RIO CORDON INSTRUMENTED BASIN: MONITORING AND INVESTIGATION OF JUNE 2014 BEDLOAD EVENT

In mountain environments, the analysis and quantification of bedload process are of fundamental importance for hazard assessment, understanding the morphodynamics of higher order channels, planning and designing reservoir sedimentation. The importance of bedload knowledge contrasts with the fact that it is difficult to monitor, especially in small, steep mountain basins, due to its impulsive nature. Consequently, field bedload data are relatively scarce, and monitoring programs maintained continuously over long-term periods are particularly rare. This work aims to investigate the bedload event occurred on June 9, 2014 in the Rio Cordon instrumented basin (Eastern Italian Alps). The Rio Cordon is a high gradient channel characterized by step-pool and riffle-pool morphology, and by a mean slope equal to 13%. The basin extends for 5 km2 and exhibits a prevalent ivo-pluvial runoff regime. Since 1986, the catchment is equipped with a monitoring station, that continuously records water discharge, bedload and suspended load (at 1 hr intervals, and 5 min intervals during floods). Currently, the structure is managed by ARPA Veneto – Regional Department for Land Safety. The data recorded by the station during the 2014-flood allowed the hydrological features of the event to be characterized. Moreover, multiple post-flood surveys were carried out to determine amount and grain size characteristics of the coarse material transported, as well as to detect its source area. Bedload occurred during a mixed snowmelt-rainfall event lasted more than 14 hours, peaking to 2.06 m3 s-1 (RI = 1.7 years). Overall, 65.3 m3 of coarse material were mobilized. In terms of bedload magnitude, the flood event appears as the sixth most important recorded event. To assess the grain size characteristics of bedload material, 262 transported clasts were measured. The largest particle mobilized is characterized by a b-axis equal to 230 mm, while D50 is 41 mm. In terms of particlesize, the coarse material appears comparable with that transported by the most recent bedload events, especially if supplied by hillslope processes (i.e. November 2012). Field evidences (i.e traces of moved loam) suggested that a debris flow channel located in the median part of the basin was the main source area. The results suggest that the Rio Cordon creek, despite a channel-bed strongly armoured, may transport large amounts of sediment also during no extraordinary flood events. In this sense, the morphological setting of the basin and the presence of many areas potentially unstable may trigger a significant sediment supply.