LAUSR

Abstract. Pore pressure is crucial in triggering debris slides and flows. Here we present measurements of ground water pore pressure and temperature recorded by a piezometer 1.6 m below the surface on a slope susceptible to debris flows in Western Norway. One of the largest oscillations in data collected over four years coincided with a debris flow event on the slope that occurred during storm Hilde on 15–16 November 2013. More than 100 landslides were registered during the storm. Rainfall totalled about 80–100 mm in 24 hours, locally up to 129 mm, and an additional trigger factor for the slides was a rapid rise in air temperature that caused snowmelt. On 15 November, the groundwater level in the hillslope rose by 10 cm per hour and reached 44 cm below the surface. At the same time, air temperature rose from 0 °C to over 8 °C, and the groundwater temperature dropped by 1.5 °C. The debris flow probably occurred late in the evening of 15 November, when the groundwater level reached its peak. Measurements of the groundwater in the hillslope in the period 2010–2013 show that the event in 2013 was not exceptional. Storm Dagmar on 25–26 December 2011 caused a similar rise in groundwater level, but did not trigger any failures. The data suggest that during heavy rainstorms the slope is in a critical state for a slide to be triggered for a short time – about 4–5 hours.