Shallow Landslide Activation, Pretto_Lanni
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Influence of shape, antecedent conditions and rainfall intensity on shallow landslide activation Ilaria Pretto, Cristiano Lanni University of Trento Eurac – Bolzano – 10/02/2010
description
Influence of shape, antecedent conditions and rainfall intensity on shallow landslide activation
Transcript of Shallow Landslide Activation, Pretto_Lanni
Influence of shape, antecedent conditions and rainfall intensity on
shallow landslide activation
Ilaria Pretto, Cristiano Lanni
University of Trento
Eurac – Bolzano – 10/02/2010
LEVEL OF ZONING: advanced
TRIGGERING FACTOR: induced rainfall
TYPE OF ZONING: susceptibility and
hazard maps
TYPE OF LANDSLIDE: shallow landslide
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
if < 0 (positive pore-water pressure)
account the contribution of negative pore-water pressure on soil shear strength
SUCTION
Infinite slope model
The failure criterion, according to Bishop (1959) is:
The safety factor SF is defined as:
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
pore water pressure for each discertized soil layer
coupling the infinite slope stability model with the phisical based hydrological model is possible to investigate the evolution of the hillslope SAFETY FACTOR
3-dimensional form of Richards’ equation
output maps of GEOtop hydrological model
follow the evolution of soil-pore pressure during the rainfall event
each discertized soil layer
every time step
GEOtop hydrological model [Rigon et al. (2006)] Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Mualem (1976) parametic formula for hydraulic conducibility
Mualem-Van Genuchten (1978) parametric formula for the retention curve
Where: Se is the effective saturation ϑ is the saturation degree ϑr is the residual water content Ψ is the piezometric load
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
The role of the ANTECEDENT CONDITION
The role of the HILLSLOPE SHAPE
soil wetness degree
plan shape profile curvature
high medium low
The role of the RAINFALL INTENSITY
simplified model vs complex one
Comparison between hydrological models: SHALSTAB and GEOtop
Overview
Failure criterion
Objectives
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
silty-sand soil, internal friction angle 38°, saturated hydraulic conductivity Ks=10-4 m/s soil depth 2m, area 900m2, average slope 35° impermeable bedrock
3 different rainfall intensity:
2 different soil-moisture antecedent conditions:
shallow landslisde, unpredictable with instruments
low 6mm/h (1.5% Ks) medium 18mm/h (5% Ks) high 36mm/h (10% Ks)
dry (saturation degree = 25%) wet (saturation degree = 75%)
LANDSLIDE CARACHTERISTCS:
SOIL COMPOSITION AND CHARACTERISTICS
ANTECEDENT CONDITIONS AND RAINFALL
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Evans formulation
3 plan shapes: parallel, convergent, divergent 3 profile curvatures: straight, convex, concave
TOPOGRAPHIC SHAPES Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Rainfall intensity = 18 mm/h Rainfall time = 10 ore
Slope maps
INITIAL CONDITION
SAFETY FACTOR
PLANARE Percentuale instabile = 50%
CONCAVO Percentuale instabile = 85.73%
CONVESSO Percentuale instabile = 28.57%
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
• Convex profile curvature
• Straight profile curvature
• Concave profile curvature
FS water table slope
FS
FS
non saturo
saturo
non saturo
saturo
non saturo
saturo
tanα< tan Φ’
tanα> tan Φ’
tanα< tan Φ’
tanα> tan Φ’
tanα< tan Φ’
tanα> tan Φ’
water table slope
water table slope
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Slope maps
INITIAL CONDITION
Rainfall intensity = 18 mm/h Rainfall time = 10 ore
PARALLELO Percentuale instabile = 50%
CONVERGENTE Percentuale instabile = 72.5%
DIVERGENTE Percentuale instabile = 34.9%
SAFETY FACTOR
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
• Parallel shape
• Divergent shape
FS
FS
FS
non saturo
saturo
non saturo
saturo
non saturo
saturo
tanα< tan Φ’
tanα> tan Φ’
tanα< tan Φ’
tanα> tan Φ’
tanα< tan Φ’
tanα> tan Φ’
water table slope
water table slope
water table slope
• Convergent shape
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
INITIAL CONDITION
Total rainfall volume = 180 mm
Rainfall intensity = 36mm/h Percentuale instabile = 81.1%
SAFETY FACTOR (convex-convergent hillslope)
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Rainfall intensity = 6mm/h Percentuale instabile = 16.8%
Rainfall intensity = 18mm/h Percentuale instabile = 65.7%
Time needed to achieve specific percentages of destabilized hillslope area for a continuous rainfall simulation of 5 days event
Hillslope propensity to
landslide
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
For high percentage, CONVERGENT hillslope reaches instability earlier
SHAPE has no influence for small percentage of instable area
Rain-volume and Total-volume needed to achieve specific percentages of hillslope area for a continuous rainfall simulation of 5 days event
For DRY scenarios the RAINFALL intensity influences more the instability process
The TOTAL VOLUME of water to reach a specific percentage of unstable area is the same for both WET and DRY scenarios
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
THRESHOLD BEHAVIOUR
Long rainfall time is needed to achieve a small percentage of unstable area (5%) but only a short time is therefore needed to quickly increase the destabilized area for both wet and dry cases
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
DRY SCENARIOS: pore water pressure evolution
low rainfall intensity high rainfall intensity pore water
pressure
pixel number
pixels numeration
Critical pressure head
FAILURE!
1) SUBSURFACE NORMAL FLOW
2) SUBSURFACE LATERAL FLOW: very significant in cases of HIGH rainfall intensity
pixel number
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
doesn’t consider sliding processes caused by saturation from above. Instability conditions can be reached only from below
consider sub-superficial flow in steady-state conditions
doesn’t take into account the shear resistance in unsaturated zone
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
Unconditionally unstable
Unconditionally stable
Stable
Unstable
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
An example on the usage of JGrass consolle
Instability propension:
Safety Factor with SHALSTAB formulation
JGrass consolle
3D visualization
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
effective Ip GEOtop
GEOtop
GEOtop
concave-parallel
concave-convergent
concave-divergent
real Ip
effective Ip real Ip
effective Ip real Ip
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
• the WETNESS DEGREE plays a fundamental role on the instability: when 5% of hillslope area reaches instability (relevant soil wetness reached) failure propagation velocity increases • WET antecedent moisture condition induces faster development of LATERAL SUBSURFACE FLOW
the DRYER the ANTECEDENT CONDITION is and the LOWER the RAINFALL INTENSITY is, the more the HILLSLOPE SHAPE is relevant on the triggering of shallow landslides
• the controller is the TOTAL VOLUME • the HIGHER the RAINFALL INTENSITY is, the HIGHER total volume is needed • when RAINFALL INTENSITY is MEDIUM-HIGH, the triggering of shallow landslide occurs BEFORE STEADY_STATE CONDITIONS are reached
Simplified models need simple parameters which are difficult to estimate
VERTICAL and LATERAL subsurface flow
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
the contribution of unsaturated soil condition provide an additional shear strenght to the soil so that it may ensure the stability of steep slope (in areas where the internal friction angle in smaller than the slope inclination)
[email protected] [email protected]
Overview
Failure criterion
Models
Setup
Shape
Rainfall Intensity
Time
Volume
Shalstab
Take home
