Rockfall hazard analysis for Hong Kong based on rockfall inventory

This paper compiles and analyzes the rockfall data in Hong Kong in the last fifty years. A simple rockfall hazard analysis is presented based on this rockfall inventory. A frequency-magnitude relation, which is analogous to the Gutenberg-Richter relation for earthquake occurrence, is proposed for rockfall, and direct correlation between rockfall frequency and the daily rainfall is observed. Data analysis shows that a threshold daily rainfall of about 150-200 mm is expected in order to trigger rockfall events in Hong Kong. Among the 368 rockfall events in the 13 year period from 1984 to 1996 in Hong Kong, 35% of the incidents lead to blockage of or damage to roads, 22% lead to damages or evacuation of squatter huts, 21% lead to blockage of pedestrian pavement and footpath, and 15% affect buildings, such as housing apartments and schools. Only 15% of these rockfalls fell onto open space and caused negligible effects on human activities. Most of these rockfall events occurred during heavy rain and when "landslide warning" should has been issued by the Hong Kong Observatory, thus only 6% of these events led to injury or casualty, car damages, and damages to public utilities. Rockfall hazard zonation maps in terms of the spatial distribution of previous rockfalls are proposed for both Kowloon Peninsula and Hong Kong Island.