Impacts of September 21, 1999 Chi-Chi earthquake on the characteristics of gully-type debris flows in central Taiwan

Debris flows are more frequent in central Taiwan, because of its mountainous geography. For example, many debris flows were induced by Typhoon Herb in 1996. The Chi-Chi earthquake with a magnitude of 7.3, which took place in 1999 in central Taiwan, induced many landslides in this region. Some landslides turned into debris flows when Typhoon Toraji struck Taiwan in 2001. This study investigates the characteristics of the gullies where debris flows have occurred for a comparison. Aerial photos of these regions dated in 1997 (before the earthquake) and 2001 (after the earthquake) are used to identify the occurrence of gully-type debris flows. A Geographic Information System (GIS) is applied to acquire hydrological and geomorphic characteristics: stream gradient, stream length, catchment gradient, catchment area, form factor, and geology unit of these gullies. These characteristics in different study regions are presented in a statistical approach. The study of how strong ground motion affects the debris flows occurrence is conducted. The characteristics of the debris flow gullies triggered by typhoons before and after the Chi-Chi earthquake are quantitatively compared. The analysis results show that a significant transformation in the characteristics was induced by the Chi-Chi earthquake. In general, the transformation points out a lower hydrological and geomorphic threshold to trigger debris flows after the Chi-Chi earthquake. The susceptibility of rock units to strong ground motion is also examined. The analysis of debris flow density and accumulated rainfall in regions of different ground motion also reveal that the rainfall threshold decreases after the Chi-Chi earthquake.     

Landslide and debris flow initiated characteristics after typhoon Morakot in Taiwan

Typhoon Morakot brought extreme rainfall and initiated numerous landslides and debris flows in southern Taiwan in August of 2009. The purpose of this study is to identify the extreme rainfall-induced landslide frequency-area distribution in the Laonong River Basin in southern Taiwan and debris flow-initiated conditions under rainfall. Results of the analysis show that debris flows were initiated under high cumulative rainfall and long rainfall duration or high rainfall intensity. The relationship of mean rainfall intensity and duration threshold could reflect debris flow initiation characteristics under high rainfall intensity in short rainfall duration conditions. The relationship of cumulative rainfall and duration threshold could reflect debris flow initiation characteristics under high cumulative rainfall in long rainfall duration. Defining rainfall events by estimating rainfall parameters with different methodologies could reveal variations among intermittent rainfall events for the benefit of issuing debris flow warnings. The exponent of landslide frequency-area distribution induced by Typhoon Morakot is lower than that induced by the Chi-Chi earthquake. The lower exponent of landslide frequency-area distribution can be attributed to the transportation and deposition areas of debris flow that are included in the landslide area. Climate change induced high rainfall intensity and long duration of precipitation, for example, Typhoon Morakot brought increased frequency of debris flow and created difficulty in issuing warnings from rainfall monitoring.     

A formation model for debris flows in the Chenyulan River Watershed, Taiwan

Many debris flows were triggered in the Chenyulan River Watershed in Taiwan in a rainstorm caused by the Typhoon Toraji. There are 117 gullies with a significant steep topography in the catchment. During this Typhoon, debris flows were initiated in 43 of these gullies, while in 34 gullies, it was not certain whether they have occurred. High-intensity short-duration rainfall was the main triggering factor for these gully type debris flows which are probably entrained by a “fire hose” mechanism. Previous research identified 47 factors related to topography, geology, and hydrology, which may play a role in the formation of gully type debris flows. For a better understanding of the probability of the formation of debris flows, it is proposed to represent the factors related to topography, geology, and hydrology by one single factor. In addition to the existing topographic and geological factor, a normalized critical rainfall factor is suggested with an effective cumulative precipitation and a maximum hourly rainfall intensity. In this paper, a formation model for debris flows is proposed, which combines these topographic, geological, and hydraulic factors. A relationship of these factors with a triggering threshold is proposed. The model produces a good assessment of the probability of occurrence of debris flows in the study area. The model may be used for the prediction of debris flows in other areas because it is mostly based on the initiation mechanisms and not only on the statistical analyses of a unique variety of local factors. The research provides a new and exciting way to study the occurrence of debris flows initiated by a “fire hose” mechanism.     

