A Roundup of Recent Debris Flow Events in Taiwan

Transcription

1 2015 International Workshop on Typhoon and Flood APEC Experience Sharing on Hazardous Weather Events and Risk Management A Roundup of Recent Debris Flow Events in Taiwan Hsiao-Yuan(Samuel) Yin Ph.D. sammya@mail.swcb.gov.tw Soil and Water Conservation Bureau, Council of Agriculture May 28,

2 Outline 1. Background Introduction to Debris Flow Management in Taiwan 2. Recent Debris Flow Events Eastern: Hoping village, 2012 Central: Touken torrent, 2012 On-site debris flow monitoring Central: Shenmu village, Conclusions 2

3 Source area Landslide Transportation part Channel erosion Deposition zone Affected (fan)area 3

4 1,673 Torrents Feedback 47,830 People Hardware Potential debris flow torrents Delineation of affected areas Inventory of protected targets Engineering 1. Prevent vulnerability factors 2. Mass energy transformation 3. Diversion 4. Suppression works 5. Restrain works Assessment Land use restrictions Delimitation Announcement Restriction Where? How big (far)? Software Residential Relocation Evacuation Location Coordination Relocation Debris Flow Disaster Management Framework Planning Drill Promotion When? Warning Evacuation Disaster analysis Zoning Rainfall-based Warning system Rainfall > Threshold Feedback Disaster Info. 4

5 1st Debris Flow Event Eastern Taiwan: Hoping village,

6 Typhoon Saola Invaded Taiwan during Aug. 1-3, 2012 Hourly rainfall(mm/hr) Debris flows occurred at 4 am on Aug. 2, R=718 mm; I=120 mm/hr Debris flow warning at 11 pm on Aug. 1 Accumulated Rainfall (mm) 6

7 3 Debris Flow Events in Hoping Village, Hualien County Hualien-DF026 Hualien-DF166 7

8 Disasters of Hualien DF-166 Potential Debris Flow Torrent Geology: marble, gneiss, shallow topsoil with many folds and joints. There used to be mining industry. Channel length: 2.88 km ;Slope: 10 degrees; Catchment area: ha Over 70,000 cubic meters of sediments were transported to the downstream area and buried 80 houses. The channel was broaden from 5 m to 70 m in width due to strong erosive effect. 8

9 Causes of Debris Flows Intense rainfall The debris flow occurred at 4 am on Aug. 2 with R=718 mm; I=120 mm/hr. The return period of 12hr-rainfall (567mm/12hr) is over 200 years. A lot of existing debris materials scattered in the upstream areas of the catchment. The channel was blocked by a mound of sediments and giant boulders at the mild portion. The communities are located at the low-lying land and even lower than the potential debris flow torrent. 9

10 Reconstruction Works after Debris Flow Disasters Check dam Sedimentation pond Protecting embankment Channel dredging Debris flow simulation using FLO-2D 10

11 Engineering Features 11

12 2nd Debris Flow Event Central Taiwan: Touken torrent,

13 Debris flow caused by torrential rain in Touken torrent on June 11, 2012 Location: Touken torrent (Nantou DF-202) Shenmu village, Nantou County Geology: shale, sandstone and siltstone Channel length: 3.85 km Average slope: 15 degrees Catchment area: 447 ha The debris flow destroyed 3 houses and 10m roads Slit dam Fan area 13

14 Hourly rainfall (mm/hr) Soil and Water Conservation Bureau (SWCB) Debris flow warning at 5 pm, June 10 Debris flow, 7 pm, June 11, R=435mm; I=18.5mm/hr Accumulated Rainfall (mm) Fan(natural dam) length: 300 m width: 150 m depth: 5 m volume: 225,000 m3 14

15 Dammed lake in 2009 by typhoon Morakot Dammed lake in 2012 by torrential rainfall

16 3rd Debris Flow Event Central Taiwan: Shenmu Village,

17 Shenmu Debris Flow Monitoring Station, Nantou County Aiyuzih creek (Nantou DF-226) Geology: weathered shale and sedimentary rock channel length: 3.81 Km Average slope: 14 degrees Drainage area: 410 ha 17

18 Landslide Changing from Aerial Images (Upstream area of Aiyuzih creek)

19 seismometer geophone Soil and Water Conservation Bureau (SWCB) Debris Flows Observation Data Torrential rain in Shenmu monitoring station, May 20, 2014 Seismic signals are about 4 min ahead of geophone signals Geophone initiated at 12:53:43 Wires broke at 12:53:44 Upstream, I=54 mm/hr, R=58.5 mm Downstream, I=24.5 mm/hr, R=26 mm Front surge velocity = 4.9 m/s spectrum 19

20 Conclusions 1. Due to the extreme weather conditions, we are facing more and more sediment-related disasters recently in Taiwan as well as other countries in the world. 2. A reliable early warning system ( mostly rainfall-based) is crucial for the evacuation operation. Besides, the warning criteria should be revised regularly according to the feedback of new debris flow events and on-site debris flow monitoring data. 3. Not only debris flows, but shallow, deep-seated landslides and landslide dam problems should be also considered in order to mitigate the possible disasters due to climate change impact in the future. 20

21 Thank You for Your Attention Soil and Water Conservation Bureau Always Working with You 21