"Impact on infrastructure by Gorkha earthquake 2015 induced landslides"

Transcription

1 J-Rapid Final Workshop 21 June, 2016, Kathmandu Inventory mapping of landslides induced by the Gorkha earthquake 2015 and a proposal for hazard mapping of future landslides for making a plan of better reconstruction "Impact on infrastructure by Gorkha earthquake 2015 induced landslides" Masahiro CHIGIRA Masahiro CHIGIRA Professor, Disaster Prevention Research Institute, Professor, Disaster Prevention Research Institute, Kyoto University Kyoto University Vishnu DANGOL Vishnu DANGOL Professor, Department of Geology, Professor, Department of Geology, Tribhuvan University Tribhuvan University

2 Objective 1. to make an inventory mapping on landslides, cracks, and landslide dams induced by the Nepal earthquake and to investigate their formative mechanisms 2. to detect displaced areas of slope surfaces, of which future susceptibility to landslides would be evaluated on the basis of geology, geomorphology, and groundwater conditions. 3. propose a methodology of hazard mapping for earthquake-induced landslides in Nepal.

3 Members (Japan side) 1. Masahiro CHIGIRA (Kyoto Univ.) PI Applied Geology 2. Daisuke HIGAKI (Hirosaki Univ.) Landslide control 3. Hiroshi YAGI (Yamagata Univ.) Landslide susceptibility mapping 4. Akihiko WAKAI (Gunma Univ.) Geotechnical analysis of landslide 5. Hiroshi, P. SATO (Nihon Univ.) Remote sensing 6. Go, SATO (Teikyo Heisei Univ.) Geomorphology 7. Ching-Ying, TSOU (Hirosaki Univ.) GIS analysis 8. Akiyo YATAGAI (Res. Inst. Humanity and Nature) Meteorology

4 Members (Nepali side) 1. Vishnu DANGOL (Tribhuvan Univ.) PI Applied Geology 2. Smajwal BAJRACHARYA (ICIMOD) Remote sensing 3. Shanmukhesh Chandra AMATYA (DWIDP) Hydrogeology 4. Tuk Lal ADHIKARI (ITECO-Nepal) Geotechnical

5 Field survey Trishuli River catchment from Trishuli to Syabrubesi (29 October to 1 November, 2015) Sun Kosi and Bhote Kosi River catchments from Bansaghu to Kodari. (2 to 4 November, 2015) Budhi Gandaki River catchment from Arughat to Litin (6 and 7 March, 2016) Dauradi River catchment from Jhulunge Bajar to Barpak (7 and 8 March, 2016)

6 Inventory mapping of landslides Inventory Complied Data Used Note Topographic classification Knickpoint ASTER GDEM Data contributed by METI and NASA Slope break AW3D DEM derived AW3D DEM of JAXA Constant Vertical Exaggeration Stereoscopic Map (CVES Map) Deep-seated landslide Deep-seated landslide Stereo-pair aerial photos Complied by Dr. Go Sato Slope failure with scar The 2015 newly formed scar Earthquakeinduced slope enlargement of a preexisting failure scar Pre-existing slope failure not enlarged by the earthquake CVES Map Pre-earthquake: Google Earth Imagery (Dec. 12, 2014) Post-earthquake: Google Earth Imagery (May. 3, 2015) & LANDSAT 8 satellite imagery (June 1, 2015/covering the upstream Trishuli River) Used for the area not covered by stereo-pair aerial photos Complied with the reference inventory of the Japan Landslide Society Presented as points

7 Geological setting of the affected area Higher Himalaya MCT Lesser Himalaya Kathmandu

8 Mapped landslides 6,527 new landslides 558 enlarged landeslides 5,159 (ICIMOD-NELS)

9 Cumulative landslide frequency and the landslide size Data added to Chigira et al. (2010, Geomorphology)

10 What type of landslides occurred?

11 Shallow disrupted landslides in most cases No gigantic landslides like those induced by the 2009 Wenchuan earthquake Bhote Koshi catchment

12 Rockfall 400m fall from a slope break Dadakateri From a river terrace (Kodari)

13 Cracks Mostly have not developed after the quake Cracks on a sharp ridge in Barpak Cracks on debris slopes (Bhote Koshi catchment)

14 Large landslides, which had been continually moving were not reactivated. Dahlan landslide Mica schist

15 Where did they occur?

16 Inner valleys (valley in valley) Inner valley

17 Inner valley

18 Landslides

19 Inner valley development Loosening of the valley wall Rapid River incision River Incision Valley widening Loosened rock masses have been removed by the shaking

