten (1981)
w
r
⇥s
r
from slide 7
!22
Rigon et al.

23. Geomechanics and vadose zone hydrology
Therefore
The determination of the SWRC is of fundamental
importance, and the key to predict both soils
suction and stress evolution in hillslopes
What about hillslope
failure ?
!23
Rigon et al.

24. Geomechanics and vadose zone hydrology
FoS
Factor of Safety
Both Material Strength and Design Loads, in
the hillslope game are function of the stresses
Details in Lu and Godt, 2012
and references therein
!24
Rigon et al.

25. !
To sum up
We have a sound and comprehensive theory
!25

26. GEOtop and its brothers and sisters

27. So what ?
We have the theory (and the equations)!
However we have to
implement it in sound numerical code
to identify the parameters (and there are quite a few)
treat the input and the output data
!27
Rigon et al.

28. GEOtop as a tool of this project
Rigon et al., 2006 , Endrizzi et al., 2013
GEOtop
!
• it models:
!
- subsurface saturated and unsaturated flows
!
- surface runoff
!
- turbulent fluxes across the soil-atmosphere
interface.
Rigon et al.
!28

29. GEOtop as a tool of this project
Rigon et al., 2006 , Endrizzi et al., 2013
GEOtop
• 3D physically based finite-difference model
!
!
• spatially distributed
!29
Rigon et al.

30. GEOtop as a tool of this project
Model integration with OMS
The Object Modeling System is a modular modeling framework that,
using an open source software approach, enables all members of the
David et al., 2013
scientific community to address collaboratively the many complex
issues associated with the design, development, and application of
distributed hydrological and environmental models.
Features
Components interoperability
Data interoperability
Language interoperability
!30
Rigon et al.

31. Other tools
In reality GEOtop is the most complex of a series
of models
and one scope is to compare many of them, and eventually use them
for different purposes. In red those already implemented for the
project.
!31
Rigon et al.

32. Every is free … as “free of speech"
GEOtop:
code at:
https://code.google.com/p/geotop/
information at:
http://abouthydrology.blogspot.it/search/label/GEOtop
CISLAM and SHALSTAB:
code at:
https://code.google.com/p/jgrasstools/
information at:
http://abouthydrology.blogspot.it/2012/09/my-past-research-on-shallow-landslide.html
!32

33. Questions
Research but also practical questions
Many landslides models with many physical approximations
!
• Does exist a minimum physical degree of simplification to
model a landslide?
!
• Is it possible create a unique modeling framework where
different models can be executed and compared?
!
!
!
!
!
!
!33
Rigon et al.

34. It works!
Waiting for the data
… an application where we have some
Tuostolo
Rigon et al.
!34

35. It works!
Some results
Measurement station
!35
Rigon et al.

36. It works!
Tuostolo - Campania- Italy
Measurement station
• Suction [KPa] simulation at 0.35 m depth
!
• Simulation period:
01-12-2007 to
01-05-2008
Rigon et al.
PBIAS=1.90
NSE=0.83
!36

37. Tuostolo - Campania- Italy
• Soil moisture
simulation at 0.35 m
depth
!
• Simulation period:
01-12-2007 to
01-05-2008
!
• Time step: hourly
PBIAS=2.1
NSE=0.70
!37

38. Drake River - CO - USA
C3
C1
Input data:
• Radiation
• Rainfall
• Air Temperature
• Relative Humidity
!
Output:
• soil moisture at
different depths
G. Formetta, Trento 17 June 2011
24
!38

39. Drake River - CO - USA
Simulation period:
01/09/2008-01/02/20
09
!
Simulation timestep:
hourly
!39

40. Drake River - CO - USA
Suction (mm)
Simulation period:
01/09/2008-01/02/20
09
!
Simulation timestep:
hourly
30 cm
depth
50 cm
depth
!40

41. Drake River - CO - USA
Suction (mm)
Simulation period:
01/09/2008-01/02/20
09
!
Simulation timestep:
hourly
30 cm
depth
50 cm
depth
!41

42. Drake River - CO - USA
Suction (mm)
Simulation period:
01/09/2008-01/02/20
09
!
Simulation timestep:
hourly
30 cm
depth
50 cm
depth
!42

43. Drake River - CO - USA
Safety factor
!43

44. It works!
Comments
• GEOtop seems able to reproduce decently suction and water contents
• Therefore it can be reliably used for trying a forecasting of these quantities
• Certainly more analysis and data are required
!44
Rigon et al.