Landslides and Rainfall Characteristics Analysis in Taipei City during the Typhoon Nari Event

The severe Typhoon Nari ended on September 15, 2001 with a high-intensity and high-accumulation storm that dumped up to 1249 mm of rain in Taipei City, Taiwan. The high-intensity and high-accumulation event caused flooding and triggered more than 400 soils slips and debris flows and large, complex landslides. Detailed information on 63 events, including rainfall, initiation time, and magnitude of landslides were documented and analyzed to identify the landslides and rainfall characteristic in Taipei City during Typhoon Nari. The result reveals that slump, slide, and debris flow events are associated with the situation of high-intensity or high-accumulation rainfall. The rainfall intensity-duration condition resulted in smaller magnitude and shallow failures. Medium to massive landslide were mainly related to the high-accumulation rainfall. A landslide regionalization process based on rainfall, geomorphologic and geologic characteristics is proposed. Results of the proposed process show good agreement with landslide events observed in the Taipei City during Typhoon Nari.     

Characteristics analysis for the flash flood-induced debris flows

Typhoon Haitang caused landfall on Taiwan during 15–21 July, 2005 and brought 2,279 mm of maximum cumulative rain with a maximum intensity of 176 mm/h. The torrential rain was mainly distributed from the central mountain range to southern Taiwan and triggered 222 slopeland-related hazards. Among the hazard events, there were 17 debris flows, 157 cases of traffic cut-off, three large-magnitude deep-seated landslides, and 10 villages isolated in the off-track mountainous areas. The debris flows initiated in southern Taiwan were associated with torrential rain, short channel length (<2 km), and small basin area (<3 km²), and were speculated to be induced by flash flood. These flash flood-induced debris flows have a higher rainfall intensity-duration threshold for initiation than in other areas. The deep-seated landslides, isolated villages due to traffic cut-off in off-track mountain areas, and recurrent hazards in areas affected by the M_L 7.6 Chi-Chi earthquake in 1999 are characteristics of slopeland hazards in Taiwan in recent years. One of the most urgently needed mitigation strategies in response to slopeland hazards is the plan for enhancing self-rescue disaster resistance in off-track mountainous villages in Taiwan.     

浅层滑坡诱发沟谷泥石流的地形和降雨条件

2011年贵州省望谟县打易镇的大范围浅层滑坡诱发的沟谷泥石流提供了研究这类泥石流地形和降雨条件的机会。在地质条件一致和小区域内的降雨条件基本一致的情况下,地形条件就是这些泥石流暴发与否的唯一决定因素。对比一些重要的地形因素与泥石流暴发的关系,得出了由流域面积、沟床纵比降和25°~45°山坡坡度面积比组成的泥石流综合地形因子T。在地形因子T的基础上,研究获得了由前期降雨量、1 h降雨强度、年平均降雨量等组成的降雨因子R。由地形因子T和降雨因子R获得的临界条件P可以判断该区域的泥石流暴发。由于研究工作部分基于泥石流的形成机理,研究成果还可用于其他区域的泥石流形成预测,为泥石流的预测预报提供了一个较好的方法。     

