| 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. -- J.J. Major, 1996 |
