INCITAP   20787
INSTITUTO DE CIENCIAS DE LA TIERRA Y AMBIENTALES DE LA PAMPA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Fluvial responses to logjams breakout: aftermaths of the 2008 Chaitén volcano eruption, southern Chile
Autor/es:
UMAZANO, A. M.; MELCHOR, R. N.; BEDATOU, E,; BELLOSI, E. S.; KRAUSE, J.M.
Lugar:
Puerto Madryn
Reunión:
Congreso; 14°Reunión Argentina de Sedimentología; 2014
Institución organizadora:
Asociación Argentina de Sedimentología
Resumen:
The rhyolithic Plinian eruption of the Chilean Chaitén Volcano, initiated on May 2, 2008, suddenly introduced abundant pyroclastic sediments in several fluvial basins including the Blanco River catchment area (Fig. 1), which experienced important modifications. Before May 2, this river was characterized by gravelly and moderate to low-sinuosity channels crossing a vegetated and locally urbanized (Chaitén City) floodplain (Fig. 2). The river had two different configurations along the track: straight-like near the volcano and meandering-like in a more distal position. This river, limited by steep and densely forested highlands, was connected with the Pacific Ocean via a tidally-influenced delta plain. After heavy rains in May 11-20, the river discharge increased and triggered several responses including formation and breakage of several logjams, episodic crevassing, avulsions, channel abandonment (hereafter palaeochannel), changes in the pattern and dimensions of channels, and construction of a new delta plain area (Fig. 2). Particularly, the channels widened, except in those sectors controlled by rock embankments, and acquired a braided configuration. The avulsion point is positioned 7.5 km downstream of the logjam zone, a site where the logjams were concentrated. In this context, the goals of this contribution were: i) to document the syn-eruptive sedimentological processes within a detailed geomorphic framework and ii) to understand the influence of logjams on fluvial dynamics, mainly on both flood and avulsion events. Upstream of the logjam zone, the deposits are mostly composed of reworked ash and lapilli with abundant palaeovolcanic (epiclastic) sediments, which were produced by dilute currents with different sediment concentration, and debris flows. Downstream of the logjam zone, deposits are composed by reworked ash and lapilli, both pumice-rich and lacking important participation of older (epiclastic) sediments. The abandoned and filled palaeochannel, and the proximal part of crevasse splays experienced transient dilute flows with variable sediment-water ratio and, subordinately, hyperconcentrated flows. The distal sectors of crevasse splays mostly record settling from suspension and very scarce dilute flows. At the delta plain, tephra transported by the Blanco River, both in the palaeochannel and the current course was mixed with older sediments by tide and wave action. Logjams (the key elements of disturbances) were produced where floodwaters transporting logs found standing trees near the channel. Logs were stranded against upright trees with the largest axis perpendicular to the current along with large blocks or pebbles. In this way, river discharge was reduced or prevented by obstruction of channels, and sediment and water was trapped in the dam. Devastated forest due to volcanic blasts, burning and subsequent mass wasting phenomena related to heavy rains, in addition to dead wood material lying in the forest, sourced the logjams. Logjam breakage produced a sudden release of water, sediments and logs feeding further overbank flows in the downstream area (crevasse splays), and triggered channel avulsion. We conclude that immediately after eruption, both geomorphic and sedimentary processes of the river were mainly controlled by a combination of high availability of incoherent pyroclastic sediments on steep slopes, abundant rains, large logs that jammed the river and huge areas of devastated forest. Logjams played an important role in the river response to the volcanic eruption; they were responsible of the marked compositional change recorded upstream and downstream of the logjam zone and its breakage resulted in downstream flooding and avulsion. The likelihood of formation of logjams in rivers draining forested volcanic areas should be considered in the evaluation of volcanic hazards related to Plinian eruptions.