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.
Rivers draining the flanks of Mount Pinatubo, Philippines, were blanketed in pyroclastic flow and ash deposits during the June 1991 eruption, leading to record high sediment yields on rivers draining the flanks of the volcano. Although most sediment was moved initially by lahars, rivers transported high sediment loads even at low flow through the development of smooth, highly-mobile sand beds. Rivers are evolving back into gravel-bedded channels as sand-rich pyroclastic flow deposits are stabilized or removed, leading to bed coarsening and decreased sediment mobility. This investigation focused on the coupling and feedbacks between decreasing sand supply, bed development, and sediment transport on rivers draining the flanks of Mount Pinatubo. It continued monitoring on rivers initially conducted from 1996 through 2003, extending the record out 20 years after the cataclysmic event. Most of the research focused on the Pasig-Potrero and Sacobia Rivers on the east side of the volcano. Few studies have investigated long-term impacts of volcanic eruptions on rivers. Mount Pinatubo was the 2nd largest eruption of the 20th century, but there is little long-term monitoring taking place there, particularly when compared with the wealth of research conducted on Mount St. Helens in the wake of the 1980 eruption. Results from Mount St. Helens indicate that although high sediment yields on the hardest hit rivers fell exponentially for several years, they failed to recover to pre-eruption rates and have remained 1-2 orders of magnitude higher than pre-eruption rates. Our work at Pinatubo shows a similar pattern, with an exponential decline in sediment yield for the first decade, followed by more stable, but elevated sediment yields in the second decade. For communities living within volcanic hazard zones, having a better idea of the timescale associated with river recovery will help in long-term disaster mitigation planning. One primary driver of channel evolution that is being closely monitored is the loss of sand in the system as sand-rich pyroclastic deposits are eroded and transported downstream. High sand content has been shown to increase bedload transport rates of both sand and gravel by orders of magnitude. Sand loading is not unique to volcanically-impacted channels. Many mountain rivers are subjected to episodic landsliding, and sand loading is a common concern on highly-modified channels in the wake of dam removals. Our findings show that sand content on the bed has a strong, nonlinear effect on bedload transport rates. During the dry season, when precipitation-driven inputs of sand are reduced, the bed incises and armors, dropping bedload transport rates by several orders of magnitude. In 2009-10, this led to a 2-3 meter deep valley being carved in a 4 month period following the end of the rainy season. Modeling and observations suggest that sand loading due to lateral migration into sand-rich banks may increase the depth of incision as added sand from the banks breaks up the armor layer, mobilizes sediment, and leads to further incision. Thus, sand loading to a gravel-bedded reach can actually lead to incision, rather than aggradation. Our research has also focused on the role of vegetation in stabilizing highly mobile channels. Vegetation was long absent on the valley bottom of the hardest hit rivers at Pinatubo. Any vegetation that grew during the dry season on the valley floor was wiped out by sediment-rich flows during the rainy season. Sediment transport and aggradation rates have dropped enough now that vegetation can last through the rainy season. Vegetation is starting to affect channel dynamics by controlling the location of major channels and thus the focus of aggradation. This has led to the development of a 1% lateral slope on the valley bottom as the main channels avoided the vegetated half of the valley bottom leading to higher deposition rates on the unvegetated side.
