First North America Landslide Conference – Vail, Colorado, AEG Publication No. 23 Editors: V.R. Schaefer, R.L. Schuster & A.K. Turner pp. 1475-1485 1475 METEOROLOGICAL TRIGGERING FACTORS AND THRESHOLD CONDITIONS FOR SHALLOW LANDSLIDING AND DEBRIS-FLOW ACTIVITY IN ICELAND Saemundsson Th. 1 & Decaulne A. 2, 1 1 Natural Research Center of North-western Iceland, Saudarkrokur, Iceland (nnv@nnv.is) 2 GEOLAB, UMR6042-CNRS, Clermont-Ferrand, France (armelle@nnv.is) Abstract: Shallow landslide and debris-flow activity in Iceland is frequent, often causing significant threats to many small towns and villages around the island. The main threatened areas are located in the eastern, central north and northwestern parts of Iceland. The landscape in these areas is characterized by narrow glacially eroded fjords and steep mountain slopes, often covered by debris talus. The triggering factors and threshold conditions for shallow landslides and debris flows vary in these three areas, mainly due to precipitation distribution. In eastern Iceland, shallow landslides and debris flows occur after long-lasting rainfall or intense rainfall. In central northern Iceland, snowmelt is the main triggering factor, then rain- on-snow, and long-lasting rainfall. In the northwestern part of the island long-lasting rainfall and rain-on-snow are the dominant triggering factors. Measurements of the amount of precipitation needed to trigger a shallow landslide or a debris flow has been documented in northwestern, northern and eastern Iceland to define the threshold conditions. These measurements show, for example, that in the eastern part of the island approximately 100 mm of rain falling over 24 hours is needed, while precipitation lower than 50 mm and/or rapid snowmelt are more suitable triggering factors in the northern and northwestern parts of the country. INTRODUCTION The landscape in northwestern, northern and eastern parts of Iceland display steep slopes with an average height of 6-800 m. The upper parts of mountain slopes are often vertical, with fractured and loose bedrock due to frequent freeze-thaw processes (Saemundsson 1980; Johannesson & Sæmundsson 1989). The lower parts of the slopes are covered with thick talus material, as a result of intense activity on slopes. The climate in Iceland shows contrasted influences of Atlantic and Arctic areas, and is characterized by sudden variations both in temperature and precipitation, which may have direct effects on the occurrence of shallow landslides and debris-flows. With such prerequisites, shallow landslides and debris flows are frequent and widespread in Iceland, directly threatening many towns, villages and farms (figure 1). The international literature is rich of case studies on meteorological control of debris-flows and shallow landslides all over the world, chiefly pointing out the determinant role of intense rainfall (e.g. Rapp, 1964, 1985 and 1995; Johnson and Rahn, 1970; Johnson and Rodine, 1984; Rapp and Nyberg, 1981; Addisson, 1987; Innes, 1989; Luckman, 1992; Becht, 1995; Cannon and Reneau, 2000; Saemundsson et al., 2003). These different studies documented heterogeneous threshold values and conditions for debris-flow triggering, varying from 25 mm during 15 minutes on freshly burned slopes (Cannon and Reneau, 2000) to 175 mm in 3 days falling on water saturated slopes after 3 months of rain (Rapp, 1964). To complement previous research on debris-flow triggers, this paper documents the meteorological aspects that are conducive to slope failures and debris flows in the Icelandic fjords, i.e. in the northwestern, central northern and eastern parts of the island (figure 1). Also, this work defines the triggering threshold conditions by examining the recorded meteorological data nearby the source-area of these slope processes.