Lahar initiation and sediment yield in the Pasig-Potrero River Basin, Mount Pinatubo, Philippines.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A systemic basin-wide consideration of the complex interaction of many hydrogeomorphic and volcanic processes involved in lahar initiation emphasizes the role of rill erosion, both on tephra-covered hill slopes and on the valley-filling pyroclastic-flow deposits. Rill erosion is found to be as important as erosion in the main river channel in terms of sediment contribution. In addition, erosion is fed by mass-wasting processes that are profoundly affected by secondary explosions. The observed decrease in lahar activity is inferred to be related to the depletion of the pyroclastic deposits, which in turn has affected the frequency of secondary explosions, and allowed the development of relatively stable river channels that are in synchrony with their tributary rills and gullies. The decrease in secondary explosion activity has also allowed hill slope recovery that has resulted in reduced rill erosion. Rainfall-normalized sediment yield and the lahar-triggering rainfall threshold in the Pasig-Potrero River are analyzed using near-complete data from rain gauges and acoustic flow monitors (AFM), with AFM data as proxy for lahar hydrographs. Data for coincident lahars and rainstorms show the river's hydrologic response to two major watershed disturbances: the emplacement of 300 million m3 of pumiceous pyroclastic debris during the June 1991 eruption, and the capture of the upper Sacobia catchment in October 1993. Sediment yield peaked immediately after each major watershed disturbance, followed by a non-linear decrease through time. The systematic decline is inferred to be related to the depletion of the source sediments and the development of the drainage network, particularly the decrease in drainage density and increase in channel width. These parameters influenced the volume of pumiceous sediments in contact with runoff. The rainfall threshold at which lahars are generated was found to remain at about the same low level until 1995, after which it increased progressively until 1997. No major lahar has been observed or detected by the AFM since then. The triggering rainfall is postulated to be a function of the erodibility and infiltration capacity of the surface, which together control the amount of runoff and entrainable sediments. These factors in turn are largely controlled by the areal distribution of pumiceous 1991 pyroclastic deposits, including both flow and fall deposits. Secondary explosions have played a key role in this regard by intermittently blanketing large areas with tephra, therefore momentarily negating the effects of vegetation during the rainy season. Thus, the triggering rainfall increased only when erosion has reached the pre-eruption surface and secondary explosions have decreased substantially. The trends in laharic sediment yield and lahar-triggering rainfall together demonstrate the progressively increasing rainfall requirement for lahar generation through time. Cautious extrapolation of the data suggests that to trigger and sustain a modest lahar (<10 million m3) now requires a rainstorm with a return period exceeding 100 years. Lahars have also been initiated by the sudden failure of temporary lakes that have been formed by the blockage of tributaries by lahar or pyroclastic-flow deposits. Many such pyroclastic-flow or lahar dams have been observed to repeatedly form and breach, often resulting in catastrophic lahars. A few, however, have survived for many years and may pose lahar hazards in the future. Four conceivable mechanisms for the failure of pyroclastic dams are considered: (1) erosion of the dam by flows along the lahar channel; (2) gravitational collapse and/or piping; (3) lake overtopping; and (4) secondary explosions. Analyses show that the blockages are naturally susceptible to overtopping failure through breach erosion. However, material lost to bed erosion along the breach-channel may be initially compensated for by sediments delivered onto the dam by lahars or local runoff. Once sediment supply to the dam is cut off, e.g., due to the depletion of source sediments, by revegetation, or by stream beheading, the dam is predicted to fail by breach erosion. It can thus be expected that, barring engineering intervention, all pyroclastic-flow and lahar dams around Mount Pinatubo will eventually fail, as suggested by their absence before the 1991 eruptions.