指導教授 ： 張光宗 林峻德 趙逸幃 Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: example from the Chenyulan River watershed, Nantou, Taiwan 1 Ching-Weei Lin, Chjeng-Lun Shieh, Bee-Deh Yuan,Yeou-Chung Shieh, Shou-Heng Liu, Sen-Yuan Lee Engineering Geology 71 (2003)49-61

1 、 Introduction 2 、 Study methods 3 、 Geological setting 4 、 Impact of the Chi-Chi earthquake on landslides 2 5 、 Occurrences of debris flows This paper is divided into nine main sections 6 、 Critical conditions triggering debris flow 7 、 Possible mechanisms for the change in critical conditions triggering debris flow 8 、 Discussions 9 、 Conclusions

1 、 Introduction After the Chi-Chi earthquake, over 20,000 landslides totaling approximately 113 km 2 had occurred in an area of 2400 km 2 in central Taiwan (Wang et al., 2000). More than 90% of the landslides were smaller than 0.01 km 2 in scale, and most are shallow debris slides, although a few being large and deep-seated (Hung et al., 2000, Huang et al., 2001 and Wang et al., 2000.) Consequently, a great deal of loose sediment was induced which in turn promoted heavy debris flows during subsequent typhoons and heavy rains. 3

1 、 Introduction The present study investigates the Chenyulan River watershed, where hazardous debris flows occurred during Typhoon Herb on July The study area covers about 21×36 km, and is located about 12 km east of Chi-Chi, the town closest to the epicenter of the September 21, 1999 earthquake. Since geologists and engineers have studied extensively in this watershed on the occurrence, mechanism, and hazard mitigation for debris flows after the Typhoon Herb (e.g., Lin et al., 2000, Chen, 2000 and Chen and Su, 2001), more morphologic, geologic, and hydrologic information have been collected here than anywhere else on Taiwan. 4

2. Study methods To compare the rainfall-induced landslides that took place before and after the Chi-Chi earthquake, eight SPOT images taken between 1996 and 2001 were analyzed. Only those images taken after typhoons with accumulated precipitation of over 100 mm were selected. 5

6 SPOT image taken on January 8, The study area is marked by the white boundary. 合社 豐丘 信義 神木 郡坑 龍神橋

2. Study methods Landslides, especially shallow debris slides, are easily identified from SPOT images, and were checked by aerial photos when available. The resolution of the SPOT images is such that only landslides with a minimum area of 0.3 ha can be recognized. A 40×40-m grid Digital Terrain Model (DTM) established before the Chi-Chi earthquake was applied for calculating the slope distribution (with a 10° interval) of the study area. 7

2. Study methods To understand the relationship between landslide and the intensity and accumulated amount of precipitation in the study area, and to study the conditions that set off debris flow. Data from 5 measurement stations set up by the Bureau of Weather in the study area were collected. Rainfall data recorded at the station closest to the gully were chosen to represent the precipitation condition for the debris flow that occurred there. 8

5 measurement stations 9

3. Geological setting The Chenyulan watershed area is in central Taiwan, and situated at the junction where the metamorphic Shuehshan ( 雪山 )Range juxtaposes against the unmetamorphosed Western Foothills. The Chenyulan River which closely follows the Chenyulanchi fault line. The Chenyulanchi fault separates the Neogene sedimentary rocks of the Western Foothills from the Paleogene metamorphic rocks of the Shuehshan Range. Differential uplifting along this fault has generated great topographic relief and abundant fractures that resulted in frequent landslides and debris flows. 10

11 水長流層 南港層 南莊層 陳有蘭溪斷層 十八重溪層 白冷層 達見砂層

4. Impact of the Chi-Chi earthquake on landslides 12 Shallow debris slides are commonly the easiest and most reliable type among different sliding phenomena to be detected on a SPOT image since they strip off the vegetation cover from the steep slopes, which makes them readily discernable. Therefore, only shallow debris slides were utilized in this study, while deep-seated slides and lateral erosion along the gully bed caused by debris flows are excluded.

4. Impact of the Chi-Chi earthquake on landslides 13

4. Impact of the Chi-Chi earthquake on landslides 集集大地震 1996,Typhoon Herb

4. Impact of the Chi-Chi earthquake on landslides 15 Since there was no abnormally high precipitation after the Chi-Chi earthquake, the abrupt increase in landslide area was most likely attributed to the Chi-Chi earthquake. Field survey revealed that in most slope failures, only 1– 5 m of sediments or rock debris were striped off during the earthquake.

4. Impact of the Chi-Chi earthquake on landslides 集集大地震

This implies that a strong earthquake such as the Chi-Chi did more than cause severe landslide, it also substantially induced landslide during future heavy rains. 4. Impact of the Chi-Chi earthquake on landslides This shows that the landslide area increased drastically not only immediately after the earthquake, but also expanded significantly 2 years later, occurring as rainfall- induced landslides. We believe this is due to numerous extension cracks generated on hill slopes during the earthquake, which accelerate landslides during subsequent downpours. 17

5. Occurrences of debris flows Debris flows in the study area were first reported during Typhoon Wayne (August 22, 1986). Since 1986, no debris flow was reported until Typhoon Herb (July 29, 1996), in spite that 36 typhoons landed on Taiwan within that interval. During Typhoon Herb, debris flows happened in more than 32 gullies. On July 30, 2001, Typhoon Toraji brought in intense rainfall, with accumulated precipitation of 167–426 mm within several hours, which resulted in severe debris flow in the study area. 18

19 Bold solid lines represent gullies in which debris flow occurred during Typhoon Herb. Shaded areas represent landslides identified on the SPOT image. 1: 龍神橋 2: 信義 3: 和社 4: 望鄉 5: 神木 實心三角數表示的雨量測量站 Bold solid lines represent gullies in which debris flow occurred during Typhoon Toraji. Shaded areas represent landslides identified on the SPOT image.

