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2012 seoul umyeonsan(ikeda, japan) Presentation Transcript

  • 1. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 Preliminary Study on Occurrence of Debris Flow at Umyeonsan(Mt.) Sabo & Landslide Technical Center, JAPAN Akihiko IKEDA SABO & LANDSLIDE TECHNICAL CENTER
  • 2. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 Issues in dispute 1) Whether adequate countermeasures were established for the Umyeonsan(Mt.) area, after the part of the area experienced landslides in 2010. Dukwooam valley, for example, suffered even more severe damages in 2011 again, for which restoration works have been done after landslides occurred in 2010. 2) Whether the landslides occurred around, and the drained water from the Air Force base, located at the summit of Umyeonsan(Mt.), aggravated the damages of the disaster in 2011. 3) Whether the existence and/or misadministration of the Eco- Eco- park reservoir in Hyeongchon valley added the damages in the lower catches. 4) Whether the blasting executed for tunneling underneath the Umyeonsan(Mt.) caused the landslides in the surrounding area. SABO & LANDSLIDE TECHNICAL CENTER 2
  • 3. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 Contents 1. Property of Debris Flow (in general) 2. Precondition Data of Umyeonsan Debris Flow - Based on technical report - 3.Consultation about the process of Debris flow occurrence 4.Comments SABO & LANDSLIDE TECHNICAL CENTER 3
  • 4. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) ■Classification of Debris Flows (1) Gravel-filled debris flow Gravel- (2) Mud-filled debris flow Mud- SABO & LANDSLIDE TECHNICAL CENTER 4
  • 5. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (1) Gravel-filled debris flow Gravel- A. The tip of a debris flow moves quite straight with a large concentration of gravel and boulders that are two to five meters in diameter, or larger in some cases. B. Behind the tip of a debris flow follows a muddy water flow (a subsequent flow) that contains less gravel than the tip of a debris flow. June 27,1999 ; Name River, Nagano Pref. , JAPAN 27, Pref. C. Boulders are concentrated at the tip of a debris flow. Maximum velocity: 8.48m/sec D. Debris flows rise and flow in an outer direction around the corner of a watercourse. SABO & LANDSLIDE TECHNICAL CENTER 5
  • 6. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (2) Mud-filled debris flow Mud- A. The Mud-filled debris flows are Mud- easily generated in areas with fine- fine-grained basement rock. This type of debris flow contains boulders, but it consists mostly of fine-grained sand. fine- B. Mud- Mud-filled debris flows move rampantly entirely from the tip to subsequent flows, and they flow in surges. C. Mud- Mud-filled debris flows flow easily compared with gravel- gravel- filled debris flows, so there are cases where mud-filled debris mud- flows reached the sea. SABO & LANDSLIDE TECHNICAL CENTER 6
  • 7. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) ■Classification of Debris Flows Source Based on the location, scale of the landslide and the large amount of sediment discharge from the river course. (1) Landslide-type debris flow; Landslide- the debris flow generated from the slope failure (2) Riverbed flow-type debris flow; flow- the mobilization of riverbed deposits turns into debris flow (3) Landslide Dam-type debris flow; Dam- the debris flow are assumed to occur under the effects of a landslide dam SABO & LANDSLIDE TECHNICAL CENTER 7
  • 8. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (1) Landslide-type debris flow Landslide- Minamata City, Kumamoto Prefecture, in 2003 SABO & LANDSLIDE TECHNICAL CENTER 8
  • 9. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (2) Riverbed flow-type debris flow flow- Inabe City, Mie Prefecture, in 1999,2002,2003 SABO & LANDSLIDE TECHNICAL CENTER 9
  • 10. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (2) Riverbed flow-type debris flow flow- Okaya City, Nagano Prefecture, in 2007 SABO & LANDSLIDE TECHNICAL CENTER 10
