A multi-method monitoring of timing, magnitude and origin of rockfall activity in the Japanese Alps

Abstract

A multi-method monitoring was conducted to detect the timing and trigger of rockfall activity on an alpine rockslide cliff composed of Cretaceous sandstone and shale in the southern Japanese Alps (Aresawa rockslide, 2900 m above sea level). The monitoring programme includes manual measurements of flaking from painted rock surface and collection of fallen debris, daily time-lapse imaging of the rockwall, automated recordings of rock temperature, rock moisture and influencing meteorological factors (air temperature and precipitation). A stereographic pair of sequential photographs is used to visually identify the location of new erosion and semi-quantitative evaluation of detached materials at daily resolution. Six years (2010–2016) of debris trapping show major rockfall activity in winter (between November and May) and occasional activity associated with heavy rains in summer, and yield an average rate of rockwall retreat on the order of 1 mm a−1. The rockwall shows heterogeneous debris production mainly reflecting joint spacing. Time-lapse imaging displays at least 10–15 rockfall events per year within the shot area (~500 m2). The integration of multiple data suggests at least five types of triggering processes towards rockfalls, which recur annually. (1) In summer and early autumn, heavy rainfalls raise the moisture content of the shallow bedrock, often triggering significant rockfalls, probably due to raised water pressure in rock joints or lubrication of joints. (2) In late autumn and late spring, light or intermediate rainfalls are sometimes followed by high moisture, shallow freezing, rapid thawing and eventually by different scales of rockfalls. (3) In winter and early spring, the same process as in the second case, but rainfall is replaced by snowfall, triggers submeter-scale rockfalls. (4) Deep seasonal freezing to about 5 m enables release of meter-scale rockfalls during seasonal thawing periods from late spring to early summer. (5) Short-term large thermal fluctuations that produce cyclic thermal stress may also contribute to submeter-scale rockfalls, mainly in winter locally enhanced by freeze-thaw action.