2012 10-12 seoul unmyongsan comments(fukuoka, japan)
DPRI, KYOTO UNIV.Comments on the report of the 2011 Unmyong-san disaster Hiroshi FUKUOKA Associate Professor Research Center on Landslides Disaster Prevention Research Institute Kyoto University
DEBRIS SLIDE – DEBRIS FLOW EXPERIMENT IN JAPAN, 2003 by giving artificial rainfall of 80 mm/h x 5 hours to natural slopeArtificial landslide experiment using rainfall simulator (front view) (Forestry and Forest Product Research Institute)
Artificial landslide experiment using rainfall simulator (side view) (Forestry and Forest Product Research Institute)
Comments on the “6. Concluding Remarks”• 1. Widely distributed thick colluvial deposits and shallow ground water table which frequently appears suggests high risk.• 2. (recovery work at Dukwooam source area is not clear)• 3. Topography and other geomaterial condition on the border of Air Force before the disaster is still unknown, then their responsibility is not clear. (Problem is not the # of slides.)• 4. Check dam and other large-scale structured countermeasure construction needs more than a year. Complete coverage needs gigantic costs (unrealistic). Damage to the properties in 2011 could NOT be avoided, but early warning could have helped more lives.• 5. Reservoirs sometimes contributed to longer debris flow runout.• 6. Ground motion due to underground blastings never caused slides, however, may have contributed to more underground cracks. Groundwater veins, pipings may have been affected, and long-term slope stability could be affected, too.
Present risk of artificial fill on the border of Air Force Base
Proposed slitwork against upcomingdebris flows
Comment (1)• Extra-ordinary intense rainfall of 120-years return period is the primary triggering factor of the 2011 disaster.• Frequency of those extreme weather events in limited area (a few km2) is apparently increasing in most countries. Could expect more events in the future.• Structured countermeasure for those events everywhere cost so high and not realistic.
Numerous landslidesin Shobara city,Hiroshima Prefecture,Japan, induced byextreme rainfall inJuly 2010. Thisdisaster is localizedin a small area ofabout 5 km x 3 km.
Debris flow distribution in Shobara city (Asia Air Survey, Co.)
A number of landslides were induced by the intense rainfall
A panoramic photo of the head part of theOhto river. Shallow landslides took place and covered almost 360 degrees. C
Debris flow distribution in Hofu city interpreted by airphotos
Hourly precipitation and 10 minutes precipitation as well as cumulative in Hofu and Yamaguchi city
Calculated return period of hourly, 3-hours, 6- hours, and 24 hours precipitation Yamaguchi city: return period of max 6 hour rain = 601,7 years. Hofu city: max 6-hours rain: 245,9 years.The key factor was extraordinary large 6-hours precipitation
Comment (2)• Micro-landslide scars could be extracted by detailed interpretation of LiDAR-based map combined with appropriate image processing.• Precursor depression could NOT be extracted by present sensors yet, except ground-based precise sensors.• Comparison of before and after the disaster can give volume estimates of initiation, erosion and deposition.
Airborne laser scanner penetrating forests to detect old landslide scarsExample : July2009 Hofu citydebris flow disasterarea, western JapanGPS Satellites Pulse laser penetrates to Helicopter the ground surface hidden under the forestReference pt.
Comment (3)• Initiation mechanism: penetration of rain water + sliding surface liquefaction, otherwise unlimited displacement can not be expected. ß geotechnical characterisitcs• Evolving mechanism: Fluidizing or scraping of torrent deposits by undrained impacting loading by slide body.
Air photo of a slide-debris flow in Hiroshima (1999.7.14 Sassa）and its plan map （after Yokota et al., 1999）
The source volume is about 300 m3,the deposits is 20 times larger.
Ring-Ring-shear test for reproduction of landslide movement Simulating the sliding surface Normal axis Normal stress stress res s Shear st Sliding Sample surface 24 Un-drained shear box
A slide- debris flow disaster in Izumi city, 1997
体積減少 Volume decrease せん断ゾーン Shear zone すべり面における粒子破砕とすべり面液状化Grain crushing takes place in the shear zone, volume tends todecrease, then excess pore pressure is generated up to almostsame level as the normal stress (liquefaction). This condition isnamed as “Sliding-Surface Liquefaction” Illustration of sliding surface liquefaction （Sassa, 1996 and Sassa et al.1996）
Landslide Ring-Shear Simulator to reproduce high speed shearing at a sliding surface （Normal Stress 200 kPa , Shear Velocity 200cm/sec） Grain crushing causes muddy water and sliding surface liquefaction
Sliding Surface Liquefaction can be compared to….3rd floor ※Rooms are filled with water and undrained2nd floor condition is maintained1st floor Big reduction of apparent friction High water pressure is generated
Model of the landslide-triggered debris flow (Sassa et al. 1997)
Illustration of moving landslide mass along a torrent (after Sassa et al 2004a)
Results of a test tosimulate undrainedloading to the tuffbreccia deposits inMinamata landslide(BD = 0.89) (afterSassa et al 2004a).