Watershed and Channel Recovery Processes Following Extreme Sediment LoadingEPA Grant Number: U915964
Title: Watershed and Channel Recovery Processes Following Extreme Sediment Loading
Investigators: Gran, Karen B.
Institution: University of Washington
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 2001 through January 1, 2006
Project Amount: $91,567
RFA: STAR Graduate Fellowships (2001) RFA Text | Recipients Lists
Research Category: Fellowship - Geology , Academic Fellowships , Ecological Indicators/Assessment/Restoration
Following the 1991 eruption of Mount Pinatubo, sediment yields in rivers draining the flanks of the volcano were the highest ever recorded. The mechanics of how channels respond to extreme sediment loading and adjust as sediment yields decline are not well understood, primarily because most sediment transport studies focus on streams with a low sediment supply relative to transport capacity. Although the bulk of the sediment at Pinatubo has been removed from the system through lahars, substantial low-flow sediment transport still occurs. Through time, sediment yields are declining, and as they decline, the channel bed should coarsen, bedload transport rates should decline, and the channel profile should reflect these changes. The objective of this research project is to study five basins that experienced varying amounts of sediment loading at Mount Pinatubo to test the following hypotheses: (1) bed coarsening and armoring are occurring in response to a decrease in sediment supply and will lead to changes in bedload transport; (2) recovery rates will vary depending on the initial amount of sediment loading; and (3) recovery on the bed will be reflected through changes in longitudinal profile concavity.
On each of the five rivers, I will conduct cross-sectional surveys on at least two reaches upstream of the alluvial fanhead. Longitudinal bed profile and grain size distribution analyses will be conducted only on the three basins with the greatest sediment loading: the Pasig-Potrero, Sacobia, and O’Donnell. Sediment budget and grain mobility studies will be limited to the Pasig-Potrero and Sacobia. Bedload and suspended load measurements will be conducted on the Pasig-Potrero only. The rationale behind these decisions is based primarily on the desire to concentrate my efforts on basins that suffered the greatest amount of sediment loading and for which I have baseline data from 1996-2000. Changes already documented led to the prediction that the next several years will be pivotal for documenting mechanisms of channel recovery at the Pasig-Potrero. I plan to continue this research project for the next 3 years, and while I cannot guarantee that I will capture significant recovery of the Pasig-Potrero and other Pinatubo channels, the next several years may be the best time to try. Simple confirmation and extension of the record of ongoing changes already documented will test the hypotheses identified above and enhance our understanding of how fluvial systems recover from extreme sediment loading. Increased knowledge of sediment transport under conditions of relatively high sediment supply is important, not only in volcanic hazard zones such as Mount Pinatubo, but also in nonvolcanic systems where sediment supply is locally increased because of dam removal, catastrophic floods, intensive logging, mining operations, or other processes that transport excess sediment to stream channels.