45. The IWL3 Round Robin

46. GEOtop in the lab
Thanks to Neaples Group: the IWL3 experiment
R. Greco1, L. Comegna1, E. Damiano1, A. Guida1,2, L. Olivares1, and L. Picarelli1
1Dipartimento di Ingegneria Civile Design Edilizia e Ambiente, Seconda Università di Napoli, via Roma 29, 81031
Aversa (CE), Italy
2Centro Euro-Mediterraneo sui Cambiamenti Climatici, via Maiorise, Capua (CE) 81043, Italy
!46
Rigon et al.

47. t
GEOtop in the lab
Soil
Thickness
(cm)
Slope
Length
(cm)
Initial
porosity n0
Rainfall
intensity
(mm/h)
Initial mean
s uction
(kPa)
Duration
of test
(min)
10.0
100
0.75
55
17.5
36
10.0
120
0.76
56
41.0
30
The inclination of the slope is 40°.
!
The test are carried out with constant and spatially homogeneous
rainfall intensity.
Several devices (tensiometer, pore pressure transducer, TDR and laser
!47
Rigon et al.

48. Analysis of the data
Suctions and pressures
failure
first displacement
first displacement
.
-5 cm
!
!
factor of safety here is 1.2
-10 cm
!48
Rigon et al.

49. Analysis of the data
Water Content
!49
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50. Analysis of the data
Water Content talks
Hydraulic conductivity was measured in the lab. The value given was
around one order of magnitude less than the artificial rainfall
So we expect an Hortonian flux: saturation at the top and
movement downward.
Which we do not have!
!50
Rigon et al.

51. Analysis of the data
So we expect an Hortonian flux: saturation at the top and
movement downward.
red line is more ore less what we expect just after the beginning
of irrigation in a Hortonian interpretation of infiltration
Rigon et al.
!51

52. Questions
What about the Darcy scale here ?
!52
Rigon et al.

53. Analysis of the data
Water Content talks
Is irrigation really stationary ? What happens after the 28th minute ? Lateral flow
triggers ?
!53
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54. Let’s go !
Two hydraulic conductivities
One hypothesis we did is that, despite the homogeneity of the
preparation of the experiment, hydraulic conductivity (at
saturation) at the bottom is different from hydraulic conductivity at
the top of the mock-up.
Due to packing of particles ? Due to some unavoidable imperfection
in preparation ? Due to avoidable imperfection of the preparation ?
What else ?
!54
Rigon et al.

55. Which parameters ?
Suction talks
Both suction and water content data were used to calibrate van Genuchten
parameters. Also the hydraulic conductivity is among
Se = [1 + (
Se :=
n
⇥) )]
m
w
r
⇥s
r
Also hydraulic conductivity at saturation is a calibration parameter
K(
w)
= Ks
⇧
Se
⇤
1
(1
Se )
1/m
⇥ m ⌅2
!55
Rigon et al.

56. Which parameters ?
Calibrated Parameters
alfa
0.052
n
m
1.805 0.445983
Ksat_layer superficiale (0-5cm) = 0.178 mm/s
Ksat_layer di fondo (5-10cm) = 0.117 mm/s
!56
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57. Suctions
!57
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58. Water content
Averaging does not get the right result
even if water contents are reproduced fairly well until the 21th minute
Rigon et al.
!58

59. Analysis of the set up
Discussion
The friction angle of the material is 38 degrees. Therefore the stability
depends mainly on suction.
The FoS has been calculated into 2 different ways, by using the Fos
below that includes both the “classical” and “Lu’ Fos:
!59
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60. Stresswise
Stresses (FoS)
!60
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61. Stresswise
Stresses (FoS)
!61
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62. Stresswise
Comment
The way you calculate the FoS matters. While with the classical approach,
FoS remains consistently higher than 1 (close to 2), with Lu approach the
FoS comes close to 1.
In a sense, also this is not satisfactory, since we would like to have a
safety factor well below 1.
Do we neglected something ?
!62
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63. What we gain
Conclusion
We are close … but also far away …
The project allowed to set up an infrastructure that can be used to
investigate real cases, and to better tune experiments
On the present infrastructure can be incrementally built
“Real Time” physical modelling of these phenomena is feasible
!63
Rigon et al.

64. Closing
G.Ulrici, 2000 ?
Thank you for your attention
La presentazione può essere scaricata da http://abouthydrology.blogspot.com
!64
R. Rigon

65. Closing
Questions
The theory is sound … where it fails ?
Do the physics simplify at hillslope scale ?
Which measures can we envision to make a verification of the theory ?
Can lab measures reproduce field ?
How we characterise spatial patterns (needed to check real cases) ?
What the relation between mass waste and erosion ?
What is the influence of these phenomena on landscape evolution ?
How can we validate the overall process ?
Rigon et al.
!65

66. Closing
Questions
What the role of plants’ roots in stability?
!66
Rigon et al.