汶川震区北川9.24暴雨泥石流特征研究

2008年9月24日汶川震区的北川县暴雨导致区域性泥石流发生,这次9.24暴雨泥石流灾害导致了42人死亡,对公路和其他基础设施造成严重损毁。本研究采用地面调查和遥感解译方法分析地震与暴雨共同作用下的泥石流特征,获取的气象数据用于分析泥石流起动的临界雨量条件。本文探讨了研究区泥石流起动和输移过程,并根据野外调查,分析了泥石流形成的降雨、岩石和断层作用,特别是强降雨过程与物源区对泥石流发生的作用。根据应急调查发现北川县境内暴雨诱发的泥石流72处,其分布受岩石类型、发震断层和河流等因素控制。根据对研究区震前和震后泥石流发生的临界雨量和雨强的初步分析,汶川地震后,该区域泥石流起动的前期累积雨量降低了14.8%～22.1%,小时雨强降低25.4 %～31.6%。震区泥石流起动方式主要有二种,一是由于暴雨过程形成的斜坡表层径流导致悬挂于斜坡上的滑坡体表面和前缘松散物质向下输移,进入沟道后转为泥石流过程;二是消防水管效应使沟道水流快速集中,并强烈冲刷沟床中松散固体物质,导致沟床物质起动并形成泥石流过程。调查和分析发现沟内堆积的滑坡坝对泥石流的阻塞明显,溃决后可导致瞬时洪峰流量特别大。研究结果表明了汶川震区已进入一个新的活跃期。因此,应该开展对汶川地震区的泥石流风险评估和监测、早期预警,采取有效的工程措施控制泥石流的发生和危害。     

四川泸定MS6.8级地震区湾东河流域泥石流活动性预测

2022年9月5日四川泸定县发生M_S 6.8级地震,地震诱发大量同震崩滑体,并导致湾东河断流。基于现场调查、影像解译和区域地质资料分析,采用空间统计和水文计算的方法,对湾东河流域同震崩滑体分布特征和潜在泥石流危险性进行了研究。结果表明：湾东河流域内同震崩滑体主要分布在地震烈度Ⅸ度区,规模以中小型为主,主要沿沟道两侧展布,尤其是单薄山脊两侧临空面发育密度较大,距断层距离和坡度对其分布具有明显的控灾效应;未来湾东河流域暴发溃决型泥石流的冲出量可能为同等触发条件下震前泥石流的约两倍。依此提出了加强流域内溃决型泥石流风险防范,尽快通过综合监测预警获取泥石流发生的临界雨量值,在泥石流防治工程设计中应充分考虑泥石流规模放大系数等防灾减灾建议,为泸定地震后泥石流灾害防灾减灾提供科学参考。     

Variability of impact of earthquake on debris-flow triggering conditions: case study of Chen-Yu-Lan watershed,Taiwan

This study investigates the variations in the critical conditions for debris-flow occurrence before and after the Chi-Chi earthquake in the Chen-Yu-Lan watershed, Taiwan. Topographical and rainfall parameters such as the gully gradient, drainage area, rainfall intensity, cumulative rainfall, and rainfall duration in the Chen-Yu-Lan watershed were used to analyze the conditions of debris-flow occurrence over the past 25 years. A recovery equation was proposed on the basis of rainfall parameters and used to determine the variations in the critical line of rainfall that trigger debris flow after the earthquake and to evaluate the recovery period required for the rainfall threshold of debris-flow occurrence after the earthquake to return to that before the earthquake in the watershed. The critical line for the runoff parameter versus gully gradient in the watershed was also presented.     

Integrated Community-Based Disaster Management Program in Taiwan: A Case Study of Shang-An Village

Taiwan has long made efforts to increase community emergency response capability, due to its vulnerability to earthquakes, typhoons, landslides and debris flows. Not until recent major natural disasters, such as the 1999 Chi–Chi Earthquake, Typhoon Toraji and Typhoon Nari, has the government reformed its policy toward empowering the community to take actions in hazard mitigation, emergency preparedness and emergency response. A new initiatve, Integrated Community-Based Disaster Management Program (ICBDM), was launched in 2001 by the Executive Yuan to achieve the goal of strengthening community resistance. The paper, taking Shang-An Village as an example, describes Taiwan’s new community-based disaster management program. Through a participatory process, community residents have learned how to analyze vulnerable conditions, discover problems, develop solutions and establish an organization to implement disaster management tasks. Further, basic response training courses and a disaster scenario were held in order to improve their emergency response capability. Based on the case study, a phased process, including initiation, assessment, planning and practice, is generalized.     

