IAEG2006 Paper number 819 Rainfall triggered flow-like landslides: Understanding from southern hills of Kathmandu, Nepal and Northern Shikoku, Japan RANJAN KUMAR DAHAL1, SHUICHI HASEGAWA2, MINORU YAMANAKA3 & KATSUHIRO NISHINO4 1 Dept. of Safety System Construction Engineering, Kagawa University. (e-mail: [email protected]) (Department of Geology, Tribhuvan University, Tri-Chandra Campus, Kathmandu, Nepal) 2 Dept. of Safety System Construction Engineering, Kagawa University. (e-mail: [email protected]) 3 Dept. of Safety System Construction Engineering, Kagawa University. (e-mail: [email protected]) 4 Graduate School of Engineering, Kagawa University. (e-mail: [email protected]) Abstract: Flow-like landslides triggered by rainfall are very prominent in Nepal and Shikoku, Japan. In July 2002, many landslides occurred in the southern hills of the Nepalese capital, Kathmandu, because of torrential rainfall. A single flow-like landslide occurred at Matatirtha, a small village situated at the south marginal hill of Kathmandu, killing 18 people who lived at the foot of the hill. Much damage was caused to roads and houses because of landslides and debris flows in small streams. Similarly, in August, September and October 2004, strong typhoon hit the area of northern Sikoku, Japan and extensive damage occurred on hill slopes and some human casualties were also reported. Field observation showed that in northern Sikoku, many flow-like landslides occur in the thin weathering profile of igneous and sedimentary rocks, as well as in old debris materials. However, in the southern hills of Kathmandu, flow-like landslides occurred in weathered debris. During the investigation, the geotechnical properties of landslide materials were determined in the laboratory. The volume of material involved in some of the flows was calculated as per average thickness of the soil cover and area of failure. Likewise, rainfall threshold value for Kathmandu and Northern Shikoku is also evaluated. From the field investigations, it is recommended that human habitation at the foot of hills should be legally regulated by the government to reduce death from flow-like landslides triggered by torrential rainfall. It is also recommended that landslide hazard maps need to be quantified to include landslide risk assessment and management for flow like landslide also which help to develop early warning systems for flow-like landslide disasters. Résumé: Couler-comme des éboulements déclenchés par des précipitations soyez très en avant au Népal et le Shikoku, Japon. En juillet 2002, beaucoup d'éboulements produit dans les collines méridionales du capital népalais, Katmandou, en raison de précipitations torrentielles. Un simple couler-comme l'éboulement s'est produit chez Matatirtha, a petit village situé à la colline marginale du sud de Katmandou, tuant 18 peuplez qui a vécu au pied de la colline. Beaucoup de dommages ont été causés aux routes et maisons en raison des éboulements et des écoulements de débris dans de petits jets. De même, en août, septembre et octobre 2004, l'ouragan fort a frappé le secteur de Sikoku nordique, le Japon et les dommages étendus se sont produits sur des pentes de colline et quelques accidents humains ont été également rapportés. L'observation de champ a prouvé que dans Sikoku nordique, beaucoup couler-comme des éboulements produisez-vous dans le profil survivant à mince des roches ignées et sédimentaires, aussi bien comme en vieux matériaux de débris. Cependant, dans les collines méridionales de Katmandou, couler-comme des éboulements s'est produit en débris survécus à. Pendant la recherche, le géotechnique des propriétés des matériaux d'éboulement ont été déterminées dans le laboratoire. Le volume du matériel impliqué dans certains des écoulements a été calculé selon la moyenne épaisseur de la couverture de sol et du domaine de l'échec. De même, seuil de précipitations la valeur pour Katmandou et Shikoku nordique est également évaluée. Du champ des investigations, on lui recommande qu'habitation humaine au pied de collines si est légalement réglé par le gouvernement pour réduire la mort de couler-comme éboulements déclenchés par des précipitations torrentielles. On lui recommande également qu'éboulement des cartes de risque doivent être mesurées pour inclure la évaluation des risques d'éboulement et gestion pour l'écoulement comme l'éboulement également qui aident à développer la détection précoce systèmes pour couler-comme des désastres d'éboulement. Keywords: Landslide, weathering, failures, igneous rock, soils INTRODUCTION Slope failure broadly described as landslide is a consequence of a large mass surges down slope in response to the gravity. The style of mass movement is generally defined according to flow velocity, volume of the material involved, shear rates, and shear strength of the materials. The fluid and its interaction with the solid materials provide broader variations in its style. Experiences show that landslide occurrences on hill slopes have very close relationship with © The Geological Society