Olokele rock avalanche, island of Kauai, Hawaii

In October 1981 a mass of rock and soil having an estimated volume of 500,000 m³ fell as a rock fall-avalanche from a steep slope 800 m high near the head of Olokele Canyon on the island of Kauai, Hawaii. Boulders were launched into the air from a bench on the slope for a downstream distance of about 850 m. The velocity of the avalanche was rapidly diminished by impact against the valley sides, and it became a muddy debris flow that traveled 4.6 km downstream, severely eroding the valley sides. The volume of debris deposited on the canyon bottom is estimated to be 2,500,000 m³, or about four times the volume derived from the slope. Slope failure is attributed to gradual undercutting and loss of shear strength in beds of weathered ash and to joints in a thick basaltic cap rock.