1. Indonesian Institute of Sciences (LIPI)
International Center for Interdisciplinary and Advanced
Research (ICIAR)
Vegetation and landslide
mechanism
Dr. Heru Santoso
Executive Secretary ICIAR-LIPI
Research Scientist @ RC for Geotechnology, LIPI
PEDRR Workshop on Ecosystems, Livelihoods and Disaster Risk Reduction
Bonn, Germany, 21-23 September 2010 1

2. Outline
• Background: landslides in Indonesia
• Water as a destabilizing factor
• Acting forces in landslides and safety factor
• Role of vegetation
• DRR, and relevant methods and tools
• Closures
2

3. Landslides in Indonesia
Landslide susceptibility map of Indonesia
(Source: DEG, 1996)
3

4. © Tohari
• Landslides cause
infrastructure
damages, economic
lost and lost of lives © Herawati
• Landslides occur on
forested and
non-forested slopes
© Herawati
© Tohari
4
© Sugiarti

5. Bukit Sentual Area Site location
• About 20 landslides occurred in
several places in February 2007
• Hundreds of houses damaged,
Bogor Bogor Bogor
no lost of lives Distric Distric
City
• About 60% of landslides are t t
shallow type (Source: PVMBG, 2007)
• “Landslides were not problem (Source: DEG) Sub-District of
10 years ago” Babakan Madang
Photos by: PVMBG
5

6. Water as destabilizing factor
Shallow landslide Deep landslide
Rain
Pore pressure
Pore pressure
+ A B +
A C
C B A
B 0 0
- -
C
Time, t Time, t
• Accumulation of water pressure, create weak zone
• Deep landslide; an increase in water table, long duration rain
• Shallow landslide; development of transient/perch water
table, high intensity rain
6

7. Acting forces
Weight
Acting forces: gravity and shear strength
Factor affecting forces
- mechanical property: weight, slope angle, water pressure
- soil property: soil cohesion (plasticity), soil shear strength
7

8. Factor of safety (FS)
• FS = ∑F preventing failure
∑F causing failure
• There are many uncertain factors acting in the
system due to heterogeneous of soil property, effect
of vegetation, etc.
• FS gives sense of probability of failure (risk/
susceptibility to fail), a simplification of complex
uncertainty
• FS is commonly used in engineering design (risk vs.
cost)
8

9. Role of vegetation
• Roots provide additional shear strength
(better “actual” FS)
– Vegetation that have good soil anchoring
(high IRA) and soil binding (high IRB)
properties give best combined root
stabilizing effect
– Depend on vegetation species, age and local Index of Root Anchoring
condition (IRA) = ∑ Dv2/dbh2
Index of Root Binding
– Only affect soil property at root depth (IRB) = ∑ Dh2/dbh2
• Large or heavy trees could give Dv = diameter of vertical root
Dh = diameter of horizontal root
additional weight to the landslide dbh = tree diameter at breast
mechanism height
9

10. In Sumatra, this species has IRA >1.25
Results
IRA LOW MEDIUM HIGH
IRB < 1.25 1.25 - 2.00 > 2.00
Durio zibethinus Bouea macrophylla Mangifera kemanga
Psidium guajava Euodia latifolia Mangifera foetida
Index of Root Anchoring (IRA) and
Sandoricum koetjape Mengifera indica Melia azedarach
Index of Root Binding (IRB)
Eugenia polycephala Ceiba pentandra
LOW Mangifera odorata Eugenia polyantha
(< 1.00) Garcinia mangostana
Gnetum gnemon
Baccaurea racemosa
Myristica fragrans
Pinus merkusii
Persea americana Maesopsis eminii -
Parkia speciosa Pithecellobium jiringa
MEDIUM Pangium idule Artocarpus heterophyllus
(1.00 - 1.50) Nephelium lappaceum
Paraserianthes falcataria
Artocarpus altilis
HIGH Gmelina arborea Schima wallichii Lansium domesticum
(> 1.50) 10
Swietenia mahogany

11. Model of root distribution at a landscape level
(Source: Hairiah et al., 2008)
Mix of native trees (vegetation)
is probably best to
give combined strength (IRA and IRB) at landscape
level
11

12. DRR, and relevant methods and tools
• Land management
– Identification of landslide susceptible and risk areas, and type of
landslides; landslide hazard zoning method or TRIGGR (to link
with rain)
– People relocation and land use management (slope in risk of
deep landslides could be used for agricultural activity, but not for
housing)
– Selection of trees, considering IRA and IRB
• Field observation and monitoring
– Identifying changes in slope morphology (tilt meter)
– Monitoring development of cracks on soil
– Pore water pressure monitoring
– Monitoring vegetation type and coverage density
• Community based DRR, i.e. community preparedness
12

13. Closure (1)
• What we know
– Landslides can occur on both forested and non-forested
land
– Water is a destabilizing factor: shallow or deep landslides
depend on the rain pattern and soil hydrological response
– Vegetation is not normally included in the FS calculation,
but could provide additional shear strength to soil at root
depth, depending on type, age and local conditions
– DRR to landslides could be conducted by proper land
management, field monitoring and observation and
community based DRR.
13

14. Closure (2)
• What we don’t known or not sure about
– Effectiveness/ reliability (i.e limit) of tress in preventing
from landslides (at landscape scale).
• Will climate change/ weather extreme off-set the role vegetation?
• Is it effective for DRR or community resilience enhancement?
– Role ecosystem service in regulating local climate and soil
moisture, to reduce development of cracks
14

15. Thank you
Acknowledgements: Research findings and data in this
presentation are from various instituitions: CIFOR
(TroFCCA Project), RC for Geotechnology – LIPI and
Brawijaya University, Indonesia
ICIAR - LIPI
c/o. Deputy Office for Scientific Services, LIPI
Jl. Jend. Gatot Subroto, Jakarta 1270, INDONESIA
Phone : +62 (21) 5255179
Fax : +62 (21) 52907313