20 Failure within a landslide body Trishuli River

21 Steep infacing-slopes Slate, Bhote Koshi catchment

22 hat Terrace scarps

23 Along a road Before the EQ

24 Landslides & Geology Gneiss (Bhote Koshi catchment) Schist Malekhu Limestone Benighat Slate Roban phyllite Dhading Dolomite 4 km Kuncha F. phyllite Fagfog Quartzite Pinkish phyllite

25 Number of landslides by each geological zone Occupancy ratio of landslides by each geological zone Landslides are dominating in areas of gneiss, slate, dolomite, quartzite Rare in areas of schist, phyllite Geology & Landslides

26 Landslides and slope angles Landslide numbers % Areal ratios of landslides ~5 6~10 11~15 16~20 21~25 26~30 31~35 36~40 41~45 46~50 51~55 56~60 61~ ~5 6~10 11~15 16~20 21~25 26~30 31~35 36~40 41~45 46~50 51~55 56~60 61~65

27 Subsequent rainfalls enlarged some landslide scars

28 Large landslide bodies were not reactivated (Trishuli River) The 2015 Earthquakeinduced slope failure newly formed scar enlargement of a pre-existing scar Pre-existing slope failure not enlarged by the earthquake Deep-seated landslide

29 InSAR image to detect small displacements on the order of cm Second (after the EQ) observation Crustal deformation First (before the EQ) observation In the case of LoS shortened, Blue -> Yellow -> Red: The ground was uplifted or close to the satellite 29

30 Comparing before and after the earethquake Mixture of noise and crustal displacement, But the red circle shows local surface displacement. Blue->Red->Yellow, away from the satellite In the red circle

31 Bansaghu Talus slope New cracks but no development after the quake Transient adjustment of rock fragments Groundwater level was shallow but the slope was not seriously destabilized.

32 After the quake No change was found in the red circle InSAR is very helpful to monitor displacements after an earthquake

33 Quantitative seismic response analysis with FEM. Input waveform (near the base camp of Mt. Everest, Ev-K2-CNR Association) 7.0km 5.8km Discretized finite element meshes for the analytical area along the Trisuli River.

34 Calculated maximum value of horizontal acceleration at ground surface. Calculated maximum value of shear stress in surficial layer. Landslides occurred on slopes with amplified acceleration and/or mobilized shear stress

35 Annual precipitation and landslides

36 Daily and accumulated precipitation for 1 Jan 30 July, Pokhara Kathmandu mean mean_accum _accum mean mean_accum _accum (7years) Pokhar a Kathmandu Okhaldhung a Okhaldhunga Simar a mean mean_accum _accum

37 Antecedent rainfall amounts Noto Hanto Chuetsu-oki Mid-Niigata 150 Gorkha earthquake 10 days before the quake

38 Proposal of the methodology for the Hazard mapping of earthquake-induced landslides on the basis of Slope angles Inner valleys, infacing slopes, and terrace edges and Geology

39 Mapped landslides 6527 new landslides 558 enlarged landeslides

40 Slope angles

41 Slope (35 45deg)

42 Slope (35 45deg)+Inner Valley

43 Slope (35 45deg)+Inner Valley+Carbonate

44 Slope (35 45deg)+Inner Valley+Carbonate +Landslides Other carbonates, sandstone, gneiss, slate, terraces, infacing slopes are not shown

45 Conclusions Landslides induced by the 2015 Gorkha earthquake were mostly shallow disrupted landslides. Landslide dams hardly caused serious threats Landslides occurred mostly on steep slopes in inner valleys, on geologically controlled steep slopes like infacing slopes, and on terrace edges Earthquake shaking was amplified on convex slope breaks on steep slopes Most susceptible rocks to landslide were carbonate, sandstone, gneiss and slate, while phyllite may be not Some earthquake-induced landslides were enlarged by subsequent rainfalls Earthquake-induced cracks have not always developed by subsequent rainfalls Deep-seated large landslide bodies were hardly reactivated The effects of rainfalls before the earthquake were not clearly identified, but landslides would be much more when the earthquake occurred during a rainy season We propose hazard mapping of earthquake-induced landslides on the basis of topographic features and geology

46 Impact on Infrastructure by Gorkha Earthquake 2015 Induced Landslides D. Pathak 1,3, S. R. Bajracharya 1,2 and V. Dangol 1,3 1 Tribhuvan University; 2 ICIMOD, and 3 Nepal Landslide Society

47 Background The 7.6 (ML) magnitude Gorkha Earthquake of 25 April, 2015 followed by numerous aftershocks has affected 31 affected districts, among which the impact was quite serious in 17 districts. This presentation describes some damages to infrastructure like road, bridge, hydropower, settlements due to earthquake induced landslides. The work was carried out through the use of satellite imageries and field visits at selected sites by Nepal Landslide Society for ICIMOD.

