Exceptional Bedload Flux Following the 2008 Eruption of Chaitén Volcano, Chile

Abstract:

The 10-day explosive phase of the May 2008 eruption of Chaitén Volcano, Chile, draped adjacent watersheds with a few cm to >1m of tephra. Subsequent lava-dome collapses generated pyroclastic flows that delivered additional channel sediment. During waning phases of explosive activity, modest rainfall (~20 mm in 24 hours; peak intensity ~3–5 mm h^-1) triggered an exceptional sediment flush from multiple watersheds. This flush swiftly aggraded channels by many meters, mostly by fluvial transport; in some basins, earliest transport was by hyperconcentrated-flow lahars. Ten km from the volcano, Chaitén River avulsed through coastal Chaitén town owing to 7m of channel aggradation. That aggradation and delta growth downstream of the abandoned and newly avulsed channels allow estimates of post-disturbance bedload flux. On the basis of pre-eruption bathymetry of Chaitén Bay (by the Chilean Navy) and measurements of delta-surface changes from satellite images, we derived time-series estimates of averaged delta volume. The initial flush from 11–14 May 2008 deposited 1–3 million m³ of sandy sediment at the mouth of Chaitén River. By 26 May, after channel avulsion, a second delta contained about 7 million m³ sediment. Delta volume increased at a logarithmically decreasing rate until late 2011 when it reached about 16 million m³. Subsequently, delta area and volume decreased slightly by wave erosion. The deltas, especially the latter, grew through bedload transport. Sediment particles in the river channel and on deltas are composed of lithic rhyolite and poorly vesicular pumice sand. Channel width through town ranged from 50 to 75 m. Rates of channel aggradation and delta growth, channel width, and an assumed bulk deposit density of 1100–1500 kg/m³ indicate that unit-width bedload flux just before and shortly after avulsion (~14–15 May) was exceptional and perhaps as great as 100–200 kg/m/s. From October 2008 to December 2011, unit-width flux rates declined logarithmically from about 6 to 0.1 kg/m/s.