Decadal-scale channel evolution at Mount Pinatubo, Philippines

University Of Minnesota Duluth, Duluth MN

Investigators

Abstract

Rivers draining the flanks of Mount Pinatubo, Philippines, were blanketed in pyroclastic flow and ash deposits during the 1991 eruption, which was the second largest of the twentieth century. These deposits erode easily in the tropical climate and upland vegetation recovers quickly, making landscape recovery both dramatic and fast. As sand-rich pyroclastic flow deposits are stabilized or removed, rivers are evolving back into gravel-bedded channels, leading to bed coarsening and decreased sediment mobility. This research will investigate coupling and feedbacks between decreasing sand supply, bed development, and sediment transport during the last two decades at Mount Pinatubo, by continuing monitoring conducted from 1996 through 2003. River cross-sections will be reoccupied, long profiles will be resurveyed, and bedload and suspended load transport on the Pasig-Potrero River will be measured again. Monitoring will be extended to include lateral migration rates of braid channels and vegetation density in the braidplain to assess the current role of riparian vegetation on channel dynamics. As riparian vegetation density increases, it may become a primary driver of channel planform and dynamics. Sand loading is not unique to volcanically-impacted channels. Understanding how excess sand is stabilized or removed from a watershed and how the channel bed and sediment transport regime respond are also important in tectonically-active areas with episodic landsliding or highly-modified channels in the wake of dam removals. Being able to predict the time-scale for fluvial recovery could help with planning for long-term disaster relief efforts in the wake of events like the 1991 eruption. Persistent high sediment yields contribute to increased flood hazards and water quality concerns and may displace communities for years. Beyond volcanic environments, understanding how sediment transport evolves in systems experiencing sand-loading could help determine the effectiveness of future dam removals and stream restoration efforts to restore aquatic ecosystems in severely-impacted watersheds.