Alluvial fan flooding in Pocito Department. San Juan Province. Argentina.

PERUCCA, L. Y PAREDES, J.

EPISODES

International Union of Geological Sciences

Lugar: Beijing; Año: 2004 vol. 27 p. 190 - 194

0705-3797

The study consists of the identification of landforms subject to alluvial fan flooding in active sectors of the Zonda range piedmont. In the Department of Pocito, located about 5 km southwest of San Juan City, a series of alluvial fans have been identified. These alluvial fans are located downstream of the natural drainage basins covering an area of approximately 130 km2 towards the eastern slopes of the Zonda range at a median elevation of 2,000 m a.s.l. The region is predominately arid, with an annual average precipitation of approximately 120 mm. Torrential rains during summer can cause violent floods characterized by high transport speeds and relatively shallow depths. These floods can also transport a great volume of sediments and debris in a short period of time. Rainfalls usually have a short duration (less than 1 hour), a high intensity, and a reduced coveragearea. This phenomenon is aggravated when the infiltration index is reduced further due to previous rains that saturate the area. The torrents generated in the high sloping mountainous region erode the existing rocks, carrying loose materials from the outcrops. As most of the piedmont is practically barren of vegetation, runoff becomes very rapid. The extent of active channels on the Zonda piedmont has been identified from geomorphologic studies of aerial photographs and land topography. The active areas are located at lower elevations relative to the adjacent older piedmont surfaces. Within the floodplain area, where the urban and rural areas are located, the majority of the historical records of floods have been confined to the location of 13th Street and 15th Street, both oriented parallel to the slope down from the piedmont, on a west-east trend. During most rainfalls these streets have been transformed  into natural riverbeds, influencing the path of the floods andlimiting their damage to civil structures. As a result of this, other streets and a few irrigation channels laying a north-south trend have been seriously affected. The development of a predictive and early warning system, combined with public education and structural corrective measures are thought to be the means to minimize the hazard that alluvial fans floods pose over the Department of Pocito.2 towards the eastern slopes of the Zonda range at a median elevation of 2,000 m a.s.l. The region is predominately arid, with an annual average precipitation of approximately 120 mm. Torrential rains during summer can cause violent floods characterized by high transport speeds and relatively shallow depths. These floods can also transport a great volume of sediments and debris in a short period of time. Rainfalls usually have a short duration (less than 1 hour), a high intensity, and a reduced coveragearea. This phenomenon is aggravated when the infiltration index is reduced further due to previous rains that saturate the area. The torrents generated in the high sloping mountainous region erode the existing rocks, carrying loose materials from the outcrops. As most of the piedmont is practically barren of vegetation, runoff becomes very rapid. The extent of active channels on the Zonda piedmont has been identified from geomorphologic studies of aerial photographs and land topography. The active areas are located at lower elevations relative to the adjacent older piedmont surfaces. Within the floodplain area, where the urban and rural areas are located, the majority of the historical records of floods have been confined to the location of 13th Street and 15th Street, both oriented parallel to the slope down from the piedmont, on a west-east trend. During most rainfalls these streets have been transformed  into natural riverbeds, influencing the path of the floods andlimiting their damage to civil structures. As a result of this, other streets and a few irrigation channels laying a north-south trend have been seriously affected. The development of a predictive and early warning system, combined with public education and structural corrective measures are thought to be the means to minimize the hazard that alluvial fans floods pose over the Department of Pocito.