Study of 24-hour probable maximum precipitation and associated landslide hazards for Hong Kong

Abstract

Probable Maximum Precipitation (PMP), which is an extreme rainfall level widely used for dam and reservoir engineering, has been taken as a practical reference for expressing extreme rainfall events for formulating landslide scenarios in Hong Kong's landslide emergency preparedness. Landslide hazards had been assessed based on scenarios corresponding to 90% of the 24-hour PMP, which was estimated by Hong Kong Observatory in 1999. The results indicated that more than 3,000 landslides affecting buildings or roads would occur if Hong Kong Island is struck by a storm of this level of rainfall. An updating of the 24-hour PMP has recently been carried out using the method of storm transposition. In this method, the major storm associated with Typhoon Morakot, which lashed Taiwan seriously in 2009, was targeted to be transposed to Hong Kong. Based on the Step Duration Orographic Intensification Factor (SDOIF) Method, storm separation technique was applied to separate the convergence component from the orographic component of the major storm and the convergence component was transposed and then combined with the local orographic factors in Hong Kong for PMP estimate. This paper will present the method adopted in the transposition and discuss the results for the updating of the PMP estimate. The assessment of the landslide scenarios corresponding to the updated PMP will also be discussed.