Proposal for the Characterization of Torrential Behavior of Colombian Andean Basins

Torrential floods are fast flows with a high concentration of heterogeneous sediment in mountain basins that represent high risk for populations and infrastructure in the river system (river and valley). These flash floods are usually triggered by short and intense rainfall events. The risk implied by these floods, added to the low frequency and the short time lapses (usually in hours or minutes) in which they occur, suppose a great difficulty to carry out their monitoring. Therefore, sediment production rates recorded during these extreme events are often underestimated, and there is no adequate understanding of the magnitude of their transforming impact on the fluvial system, which affects the quality and relevance of watershed planning and management studies. Small mountain basins are the most susceptible to flooding, as the rainy event can be concentrated over the whole basin and the slopes are usually higher. In Latin America there is a lack of monitoring methodology for small mountain basins, which could allow to have a reliable database that reveal the true impact of extreme events. For example, in Mocoa, Putumayo (Colombia), in the early morning of April 1, 2017, a rainfall of 130 mm in 3 hours over this high slope basin triggered mud and debris flows and the overflow of 3 rivers, affecting more than half of the urban population (about 45.000 people) leaving more than 300 person dead.

The present work proposes the definition of geomorphological indicators that allow the identification of basins with higher susceptibility to torrential behavior within the Magdalena - Cauca hydrographic area (Colombia), area that is highly monitored (sediment measurement for more than 40 years) due to its great morphological and climatic diversity. This area has hydro-sedimentological modeling studies carried out by IDEAM (Instituto de Hidrología, Meteorología y estudios Ambientales), which have demonstrated the underestimation of sediment transport records during the occurrence of extreme events in small mountain basins.

The indicators will be found analyzing statistical patterns of the flow and sediment transport regime registered with the historical series. Additionally, watersheds are classified according to spatial scale and basin and channel geomorphology, with the aim of find which classification criterion explains underestimation. The error is calculated by downstream mass balance, where stations report less uncertain data. When the error by underestimation is higher than an acceptable threshold the basins are torrential and the current method of measurement must be reconsidered.