A debris-flow early-warning framework incorporating watershed morphometric indices with geology and tectonic history in two fault-controlled mountain ranges in the Philippines

In October and December 2015, two Philippine typhoons triggered debris flows in multiple watersheds in Central Luzon and Mindoro Island. A segment of the Philippine Fault Zone cuts across the base of the Northern Sierra Madre mountain range of Central Luzon and offsets many alluvial fans. The East Mindoro Fault Zone runs the length of the base of the Mindoro Mountain Range, affecting many alluvial fans. Fieldwork after the events characterized the debris flows in those areas. Morphometric indices for each watershed were initially calculated and analyzed using geographic information system (GIS) to develop a tool that could automatically delineate debris-flow susceptible alluvial fans, but limitations discovered by comparing the field data with the calculated indices necessitated scaling back the tool to semi-automation. Ultimately, qualitative factors such as geology, tectonic history, rainfall, and possible anthropogenic triggers must also be considered to properly anticipate debris flows in a watershed. After comparing the two study areas accordingly, a framework incorporating morphometry is being developed to better identify debris-flow prone Philippine alluvial fans.