USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Watershed Disturbance and Lahars on the East Side of Mount Pinatubo During the
mid-June 1991 Eruptions
Jon J. Major, Richard J. Janda, and Arturo S. Daag, 1996,
Watershed Disturbance and Lahars on the East Side of Mount Pinatubo During the
mid-June 1991 Eruptions:
IN:
Christopher G. Newhall and
Raymundo S. Punongbayan (eds.), 1996,
Fire and Mud -
Eruptions and Lahars of Mount Pinatubo, Philippines:
Philippine Institute of Volcanology and Seismology and the University of
Washington Press, 1126p.
Abstract
Densely populated alluvial fans on the east side of Mount Pinatubo were affected by widespread lahars triggered during the June 15,
1991, eruptions. The lahars were triggered by heavy rainfall. However, the lahars were not generated because of uncommonly heavy
precipitation, but rather because of radical alteration of watershed hydrology by volcanic deposits in conjunction with heavy rainfall.
Fine-grained fall and surge deposits related to eruptions that preceded the climactic-phase activity damaged vegetation, reduced the
infiltration capacity of hillslope surfaces, and smoothed the natural-scale hillslope roughness. These effects led to enhanced overland
flow that instigated hill-slope and channel erosion, triggered minor slope failures, and initiated the peak-discharge lahars. Sediment
mobilized by rilling and shallow landsliding of the mantle of pumice tephra deposited by the climactic-phase eruption contributed to
lahars interbedded with pyroclastic valley fill and to pumice-bearing recessional flow that followed peak-discharge.
The peak-discharge lahars that flooded fan channels in Tarlac and Pampanga Provinces varied in rheology, magnitude, and timing. Below
fanheads, lahars were dominantly hyperconcentrated streamflow or flow transitional to hyperconcentrated streamflow; above fanheads,
lahars were dominantly debris flow. Along the mountain front from the Gumain River to the Sacobia River, peak discharge preceded
deposition of broadly distributed plinian pumice fall; between the Sacobia River and the O'Donnell River, peak discharge largely followed
pumice fall. Variations in rheology, magnitude, and timing of peak flows reflect the interplay of variations in rainfall intensity and timing,
variations in the degree of watershed disturbance, and characteristic response times of the watersheds. Interplay of these factors affects
the amount of water that moves from hillslopes to channels to fanheads, how rapidly it moves, and how easily sediment is entrained and
transported.
Damage to habitation and infrastructure on the alluvial fans resulted primarily from lateral bank erosion and from aggradation of
mainstem channels that induced backflooding of tributary channels. Overbank flooding by lahars was generally localized and of secondary
importance except along distal alluvial plains, where primarily agricultural land was inundated.
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06/06/02, Lyn Topinka