Extreme rainfall-induced lahars and dike breaching, 30 November 2006, Mayon Volcano, Philippines

On 29-30 November 2006, heavy rains from Supertyphoon Durian remobilized volcanic debris on the southern and eastern slopes of Mount Mayon, generating major lahars that caused severe loss of life and property in downstream communities. The nearby Legaspi City weather station recorded 495.8 mm of rainfall over 1.5 days at rates as high as 47.5 mm/h, far exceeding the initiation threshold for Mayon lahars. For about 18 h, floods and lahars from the intense and prolonged rainfall overtopped river bends, breaching six dikes through which they created new paths, buried downstream communities in thick, widespread deposits, and caused most of the 1,266 fatalities. In order to mitigate damage from future lahars, the deposits were described and analyzed for clues to their generation and impact on structures and people. Post-disaster maps were generated from raw ASTER and SPOT images, using automated density slicing to characterize lahar deposits, flooded areas, croplands, and urbanized areas. Fieldwork was undertaken to check the accuracy of the maps, especially at the edges of the lahar deposits, and to measure the deposit thicknesses. The Durian event was exceptional in terms of rainfall intensity, but the dikes eventually failed because they were designed and built according to flood specifications, not to withstand major lahars.