Abstract

RODRIGUEZ, Sergio R.; MORA-GONZALEZ, Ignacio  and  MURRIETA-HERNANDEZ, José Luis. Low-concentration flows associated with extraordinarily intense rain in the southern slopes of the Pico de Orizaba volcano (Citlaltépetl), Mexico. Bol. Soc. Geol. Mex [online]. 2006, vol.58, n.2, pp.223-236. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2006v58n2a5.

During June 2 to 4 2003, heavy rainstorms struck the southern flanks of the Pico de Orizaba volcano. As a result of this extreme climatic event, on June 5 water and sediment stream flows descended along the several rivers beds that constitute the hydrologic network of the southern flank of the volcano. The largest stream flow was channeled in one of the rivers beds which begin at an altitude of 4,200 m a.s.l. In the highest zones this riverbed or gully is called “barranca Infiernillo”, in the middle “barranca del Muerto” and in the lowest zones over the valley is called Chiquito river.

The damages caused by the stream flow were of high consequences, specially at the low lands, where floods covered partially the cities of Nogales and Mendoza. However, the major damages were associated with the rupture of a gas pipe line due to the impact of the water. This led to a catastrophic scenario in the village of Balastrera, where human lives were lost and several persons resulted with severe injures, as a consequence of the gas expansion and later ignition.

The source of the debris and sediments incorporated in the stream flows is located in the high lands over the 3,000 m a.s.l., with contribution from the pyroclastic and epiclastic deposits related with the volcanic activity of the Pico de Orizaba and Sierra Negra volcanoes. These deposits constitute an important source for the potential generation of debris flows and lahars.

The sedimentology and grain size characteristics of the deposits produced by the stream flow of June 5 change along the course of the river. The coarser fractions (> 0 phi) were concentrated between the 2,400 and 3,000 m a.s.l., the finer ones (< 0 phi) were transported by the stream flow producing non cohesive deposits over the food plains on the urban zones.

An estimated population of 500,000 is living on the Orizaba-Nogales-Ciudad Mendoza area, which is also occupied by a complex infrastructure consisting of oil and gas pipe lines, high voltage electricity lines, highways and railroad. These reasons point out to the importance of making hazards and risk studies in order to mitigate the consequences of future debris flows.

Keywords : Normal streamflows; Pico de Orizaba volcano; Citlaltépetl.