Abstract

MURCIA, Hugo Fernando  and  MACIAS, José Luis. Geological record of recurrent flooding and the October 4, 2005 flooding in the city of Tapachula, Chiapas, Mexico. Rev. mex. cienc. geol [online]. 2009, vol.26, n.1, pp.1-17. ISSN 2007-2902.

On October 4th, 2005, the city of Tapachula suffered the worst disaster in historic times as consequence of intense rainfall associated with hurricane Stan. Flooding occurred as the water level of the Coatán river, which drains the western skirts of the city, gradually increased. The flood destroyed ~2,000 houses, four bridges, and the railroad track. About 100,000 inhabitants were affected, although among inhabitants there were no deceased victims. The rainfall record in the highlands around the Tacaná Volcanic Complex was 143 mm from October 1st to 3rd and almost 60 mm more only on October 4th (242 mm). The cumulative rain on that day reached a return period of 20 years. By that time, the flow discharge of the Coatán river started to flood the alluvial plain and the lower terraces, and to erode the base of the higher terraces producing their collapse in some sectors. Continuous precipitation on the highlands until October 6th reached a return period of 100 years. The intense erosion produced by the flood exposed old flood deposits on the margins of the Coatán river valley. The stratigraphic record on the new uncovered walls showed at least 10 deposits related to water and sediment flow events that took place during the past 680 years, and at least seven of them occurred during the last 105 years. Some of these young deposits contain fragments of plastic, cloth, glass and wire. The stratigraphic record also indicates that the Coatán river in the Tapachula region has maintained its course during at least 1,330 years. In the western skirts of Tapachula, along the Coatán river, important urban settlements are still located on old terraces that were partially flooded in 2005. After the flooding event, an alert system to monitor possible future floods was established, and a man-made channel was constructed.

Keywords : extreme rainfall; flood; disaster; water and sediment flow; hurricane Stan; Tapachula; Chiapas; Mexico.

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