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Fire-related debris flows in the Iberian Range, Spain
38 pĂĄginas, 2 tablas, 11 figuras[EN] Debris flows occurred three weeks after a wildfire in August 1986 in the Najerilla River valley in the Iberian Range, northern Spain. The flows were triggered by a brief, intense rainstorm (approximately 25 mm hâ 1 over 15 min) in a small area with steep slopes covered by a thick colluvium of quartzite clasts. This storm resulted in the development of several unconfined hillslope debris flows and the formation of an alluvial fan at the mouth of the PĂtare stream, which partially blocked the Najerilla River. We analysed the conditions that led to the development of the debris flows, and estimated the rainfall threshold for the debris flows to occur as well as the total volume of mobilised sediment. Four factors contributed to the debris flows: (i) the occurrence of a rainstorm three weeks after a wildfire, which had removed the plant cover from the soil; (ii) the steep slopes in the area (> 30°), which were the most affected by debris flows; (iii) the presence of quartzite scarps on the hillslopes, which favoured the development of a âfirehose effectâ involving channelised surface runoff; and (iv) the low plasticity index values of the fine material of the colluvium (indices of 7 to 8), which enabled rapid liquefaction. Estimates of rainfall intensity derived from the estimated peak flow in the PĂtare stream suggests that around 80 mm of rainfall fell in approximately 15 min, although this is clearly an overestimated value given the high proportion of sediment load transported during the peak flow. Various equations estimated a rainfall-threshold of approximately 25 mm hâ 1 considering a concentration time of 15 min. The total sediment transported by the debris flows was 10,500 m3 (15,750 Mg, 6800 Mg kmâ 2), and the PĂtare stream alone transported a minimum of 4000 m3 (6000 Mg, 2500 Mg kmâ 2). These results suggest that the rainfall threshold for initiating debris flows decreases following a wildfire, such that an ordinary rainstorm is able to trigger a severe erosion and sediment transport event. Given the absence of fresh landslide scars on the hillslopes, the origin of the fire-related debris flows in the Najerilla River valley appears to have been directly linked to increased rates of overland flow having a greater effect than infiltration for triggering debris flows.Support for this research was provided by the projects PROBASE (CGL2006-11619/HID) and INDICA (CGL2011-27753-C02-01 and CGL2011-27753-C02-02), which was funded by the Spanish Ministry of Education and Science, and ACQWA (FP7-ENV-2007-1-212250), which was financed by the European Commission. NoemĂ Lana-Renault was the recipient of a research contract (Programme âJuan de la Ciervaâ, Human Resources Mobility, funded by the Spanish Ministry of Economy and Competitivity). The authors gratefully acknowledge Dr. Susan Canon, Dr. Francisco GutiĂ©rrez-Santolalla and an anonymous reviewer for their comments and suggestions, which significantly helped to improve this paper.Peer reviewe