Increase in basin sediment yield from landslides in storms following major seismic disturbance

The landslide area along the Tachia River catchment of central Taiwan was investigated using the remote sensing images of various typhoon and earthquake events taken from 1996 to 2004 and the sediment discharge measured at hydrometric stations. Our findings indicate that 88% of the coseismic landslides triggered by the 1999 Chi-Chi earthquake were first-time occurrences. After the Chi-Chi earthquake, 59% of the landslide area was reactivated during typhoon Toraji and 66% during typhoon Mindulle. The landslides prone to reactivation were on the formations with closely spaced discontinuities. It is suggested that further rock-mass defects in the formations were added or opened by the earthquake, and that the landslide areas continued to extend until the end of 2004. Post-1999 rainstorms delivered large amounts of colluvial sediment into the main channel, leading to a 2-fold increase in post-seismic sediment discharge while the precipitation was only half that of the pre-earthquake rate.     

Geological factors for hazardous debris flows in Hoser, central Taiwan

By means of an onsite investigation, the largest debris flow in Taiwan's history is analyzed in this paper. A heavy rainfall of 1,748 mm/day occurred during typhoon Herb in the Hoser area at the end of July 1996. Aerial photographic assessment and observations of geological and geomorphological features have contributed to our understanding of this massive destruction. Mechanisms of the hazardous debris flow are explored and discussed. Data revealed that the rock discontinuities were a major factor in the voluminous loose materials in the debris flows. The heavy and rapid rainfall instantly transported massive amounts of debris flow materials into the center, and then quickly funneled them to the lower parts of gullies. The heavy slurry became an effective cutting device to erode the side walls and move large quantities of the debris materials to the end of the gullies.     

震后都汶公路沿线泥石流沟堵江危险性评估

震后泥石流松散物质显著增多,临界雨量降低,导致泥石流暴发频率增加、规模增大,从而使震后灾区泥石流堵江事件频繁发生,给灾区带来严重的二次灾害。为了评价震后泥石流堵江概率大小,本文在野外考察基础上,获得研究区泥石流沟基础数据,并根据水文模型计算不同频率下的泥石流规模; 在此基础上利用泥石流堵江公式,获取研究区域泥石流堵江危险程度(数值),并结合研究区域已发生的泥石流堵江事件,界定了泥石流堵江公式的临界值,使之能够更加准确用来判定泥石流堵江概率,并对 5·12 地震极震区都汶公路沿线的7条泥石流在不同频率下的泥石流堵江概率进行评价。评价结果显示:牛眠沟和关山沟在暴发50a及其以上泥石流时,会发生堵江; 烧房沟、红椿沟和磨子沟在暴发10a及其以上泥石流时,会发生堵江事件; 洱沟和太平沟在暴发5a及其以上泥石流时便可能发生堵江; 上述泥石流沟一旦发生堵江,便会给映秀镇和都汶公路带来严重灾难。通过本文获取的堵江临界值可以作为判定泥石流堵江的参考标准,为泥石流防治、预报提供参考,同时可以为灾后重建和预防二次灾害提供科学借鉴。     

