International Center for Interdisciplinary and Advanced

Research (ICIAR)

Indonesian Institute of Sciences (LIPI)

Vegetation and landslide mechanism

Dr. Heru SantosoExecutive Secretary ICIAR-LIPI

Research Scientist @ RC for Geotechnology, LIPI

PEDRR Workshop on Ecosystems, Livelihoods and Disaster Risk Reduction

Bonn, Germany, 21-23 September 2010

mechanism

1

Outline

• Background: landslides in Indonesia

• Water as a destabilizing factor

• Acting forces in landslides and safety factor

• Role of vegetation• Role of vegetation

• DRR, and relevant methods and tools

• Closures

2

Landslides in IndonesiaLandslide susceptibility map of Indonesia

(Source: DEG, 1996)

3

• Landslides cause

infrastructure

damages, economic

lost and lost of lives

• Landslides occur on

forested and

non-forested slopes

© Herawati

© Tohari

4

© Tohari

© Herawati

© Sugiarti

Bukit Sentual Area• About 20 landslides occurred in

several places in February 2007

• Hundreds of houses damaged, no lost of lives

• About 60% of landslides are shallow type

• “Landslides were not problem 10 years ago”

Sub-District of Babakan Madang

Bogor

City

Bogor

Distric

t

Bogor

Distric

t

Site location

(Source: DEG)

(Source: PVMBG, 2007)

5

Photos by: PVMBG

Water as destabilizing factor

Pore pressure

0

+

-

Pore pressure

0

+

-

A B CABC

Shallow landslide Deep landslide

A

B

Rain

• 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

Time, t

-

Time, t

-C

6

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

Factor of safety (FS)

•

• There are many uncertain factors acting in the system due to heterogeneous of soil property, effect of vegetation, etc.

∑∑

=failure causing

failure preventing

F

FFS

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

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 effectstabilizing effect

– Depend on vegetation species, age and local condition

– Only affect soil property at root depth

• Large or heavy trees could give additional weight to the landslide mechanism

Index of Root Anchoring

(IRA) = ∑ Dv2/dbh2

Index of Root Binding

(IRB) = ∑ Dh2/dbh2

Dv = diameter of vertical root

Dh = diameter of horizontal root

dbh = tree diameter at breast

height

9

ResultsIn

de

x o

f R

oo

t A

nch

ori

ng

(IR

A)

an

d

Ind

ex

of

Ro

ot

Bin

din

g (

IRB

) IRA

IRB

LOW MEDIUM HIGH

< 1.25 1.25 - 2.00 > 2.00

LOW(< 1.00)

Durio zibethinus Bouea macrophylla Mangifera kemanga

Psidium guajava Euodia latifolia Mangifera foetida

Sandoricum koetjape Mengifera indica Melia azedarach

Eugenia polycephala Ceiba pentandra

Mangifera odorata Eugenia polyantha

Garcinia mangostana

Gnetum gnemon

In Sumatra, this species has IRA >1.25

10

Ind

ex

of

Ro

ot

An

cho

rin

g (

IRA

) a

nd

Ind

ex

of

Ro

ot

Bin

din

g (

IRB

)

Baccaurea racemosa

Myristica fragrans

Pinus merkusii

MEDIUM(1.00 - 1.50)

Persea americana Maesopsis eminii -

Parkia speciosa Pithecellobium jiringa

Pangium idule Artocarpus heterophyllus

Nephelium lappaceum

Paraserianthes falcataria

Artocarpus altilis

HIGH(> 1.50)

Gmelina arborea Schima wallichii Lansium domesticum

Swietenia mahogany

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

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

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 responsedepend 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

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?• 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

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

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