  • 11. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) (3) Landslide Dam-type debris flow Dam- Name River, Nagano Prefecture, 1999 Landslide Dam Before debris flow(1998.11) After debris flow(1999.6) SABO & LANDSLIDE TECHNICAL CENTER 11
  • 12. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) cf. Landslide Dam <Totsukawa Village, Kii Peninsula, in 2011> SABO & LANDSLIDE TECHNICAL CENTER 12
  • 13. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 1. Property of Debris Flow (in general) ■Criteria for sediment movement form based on riverbed gradient River bed River bed 2° 10° 10° 15° 15° 20° 20° gradient Approx. Approx. 1/30 Approx. Approx. 1/6 Approx. Approx. 1/4 Approx. Approx. 1/3 Occurrence area Flow area Area Deposition area name Bed load area Debris flow area SABO & LANDSLIDE TECHNICAL CENTER 13
  • 14. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow These data are based on Korean Society of Civil Engineers, 2012, Interim Report of the “Complementary Studies on the cause of the Umyeonsan(Mt.) Landslides” (condensed version).■Location of Umyeonsan Area <2010> <2011> SABO & LANDSLIDE TECHNICAL CENTER 14
  • 15. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow■Property of Debris flow ØMoving distance: avg. 448.8m range 98-1356.1m 98- ØGradient of initiation slopes: avg. 33° range 11-52° 33° 11-52° ØVolume of slope failure: avg. 116.7m3 range 1.1-1914.8m3 1.1- ØBasin area: avg. 151,848m2 (0.15km2) SABO & LANDSLIDE TECHNICAL CENTER 15
  • 16. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow■Rainfall Analysis SABO & LANDSLIDE TECHNICAL CENTER 16
  • 17. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow■Runoff Analysis Peak discharge of the Seocho and Namhyun stations are less than 10m3/sec for the average rainfall. SABO & LANDSLIDE TECHNICAL CENTER 17
  • 18. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow■Geotechnology Entire soil layer depth in Umyeonsan is around few meters (detail data are unconfirmed). SABO & LANDSLIDE TECHNICAL CENTER 18
  • 19. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 2. Precondition Data of Umyeonsan Debris Flow■Ground water Groundwater level increase several hours after the rainfall and base flow run- run-off occurs when continuous rainfall occurs above 250mm. 250mm. <Raemian> <Hyeongchon> SABO & LANDSLIDE TECHNICAL CENTER 19
  • 20. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Based on the technical report, each valley that debris flow occurred has a slope failure at the top of valley. valley. Landslide- Landslide-type debris flow In general, the process of Landslide-type debris flow Landslide- occurrence as below, Landslide occurrence Liquefaction of collapsed soil and flow down the valley Erode riverbed and river bank, surface soil of the slope Flow down as Debris flow SABO & LANDSLIDE TECHNICAL CENTER 20
  • 21. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Landslide occurrence Ø In general, landslide classified in 2 types; shallow landslide and deep landslide. landslide. Ø Increasing groundwater, pore water pressure inside the groundwater, slope, causes the landslides. Ø Shallow landslide occurrence tends to be influenced by rainfall intensity. intensity. Ø Deep landslide occurrence tends to be influenced by cumulative rainfall. rainfall. SABO & LANDSLIDE TECHNICAL CENTER 21
  • 22. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Landslide occurrence Based on the technical report, average volume of slope failure is 116.7m3, and the range are 1.1-1914.8m3. It is estimated that 1.1- average depth of slope failure is around few meters. meters. Shallow Landslide Ø In general, shallow landslide occurrence tend to be influenced by rainfall intensity. intensity. Ø Based on maximum rainfall of 1-hour, 6-hour, 12-hour at 6- 12- Namhyun station, are over 100 year frequency, these rainfall frequency, will be the trigger of the landslide and debris flow. Necessary to confirm the hyetograph SABO & LANDSLIDE TECHNICAL CENTER 22
  • 23. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Landslide occurrence cf. Some cases in JAPAN, more than 7mm at 10-minute 10- rainfall, rainfall, riverbed flow-type flow- debris flow tends to be occur consistent with peak of the rainfall. SABO & LANDSLIDE TECHNICAL CENTER 23
