Abstract
Abstract. Pore pressure is crucial in triggering debris slides and flows. Here we present measurements of groundwater 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 4 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. Precipitation totaled about 80–100 mm in 24 h, locally up to 129 mm, and an additional trigger factor for the landslides was a rapid rise in air temperature that caused snowmelt. In the studied slope a fraction of the precipitation first fell as snow. On 15 November, the groundwater level in the hillslope rose by 10 cm/h 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 landslide to be triggered for a short time – about 4–5 h.
Highlights
It is well known that groundwater pore pressure is crucial in triggering shallow debris slides and flows (e.g., Iverson, 1997), but how exactly does pore pressure vary in a hillslope during a rainstorm? How much and how rapidly does the groundwater level rise before a landslide is triggered? And, for how long during a rainstorm is the slope landslideprone? We try to answer these questions using groundwater level data logged by an automated piezometer installed in a borehole on a hillslope in western Norway, where a debris flow occurred during heavy rainfall and snowmelt in November 2013
The flow split into three different paths (Fig. 2b) (Olsen et al, 2015) and ended in the lake (439 m altitude)
The strong warm front in the storm gave rise to a rapid temperature increase of 8–9 ◦C, initiating snowmelt that supplemented the rainfall and led to a rapid rise in groundwater. This situation triggered over 100 landslides in western Norway
Summary
It is well known that groundwater pore pressure is crucial in triggering shallow debris slides and flows (e.g., Iverson, 1997), but how exactly does pore pressure vary in a hillslope during a rainstorm? How much and how rapidly does the groundwater level rise before a landslide is triggered? And, for how long during a rainstorm is the slope landslideprone? We try to answer these questions using groundwater level data logged by an automated piezometer installed in a borehole on a hillslope in western Norway, where a debris flow occurred during heavy rainfall and snowmelt in November 2013.Such instrumental data are rarely provided during rapid landslide events. We try to answer these questions using groundwater level data logged by an automated piezometer installed in a borehole on a hillslope in western Norway, where a debris flow occurred during heavy rainfall and snowmelt in November 2013. It is well known that groundwater pore pressure is crucial in triggering shallow debris slides and flows (e.g., Iverson, 1997), but how exactly does pore pressure vary in a hillslope during a rainstorm? Such instrumental data are rarely provided during rapid landslide events. The data include the day when a debris flow occurred in this particular slope during the storm named Hilde on 15–16 November 2013
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