Rain-induced shallow landslide is the major type of landslide that happens on natural terrain in Hong Kong due to its high seasonal rainfall and deep weathering soil profile. The Geotechnical Engineering Office has been in a leading role to steer the risk management of natural terrain landslides in Hong Kong. Recently, the territory-wide rainfall-based landslide susceptibility model (in terms of landslide density per year) has been developed to predict the number of natural terrain landslides that may occur in an anticipated rainfall event. Subsequently, the storm-based landslide density has been transformed to the annual landslide frequency to compile the territory-wide landslide frequency map by incorporating the mean annual frequency of occurrence of different probable rainfall scenarios. The annual rainfall frequency of a particular rainfall scenario is derived from its return period, determined based on the abundant real-time rainfall data at a five-minute interval from 110 automatic raingauges across Hong Kong at an average density of 10 km2 /gauge. The transformation is discussed in this paper, together with the evaluation of the performance of the landslide frequency map. In addition, the potential applications of the map in Hong Kong and the pitfalls of the common evaluation methods are highlighted.

Introduction

“Natural terrain” is commonly used in Hong Kong to denote hillsides that are not significantly modified by human activities. On average, about 300 natural terrain landslides occur every year in Hong Kong and rain-induced shallow landslide is the major type of landslide due to the high seasonal rainfall and deep weathering soil profile. Heavy rainstorms in Hong Kong generally occur during the wet season from April to September and are associated with either low pressure trough or severe tropical cyclones. As a general reference, the mean annual rainfall at the Hong Kong Observatory (HKO)’s Principal Raingauge was 2398.5 mm between 1981 and 2010. Landslides from natural terrain can result in serious consequence to life and property in Hong Kong due to its dense urban development on hillsides. The problem is chronic for the impacts of extreme weather events, slope degradation and ever-increasing population and encroachment of new urban development on steep hillsides. Over the past 35 years, the Geotechnical Engineering Office (GEO) has been in a leading role to steer the risk management of natural terrain landslides in Hong Kong to reduce and contain landslide risk within an as low as reasonably practicable level that meets the needs of the public and facilitates safe and sustainable developments. The slope engineering practice and landslide risk management have evolved in response to experience and through continuous improvement initiatives and technology advances (Wong & Ko 2006).