甘肃省舟曲8.7特大泥石流调查研究

本文通过对甘肃省舟曲县城后山三眼峪沟和罗家峪沟特大泥石流灾害的现场调查,从泥石流形成的地形、地质和降雨条件入手,分析了特大泥石流灾害的特征与成因:三眼峪沟和罗家峪沟泥石流形成区在2010年8月7日23～24时的1h降雨量达77.3mm,暴雨形成强大洪水依次冲毁两条沟内的天然堆石坝和人工拦挡坝,形成规模巨大的高容重黏性泥石流,泥石流冲出总量和泥沙总量分别为 144.2104m3和97.7104m3; 泥石流携带具有强大冲击力的巨石冲毁房屋5500余间; 在白龙江内形成长约550m,宽约70m,高约10m的堰塞坝并形成堰塞湖,堰塞湖回水长3km,使县城一半被淹; 泥石流造成1744人死亡和失踪。分析研究表明,三眼峪沟和罗家峪沟泥石流如果在近期遭遇强降雨还会暴发泥石流,但规模比87特大泥石流小;如果强降雨发生在数年后,暴发的泥石流规模比87特大泥石流略小;在20a或更长的时期内,没有发生新的地震影响下,在三眼峪沟和罗家峪沟经历一次大规模泥石流暴发后,泥石流的规模将回到汶川地震前的水平。     

"5·12" 地震后汶川县泥石流特征与演化分析

2008年“5·12”汶川地震极大地改变了震区泥石流的特征,不仅增强了泥石流的活动性,同时也使得震区在相当长的时间内都要面临泥石流的威胁。本文基于前人大量的研究成果,并利用遥感解译结合现场调查等手段,分析了汶川县泥石流沟道纵坡降、沟壑密度、两岸坡度等基本发育特征;进而分析了地震前后汶川县降雨分布及泥石流相关降雨参数变化特征。结果显示,流域内泥石流沟的沟壑密度在0.2~4之间,属于微度土壤侵蚀区域,泥石流的沟床纵坡降偏大,有利于泥石流的发生;泥石流流域内斜坡坡度多为30°~40°,有利于灾害的发生;震后汶川县年均降雨量增加了5.17%,降雨多集中在7~9月份,降雨量由南及北逐渐降低;震后泥石流的降雨阈值在2008~2013年呈现缓慢回升的趋势,但2019年又有所下降,预计恢复到震前水平尚需要一定时间;同时震后汶川县泥石流物源丰富,震后物源量呈现“震荡式衰减”的演化趋势,但体量仍然很大,对泥石流仍需坚持监测预警工作。     

四川泸定县泥石流灾害成因、特征与防治建议

泸定县地处川西高原与四川盆地的过渡地带,地质环境条件复杂,分布泥石流沟127条,发育密度054条10km-2。在遥感解译和地面调查基础上,论述了泸定县泥石流灾害成因、发生特征、危害和发展趋势,提出了相关防治建议。(1)成因表现为复杂的断裂构造导致流域内松散物源丰富,较大的相对高差提供了良好势能条件,而汛期集中强降雨则提供了动能条件,且雨量条件中当日雨量与前期有效降雨量之间具有幂函数关系; (2)发生特征表现出典型的空间群发性(包括2005-06-30群发性泥石流、2005-08-11群发性泥石流和2006-07-14群发性泥石流)与时间的夜发性; (3)危害方式主要包括淤埋、冲毁、侵蚀、堵河4种,泥石流已造成74人死亡; (4)发展趋势表现为泥石流沟多处于形成期和发展期(113条,占89%),发生的年际准周期由20a缩短至不足10a,且发生条(次)由4条(次)增加至44条次(2005年); (5)泸定县泥石流防治的突出问题表现为城镇泥石流问题、泥石流堵塞主河问题、风景区泥石流问题与冲沟泥石流问题,建议通过科学规划、风险防御体系构建、土木工程治理、监测预警等措施予以防御。     

Relationships between typhoon types and debris flow disasters in Taiwan

Frequent debris flow disasters caused by heavy precipitation during the annual typhoon season are some of the most serious disasters in Taiwan. This study is on the debris flow disasters associated with the typhoons that hit Taiwan between 1986 and 2004. Typhoon data and records of debris flow disasters available for Nantou and Hualien counties in Taiwan were analyzed. The paths and rainfall characteristics of typhoons were found to have a great effect on the debris flows at these locations. Accordingly, the typhoons were grouped into four major types based on their paths and related disasters. The relationships between rainfall intensity and accumulation and debris flow are discussed for the four major typhoon types. The information may form the basis for providing useful indicators for disaster management.     

