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2 heru santoso pedrr workshop session 2

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  • 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