  • 24. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Samples of Hyetograph Landslide occurrence Occurrence of Rainfall Peak Deep Landslide Occurrence of Debris Flow Stream bed flow-type debris Flow flow- <Inabe City, Mie Prefecture, in 1999,2002,2003> <Totsukawa Village, Kii Peninsula, in 2011> Deep Landslides SABO & LANDSLIDE TECHNICAL CENTER 24
  • 25. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Landslide occurrence Changes in rainfall intensity every 6 hrs Occurrence of Deep Landslide <Totsukawa Village, Kii Peninsula, in 2011> SABO & LANDSLIDE TECHNICAL CENTER 25
  • 26. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Liquefaction of collapsed soil and flow down the valley Almost landslides flew down as debris flow. SABO & LANDSLIDE TECHNICAL CENTER 26
  • 27. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Liquefaction of collapsed soil and flow down the valley Based on technical report, Ø Maximum rainfall of 1-hour, 6-hour, 12-hour at Namhyun 6- 12- station are over 100 year frequency. Ø Peak discharge of the Seocho and Namhyun stations are less than 10m3/sec for the average rainfall. Ø Groundwater level increase several hours after the rainfall and base flow run-off occurs when continuous rainfall run- occurs above 250mm. 250mm. Ø Entire soil layer depth in Umyeonsan is around few meters (detail data are unconfirmed). SABO & LANDSLIDE TECHNICAL CENTER 27
  • 28. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Liquefaction of collapsed soil and flow down the valley The cause of liquefaction of collapsed soil and flow down the valley, influenced by huge amount of water which has been supplied from; Ø surface water due to torrential heavy rainfall Ø ground water presumably due to antecedent rainfall and cumulative rainfall, infiltrate into the slope Soil layer depth is about few meters and it will presumably has a retention of water, but in this disaster, the cumulative water, rainfall must be over the capacity of it. SABO & LANDSLIDE TECHNICAL CENTER 28
  • 29. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Erode riverbed and river bank, surface soil of the slope Based on the technical report, debris flow flew down dynamically propagating downslopes, which are captured by downslopes, video cameras at the northern side of Umyeonsan. <Site 1: Raemian> <Site 2: Shindonga> ØCatchment: 0.13km3 ØCatchment: 0.145km3 ØAverage slope: 15° 15° ØAverage slope: 14° 14° ØInitial volumes: 4,000m3 ØInitial volumes: 300m3 ØTotal volumes of ØTotal volumes of debris flow: 32,000m3 debris flow: 32,000m3 SABO & LANDSLIDE TECHNICAL CENTER 29
  • 30. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 3.Consultation about the process of Debris flow occurrence Erode riverbed and river bank, surface soil of the slope Analyzing movie files, the maximum velocity of debris flow are estimated 28.6m/sec (Site 1) and 18m/sec (Site 2). These velocity are very fast-moving, which is presumably due fast-moving, to the low sediment volume concentration of the flow, related to huge amount of water supplied from rainfall and ground water. Necessary to confirm the flow property, such as occurrence section, flow(eroded) section and deposition section, drawing the result on the longitudinal profile and plane figure. If possible, using LiDAR data and compare the elevation (topography) before and after the disaster, and calculate the volume of sediment budget. SABO & LANDSLIDE TECHNICAL CENTER 30
  • 31. Forum on the Landslide of Umyeon Mountain: Discussion with the International Experts 12 Oct.2012 4.Comments The cause of landslides and debris flow in 2010, presumably due to a huge amount of water supplying. The source of this water is supplied from a record-breaking record- torrential heavy rainfall intensity, antecedent rainfall and cumulative rainfall. The trigger of landslides and debris flow are the heavy rainfall intensity; such as 1-hour, 6-hour and 12-hour which 1- 6- 12- is over 100 year frequency. The antecedent rainfall and cumulative rainfall increased the ground water level more higher, that influenced debris flow discharge larger. SABO & LANDSLIDE TECHNICAL CENTER 31