Rainfall thresholds for debris flow initiation in the Wenchuan earthquake-stricken area,southwestern China

The Wenchuan earthquake-stricken area is frequently hit by heavy rainfall, which often triggers sediment-related disasters, such as shallow landslides, debris flows, and related natural events, sometimes causing tremendous damage to lives, property, infrastructure, and environment. The assessment of the rainfall thresholds for debris flow occurrence is very important in order to improve forecasting and for risk management. In the context of the Wenchuan earthquake-stricken area, however, the rainfall thresholds for triggering debris flows are not well understood. With the aim of defining the critical rainfall thresholds for this area, a detailed analysis of the main rainstorm events was carried out. This paper presents 11 rainfall events that induced debris flows which occurred between 2008 and 2012 after the Wenchuan earthquake. The rainfall thresholds were defined in terms of mean rainfall intensity I, rainfall duration D, and normalized using the mean annual precipitation (MAP). An ID threshold and a normalized I _MAP D threshold graph could be set up for the Wenchuan earthquake-stricken area which forms the lower boundary of the domain with debris flow-triggering rainfall events. The rainfall threshold curves obtained for the study area were compared with the local, regional, and global curves proposed by various authors. The results suggest that debris flow initiation in the study area almost requires a higher amount of rainfall and greater intensity than elsewhere. The comparison of rainfall intensity prior to and after the earthquake clearly indicates that the critical rainfall intensity necessary to trigger debris flows decreased after the earthquake. Rainfall thresholds presented in this paper are generalized, so that they can be used in debris flow warning systems in areas with the same geology as the Wenchuan earthquake-stricken area.     

中巴经济走廊泥石流活动性分析 ——以中巴公路奥布段为例

中巴经济走廊内的中巴公路奥布段泥石流频发且类型复杂,严重影响着安全出行和贸易流通。在对中巴公路奥布段沿线泥石流沟谷纵剖面形态分析的基础上,揭示其形态指数特征和活动程度,并从区域地形、地质和气象等因素方面探讨了泥石流的活动性差异成因及危害性。研究发现：公路沿线泥石流类型主要包括冰川型和降雨型两种,冰川型泥石流为27条,降雨型为26条。冰川型泥石流活动性强烈,形态指数N ≥ 1的沟谷占冰川型沟谷总数的81%,多数沟谷形态呈下凹状;降雨型泥石流活动性相对较弱,形态指数N ≥ 1的沟谷占其总数的50%,沟谷形态多呈上凸状。研究区大落差地形、不同物源供给和充沛水源条件等对泥石流的发育和活动具有重要影响,也是不同类型泥石流活动性差异的控制因素。研究结果可为研究区泥石流预测和防治提供指导,也可为中巴经济走廊区内交通工程选线和泥石流防治提供参考。     

Vulnerability assessment of rainfall-induced debris flows in Taiwan

A GIS-based decision support system, which incorporates local topographic and rainfall effects on debris flow vulnerability is developed. Rainfall at a scale compatible with the digital elevation model resolution is obtained using a neural network with a wind-induced topographic effect and rainfall derived from satellite rain estimates and an adaptive inverse distance weight method (WTNN). The technique is tested using data collected during the passage of typhoon Tori-Ji on July 2001 over central Taiwan. Numerous debris flows triggered by the typhoon were used as control for the study. Our results show that the WTNN technique outperforms other interpolation techniques including adaptive inversed distance weight (AIDW), simple kriging (SK), co-kriging, and multiple linear regression using gauge, and topographic parameters. Multiple remotely-sensed, fuzzy-based debris-flow susceptibility parameters are used to describe the characteristics of watersheds. Non-linear, multi-variant regressions using the WTNN derived rainfall and topography factors are derived using self-organizing maps (SOM) for the debris flow vulnerability assessment. An index of vulnerability representing the degrees of hazard is implemented in a GIS-based decision support system by which a decision maker can assess debris flow vulnerability.