48 Study districts

49 Damage to road from earthquake induced landslide in Dhading after earthquake before earthquake The earthquake triggered old landslide

50 Landslide triggered by earthquake at the bend in Pasang Lhamu highway, Rasuwa after earthquake before earthquake

51 Damaged Road Sections due to the EQ induced landslides Road Name Damaged length, m Araniko Highway 11, Pasang Lhamu Highway 4, Chanaute-Barpak 3, Mailung Bazar Road 3, Chaku to inner village 2, Chaku Khola HPP Access Road 2, Lamusangu-Ramechhap Highway 1, Charikot-Lamabagar Road 1, Singati-Bigu Road 1, BP Highway Listi Road Melamchi-Helmbu Road Bhirkot-Hanwa Road Besisahar-Chame Road Dolalghat-Chautara Road Namdu-Jugu Road Bhitkot-Jhule Road Helambu Road Barpak-Gumda Road Balephi-Jalbire Road Gorkha-Ghyampesal Road Busti-Mirge Road Sipring HP Access Road Mid Bhotekoshi Access Road Camp Area Upper Hadi HPP Access Road 26.10

52 Total road damaged: 38 km Araniko Highway: 11.3 km. Chanaute-Barpak road section : 3.6 km portion. Damages to roads due to earthquake induced landslides

53 Many buildings damaged by earthquake induced landslides, mainly in Sindhupalchowk, Lamjung and Rasuwa districts Buildings damaged by earthquake induced landslides

54 The avalanche at Langtang valley caused huge loss of lives and property

55 School buildings damaged by or vulnerable to EQ induced landslides

56 Hydropower Projects affected by EQ induced landslides

57 EQ induced landslides: Damages to HP Projects Landslide damages to penstock alignment of Sanima HP Project. Damages to penstock and PH sites of Bhotekoshi HP Project due to earthquake induced landslides.

58 EQ induced landslides: Damages to HP Projects PH site of Aankhu Khola HP Project affected by landslide at the upslope Landslides affecting the HP Projects on Chaku Khola

59 Landslides affecting the Bhotekoshi HP Project

60 Hydro Power Projects affected by earthquake induced landslides S.N Name of Project Location (District) 1 Upper Bhotekoshi Khola Capacity (kw) Sindhupalchowk 45,000 2 Sunkoshi Khola Sindhupalchowk 2,500 impact to Project Penstock burst due to the rock fall, power house submerged due to the penstock burst, accessibility disrupted. Landslide at penstock alignment and landslide at headworks areas, no access to power plant. 3 Baramchi Khola Sindhupalchowk 4,200 Penstock pipe burst, no access to power plant. 4 Upper Hadi Khola Sindhupalchowk Middle Chaku Sindhupalchowk 1,800 Accessibility disrupted 6 Lower Chaku Khola Sindhupalchowk 1,765 Accessibility disrupted 7 Bhairab Kunda Sindhupalchowk 3,000 8 Ankhu Khola Dhading 8,400 9 Radhi Khola Lamjung 4, Siuri Khola Lamjung 4,950 Rock fall at switchyard; Head-race steel pipe cracks at several parts. Tunnel leakage, penstock burst, switchyard damage, transmission line damage. Substation, Powerhouse fully damaged by landslide, 11 poles damaged. Headrace pipe deformed by rock fall, foundation settlement. Headrace pipe is damaged and affected due to rock fall, Anchor blocks and saddle is found have small crack and foundation settlement. 11 Sipring Khola Dolkha 9,658 Landslide at penstock alignment. 12 Mailung Khola Rasuwa 5,000 Significant damage in Headworks, desander, Penstock pipe and powerhouse.

61 Conclusions The Gorkha earthquake 2015 not only resulted in the destruction of buildings and infrastructures, but also caused landslides that damaged the infrastructures, mainly road and hydropower projects as well as residential and school buildings. Burial of the entire Langtang village, blocking of Kaligandaki River, damages to hydropower projects and damages to Araniko and Rasuwagadhi Highways connecting to China were the major noticeable events due to the earthquake induced landslide. In addition, numerous stretches of the feeder roads and trails were also damaged.