The Story of Quantified Risk and its Place in Slope Safety Policy in Hong Kong

Abstract

Each summer rainstorms trigger many landslides in Hong Kong and slope safety is a matter of some public concern. The slope problems of this former British colony and, since 1997, Special Administrative Region of China are the product of adverse terrain, severe climatic conditions, restricted land supply and phenomenal population growth in the past 50 years. Pressure on land is acute, with 7 million people occupying an area of about 400 km 2 , much of which is developed hillside. Rainfall in the urban areas ranges annually from about 1 m to 3.4 m and produces each year between about 80 and 800 reported ‘landslides’ (slides, flows, falls, retaining wall collapses, etc.). Nearly all are failures in man-made slopes – in cuttings and to a lesser extent in hillside embankments (‘fill slopes’) and retaining walls. In this chapter, the word ‘slope’ generally means ‘manmade slope’. About 90% of the reported failures yield less than 50 m 3 of debris but many of these small collapses are still quite risky, being mobile and occurring next to roads and buildings. The degree of risk is indicated in the fatality figures. In the 15 years to 2002 15 people were killed by landslides; but more than 450 people had been killed in the previous 35 years. This improvement, achieved despite population growth of about 1 million per decade, is the product of a control regime introduced progressively since the 1970s to satisfy society’s rising demand for safety. Hong Kong’s population started to grow rapidly after the Second World War. Civil war in China led to mass immigration from the north, creating an acute housing shortage