THE ASSESSMENT OF ROCKFALL HAZARD AT THE BASE OF TALUS SLOPES

Fragmental rockfall is characterized by the independent movement of individual rock fragments after detachment from a rock face. The continued operation of the process leads to the accumulation of talus slopes. On talus slopes the rockfall shadow extends beyond the base of the talus and consists of scattered boulders that have run out beyond the base of the slope. The landing probability of boulders in the shadow is examined; return periods of the order of 1000 years relative to a house site are typical. Rockfall behaviour particularly with respect to run out into the shadow can be assessed using geological evidence, empirical methods, physical modelling, and computer-based analytical models. An empirical minimum shadow angle of 27.5-degrees (i.e., the angle between the distal limit of the shadow and the top of the talus slope) is suggested and would be useful in rockfall vulnerability studies ai the base of talus slopes as a first approximation to shadow limits. It is preferable to the use of the rockfall fahrboschung as proposed by several authors. A random collision lumped mass model (ROCKFALL) is outlined. ROCKFALL uses two restitution coefficients and a transition to rolling criterion. ROCKFALL is used to analyse two fatal rockfall accidents in southern British Columbia, at Hedley in 1939 and Sunnybrae in 1983, which are documented in detail. An additional nonfatal incident is also analysed (Barnhartvale in 1974). Results based on an initial calibration were encouraging. Documentation of the three rockfall incidents shows that, in each case, rockfall fragments impacted on homes at equivalent shadow angles of 30' or more. This would suggest that a review of existing development within rockfall shadow areas at the base of talus slopes may be in order.