Rapid appraisal of rainfall threshold and selected landslides in Baguio, Philippines

Among the most landslide prone in the Philippines is the Baguio district, a center of trade, commerce, education and governance. It also has the highest recorded rainfall in the country. Rainfall-induced landslides (RILs) occur yearly and are triggered by rainfall due to southwest monsoon, tropical cyclones and their interactions; and orographic lifting by the Cordillera mountain range. Both natural and human factors contribute to the region's susceptibility to RIL. Fatalities can reach hundreds, and economic damages may reach billions of dollars per event. The rainfall and reported RIL occurrence were compiled for the period 2000-2013. The characteristics and impact of major RIL such as those instigated by Typhoon Parma in October 2009 are highlighted. Most landslides were associated with tropical cyclones, enhanced monsoon flow and 24-h rainfall that ranged from 73 to 1086 mm, corresponding to average rainfall rates of 3-45 mm per hour. The number of daily-reported landslide incidents ranged from 1 to 41. Slides, debris flows and earth flows were observed as the predominant types. As the observed landslide-triggering rainfall is below or within the range of the observed yearly maximum 24-h rainfall, which is 164-1086 mm, RIL will likely continue to occur annually and pose a major challenge for Baguio. The minimum of 70-mm daily rainfall may serve as a threshold for early landslide advisory. Given the limited number of landslides and analysis of landslide occurrence in relation to other controlling/predisposing factors undertaken, the threshold identified can only be regarded as indicative.