Pore pressure distribution in the initiation area of a granular debris flow R. GENEVOIS, Department of Geology, Padova University, Padova, P.R. TECCA, Research Institute for Hydrological and Geological Hazard Prevention, Padova, M. BERTI, and A. SIMONI, Department of Earth Sciences, Bologna University, Bologna, Italy INTRODUCTION A critical combination of hydrologic factors and material characteristics is required for debris flow initiation. The destabilising effects of the ground water flow is account for using either pore pressure distribution or seepage body forces. However, hillslope water table and groundwater flow fields, following infiltration processes during rainfall, must be properly modelled to assess failure conditions and the role of pore pressures and granular temperatures in the behaviour of the debris mass. Numerical methods for hydrological modelling of hillslope water tables are available, and they are often based on one-dimensional, vertical, transient, unsaturated finite-difference infiltration model linked to a kinematic wave equation (e.g., Buchanan et al., 1990). However, nonuniform pore pressures and seepage forces distributions may be generated by even small contrasts (one order of magnitude or less) in the hydraulic conductivity of soils modifying effective stresses ratios and increasing the failure potential especially in purely frictional soils. Common worst-case assumptions (slope-parallel ground-water flow or hydrostatic pore pressure distribution) can not account for these conditions and do not provide conservative results. As a consequence of the complexity of real groundwater flow fields, destabilising pore pressures development in connection with rainfall infiltration processes, can be detected only by continuous monitoring systems and experimental measurements in the debris flow initiation zones are of the outmost importance to define granular flow occurrence and failure mechanisms. Relations between rainfalls and pore pressure during failure of soils have been investigated in several automated monitored sites around the world, some devoted to debris flow susceptibility studies: USA, (Pierson, 1980; Keefer and Johnson, 1983; Wieczorek, 1987; Wieczorek and Sarmiento, 1988; Tanaka et al., 1989; Harp et al., 1990; Johnson and Sitar, 1990;Wilson and Wieczorek, 1995; Anderson and Thallapally, 1996), British Columbia (Fannin et al., 1997). This study has been developed in the contest of debris flow studies and monitoring activities at Acquabona Creek, near Cortina d’Ampezzo, Eastern Italian Alps, where event occur yearly. Data of pore pressure response to rainfall not triggering and triggering debris flows are analysed in this paper, allowing some preliminary considerations on initiation mechanisms. GEOLOGICAL AND GEOMORPHOLOGICAL SETTING The Acquabona catchment is located on the left side of Boite River valley (Eastern Dolomites). The climatic conditions are typical of the Alpine environment. Annual precipitation ranges from 900 to 1500 mm, occurring as snowfall from November to May; intense localised thunderstorms are common in summer and early autumn. The upper rock basin, closed at onset of the flow channel, consists of massive dolomite and limestone cliffs. A very thick talus covers the slope from the rock cliffs base. Talus deposits include heterogeneous scree, eluvial and debris flow deposits and consist of poorly sorted