of London 2006 1 IAEG2006 Paper number 819 availability of water. As a result, many varieties of landslides occur as a consequence of heavy rainfall in tropical and temperate climatic zones (Jacob and Weatherly, 2003). Flow-like landslides triggered by rainfall occur in most mountainous landscape of the world (Iverson, 2000). They pose a significant natural hazard and they have a high damaged potential (Brenner, 2003). There are many statistically meaningful analysis has been published to demonstrate threshold value of rainfall and landslide triggering (Caine 1980; Keefer et al., 1987; Wilson and Wieczorek 1995; Terlien 1998; Crosta 1998; Pasuto & Silvano 1998; Guzzetti et al., 2000; Crozier 1999; Glade et al., 2000; Wieczorek et al., 2000; Aleotti 2004; Guzzetti et al 2004; Mikos, Cetina & Brilly 2004; Shakoor & Smithmyer 2005; Xie, Esaki & Cai 2005). The first study about rainfall threshold was done by Caine in 1980. In this study, he investigated total 73 shallow landslides of world and by the help of local precipitation records and he postulated a relationship I = 14.82 D-0.39, where I is rainfall intensity (mm/h) and D is rainfall duration (h). But within last few years, there are many works available in relation to threshold rainfall for triggering landslide with hydroclimatic condition, antecedent rainfall (Crozier 1999; Glade, Crozier & Smith 2000) and hydraulic conductivity (Terlien 1998). Keeping view of these contexts, this paper will describe some of scenario of rainfall triggered flow-like landslide occurred in Kathmandu, Nepal and Shikoku, Japan. RAINFALL TRIGGERED FLOW-LIKE LANDSLIDE IN NEPAL The Kingdom of Nepal is sandwiched between Peoples’ Republic of China in the north and India in the south, east and west. Nepal is situated in the center of the long Himalayan concave chain, and is almost rectangular in shape with about 870 km length in the NWW-SEE and 130 to 260 km in N-S direction. The Himalaya has one of the most dynamic and fragile mountain landscapes. It is a live mountain with active tectonics (Figure 1). The seasonal monsoon rains, intense but improper land use practices both for cultivation and construction ensure that the Nepalese Himalayas are among the most unstable landscapes worldwide. The inherently weak geological characteristics of the rocks and soils have made the Himalaya fundamentally a very fragile mountain (Upreti 2001). The Himalaya is very vulnerable to natural and common hazards such as landslides, debris flows, and soil erosion primarily triggered by extensive rainfall of monsoon (Figure 2 and Figure 3). Moreover central Nepal, including Kathmandu, gets high intensity of rainfall than other part of Nepal. The combination of the weak geology and the monsoon climate makes each physiographic zone of Nepal very hazardous. Figure 1. Cross section of Nepal Himalaya, MCT (Main Central Thrust), MBT (Main Boundary Thrust) and MFT (main frontal Thrust) make Himalaya tectonically active and dynamic mountain of world (modified after Upreti and Yoshida, 2005 and Kizaki, 1994). Figure 2. Mean annual precipitation map and highest 24-hour precipitation (mm) Map of Nepal (DHM 2003) Although, Nepal has problem of rainfall triggered landslide, still there are no any significance works to identify the threshold value of rainfall to create shallow failure. The shallow landslide on slope that generally flows down to slope in very high velocity is found to be most devastating landslide in Nepal. Such landslide is fundamentally debris flow (Hungr et al 1984; Coussot and Meunier 1996; Guadango et al 1999, Hungr 1995; Hungr 2000; Hungr et al 2001; Wen and Aydin 2005; Jacob, Hungr &Thomson 1997) and recently very famous as flow like landslide (Dai, Lee & Wang 1999, Iverson, 2000, Hungr et al 2001, Hungr 2003, Dai, Lee & Wang 2003, Jacob and Weatherly, 2003, Malet et al 2004). Some attempt has been done to describe the threshold value of Himalayan flow-like landslides (Caine & 2 IAEG2006 Paper number 819 Mool 1982, Dahal and Kafle 2003, Gabet et al, 2004, Khanal and Watanabe 2005) but it is found to be insufficient because of extremely varied site condition. In Nepal, on an average 260 people lost their lives every year and about 30,000 families affected annually. In 2002, from mid July to late September, 52 districts affected by landslides and floods, 444 people were killed, about 44 were missed, more than 100 were injured, more than 55,000 families were affected (Dahal and Kafle, 2003). This phenomenon is also experienced in monsoon of the year 2003. In the year 2003, landslides triggered by rainfall extensively damaged the Mugling-Narayanghat road. More than 200 landslides are noticed in this road section. In which more than 60% of landslides belong to flow-like nature. Flow-like landslide in southern hills of Kathmandu In Nepal, it is felt that flow-like landslides affect human settlements, bridge and roads by destroying and burying (Figure 3). In the year 2002, many flow-like landslides occurred along southern marginal hills of Kathmandu and damaged many infrastructures.