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The 26 May 1982 breakout flows derived from failure of a volcanic dam at El Chichón, Chiapas, Mexico

J.L. Macías^,1, L. Capra^,2, K.M. Scott^,3, J.M. Espíndola^,4, A. García-Palomo^,5 and J.E. Costa^,6

¹ Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, México, D.F., México
² Instituto de Geografía, Universidad Nacional Autónoma de México, Coyoacán 04510, México, D.F., México
³ Cascades Volcano Observatory, U.S. Geological Survey, 1300 southeast Cardinal Court, Building 10, Suite 100, Vancouver, Washington 98683, USA
⁴ Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, México, D.F., México
⁵ Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán 04510, México, D.F., México
⁶ Cascades Volcano Observatory, U.S. Geological Survey, 1300 southeast Cardinal Court, Building 10, Suite 100, Vancouver, Washington 98683, USA

The eruptions of El Chichón between 28 March and 4 April 1982 produced a variety of pyroclastic deposits. The climactic phase, on 3 April at 07:35 (4 April at 01:35 GMT), destroyed the central andesitic dome and fed pyroclastic surges and flows that dammed nearby drainages, including the Magdalena River. By late April, a lake had formed, 4 km long and 300–400 m wide, containing a volume of 26 x 10⁶ m³ of hot water. At 01:30 on 26 May, the pyroclastic dam was breached and surges of sediment and hot water soon inundated the town of Ostuacán, 10 km downstream. This hot flood was finally contained at Peñitas Hydroelectric Dam, 35 km downstream, where one fatality occurred and three workers were badly scalded.

Stratigraphic and sedimentologic evidence indicates that the rapidly draining lake initially discharged two debris flows, followed by five smaller debris flows and water surges. The main debris flows became diluted with distance, and by the time they reached Ostuacán, they merged into a single hyperconcentrated flow with a sediment concentration of 30 vol%. Deposits from this hyperconcentrated flow were emplaced for 15 km, as far as the confluence with another river, the Mas-Pac, below which the flow was diluted to sediment-laden streamflow. The minimum volume of the breakout-flow deposits is estimated at 17 x 10⁶ m³. From high-water marks, flow profiles, and simulations utilizing the DAMBRK code from the National Weather Service, we calculated a maximum peak discharge of 11,000 m³/s at the breach; this maximum peak discharge occurred 1 h after initial breaching. The calculations indicated that 2 h were required to drain the lake.

Key Words: volcanic dam • breakout • debris flow • Chichón • Chiapas • Mexico

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