5. Occurrences of debris flows 20 Severe debris flows occurred widely in the study area during Typhoon Herb (1996, before the Chi-Chi earthquake) and Toraji (2001, after the quake). Therefore, the impact of earthquake on the occurrence of debris flow can be analyzed by comparing these two events. Moreover, debris flows also occurred on August 10, 2001 during Typhoon Nali. However, in the absence of SPOT images shot before Typhoon Nali, it is uncertain in relating the debris flows with individual typhoon.

5. Occurrences of debris flows 21

5. Occurrences of debris flows 22 Furthermore, before the earthquake, 2 debris flows happened between 1986 and 1996 (recurrence time of 5–10 years, based on very short records), however, after the Chi-Chi earthquake, 6 debris flow events were observed in the subsequent 2 years.

6. Critical conditions triggering debris flow 1. The data recorded at rainfall measurement stations were used to represent the precipitation condition for its nearby gully where debris flow took place. 23

6. Critical conditions triggering debris flow 2. During Typhoon Herb (July 29, 1996), most debris flow occurred in the early morning of July 31 (Fig. 8). 24 Accumulated precipitation reached more than 450 mm. Maximum hourly rainfall intensity was 50– 72 mm/h.

6. Critical conditions triggering debris flow 3. In contrast, after the Chi-Chi earthquake but before Typhoon Toraji, five debris flow events appeared after heavy rains. 25 accumulated precipitation maximum hourly rainfall intensity rainfall measurement station No.1 70–85 mm17–57 mm/h rainfall measurement station No –117.5 mm10–14 mm/h

6. Critical conditions triggering debris flow 4. Typhoon Toraji brought the most abundant rainfalls after the Chi-Chi earthquake in the study area. Consequently, extensive debris flow occurred in most gullies on the morning of July 30, Accumulated precipitation initiating debris flow in the study area was 167–426 mm. Maximum rainfall intensity was 66.5–86.5 mm/h. 26

6. Critical conditions triggering debris flow 27 地震前 地震後

6. Critical conditions triggering debris flow 5. The above criteria clearly indicate that both the accumulated precipitation and hourly rainfall intensity necessary to set off debris flow decreased after the Chi-Chi earthquake. Especially for those that took place between the Chi-Chi earthquake and Typhoon Toraji. 28

7. Possible mechanisms for the change in critical conditions triggering debris flow 29 As mentioned earlier, before the Chi-Chi earthquake, most failures occurred in places with slope between 20° and 30 °, but after the earthquake, they were concentrated in locations with slope between 40 ° and 50 °.

7. Possible mechanisms for the change in critical conditions triggering debris flow 30 In the study area, the 40 ° and 50 ° slopes are mainly composed of bedrocks covered with little or thin layer of sediments before the earthquake. After the earthquake, field survey showed that large amount of earthquake-induced cracks had developed in these rocks. The cracks allowed water to infiltrate, thus could have diminished the strength of the rocks during raining.

7. Possible mechanisms for the change in critical conditions triggering debris flow 31 For the critical condition The block will slide down-slope, if

7. Possible mechanisms for the change in critical conditions triggering debris flow 32 Thus, P/Wsinθ decreases significantly as θ changes from 20° to 40 ° or 50 ° (if holding µ as constant). This indicates that the amount of pore fluid required for a block to move decreases when the slope increases.

8. Discussions 33 The 1999 Chi-Chi earthquake provided an unprecedented amount of data for studying the impact of earthquake on landslide and debris flow occurrences. Compared to the field survey, certain errors might be created by using SPOT images to count and locate landslides. However, the vastness of the study area and poor accessibility to the disastrous region made SPOT images and DTM the most ideal tools for the current research.

8. Discussions 34 The study area, located some 12 km east of the epicenter, experienced 400–600 gal of ground acceleration during the earthquake, which caused large scale rock fracturing and landslides. This study shows that in addition to inducing co-seismic landslides, the Chi-Chi earthquake shifted the locations where rainfall- induced landslides might occur, from mid-hill (before the earthquake) to hilltops (after the earthquake). During the earthquake, although surface strata were disrupted and brought downhill, there were still abundant debris left behind on the hill tops, which would later serve as source for rain- induced landslides. The above results are consistent with most case studies of earthquake- and rainfall-induced landslides.

8. Discussions 35 Besides, both the size of the drainage basin contributing to debris flow and the debris flow recurrence time decrease drastically after the earthquake. The present study has also shown that the rate of expansion in landslide area grew almost 20 times following the Chi-Chi earthquake.

9. Conclusions 36 The present study has shown that earthquake-induced landslides provide tremendous amount of loose sediment to the gullies in the study area. In addition, numerous extension cracks were induced on hill slopes by the Chi-Chi earthquake, and these have accelerated landslides during the subsequent heavy rains.

9. Conclusions 37 In the aftermath of the most destructive earthquake in Taiwan’s history, the study area in central Taiwan was further devastated by enormous landslides and debris flows, which are likely to haunt the same region in the near future. However, to fully understand the occurrences and mechanisms triggering landslides and debris flows, and to better understand the long-term impact of earthquake on the duration and intensity of landslides and debris flows, future tracking of variations of landslides and debris flows in the study area is required.

Thanks for your listening. 38