5:15 PM - 7:15 PM
[AHW29-P05] Post-event assessment of the debris flow in Batangas Province, Philippines, triggered by Tropical Cyclone Trami on October 24, 2024
Keywords:Debris flow, Flash flood, Surface Runoff
The passage of Tropical Cyclone Trami (Local name: Severe Tropical Storm Kristine) brought heavy rainfall in the municipality of Talisay, Batangas province in the Philippines on October 24, 2024, triggering the debris flows that occurred in the western part of the municipality-the first ever recorded in the area. In the wake of the incident, a post-event assessment was conducted to analyze the factors that contributed to the occurrence of the debris flows and document the impacts of Trami in the area. Analysis of rain gauge data revealed that from midnight to 2 o’clock in the afternoon (local time, when the debris flows were reported to occur), the tropical cyclone dumped 255 mm of rainfall. More intense rains falling at an average of 41.5 mm/hr was experienced about 6 hours before the debris flow was observed to occur in the “ground zero” which is located in the westernmost portion of the municipality. The topography of the impacted areas is characterized by the steep slopes of the Tagaytay Ridge which face south toward the Taal Lake, a volcanic crater lake housing the most active volcano in the country. This, together with substantial surface runoff and river discharge in the small sub-catchments of the affected areas, facilitated the debris flows and flash floods that occurred. Accounts from residents revealed that the front of the debris flow carried large boulders and logs and was accompanied by a high-velocity runoff that lasted until early evening. The debris flows traveled generally southeast and reached the lakeshore, where high waves caused by the storm also impacted the local communities. Towards the eastern portion of the municipality, where a more gently sloping terrain is found, flash floods caused by surface runoff and rapid rise in river waters were reported to occur. Here, a number of lakeshore communities also experienced large waves that inundated and washed out several houses.