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• Sediment dynamics of a small watershed in the Northern Apennines was assessed. • The watershed is affected by rill-interrill and calanchi erosion. • A morphometrical characterisation of the basin was performed using a DEM. • A suspended sediments were assessed using a laser diffraction technique. • SSC and SMD were related to the available rainfall data. The volumes and dynamics of suspended sediments drained from a watershed are often unknown, especially in small creeks and tributaries where continuous discharge measurements are seldom available. The objective of this study was to assess in a qualitative and quantitative way the sediment dynamics of a small ungauged watershed in the Northern Apennines, Italy. We analysed and correlated the sediment volume concentration and grain-size distribution of the suspended sediments with the precipitation pattern. The study area is a small ungauged watershed (0.15 km2) that is dominated by intensive soil erosion processes and related landforms. The basin is oriented East-West with the south-facing slopes characterized by badland erosion processes. The north-facing slopes are cultivated and dominated by rill-interrill erosion phenomena. An morphometrical characterization of the basin was performed using a high-resolution DTM with a 1 × 1 m resolution. Subsequently, the physical characteristics of the topsoil were investigated based on grain size laboratory analysis. A detailed NDVI analysis of the vegetation was performed using Sentinel-2A images. Finally, we assessed the suspended sediments at the outlet of the basin using a laser diffraction technique. The Suspended Sediment Volume Concentration (SSC) and the Sauter Mean Diameter (SMD) of the eroded sediments, provide insights into the morphogenetic processes and the sediment transport dynamics of the basin. The measurements were conducted in autumn 2018 after an intense precipitation period and in spring 2019 after a dry phase. The results show a direct relationship between precipitation and SSC with a delay of about 2 h after the most intense precipitation events. Moreover, we reveal that the SMD values are inversely related to the precipitation due to turbidity effects. The SSC and SMD measurements allow for a detailed assessment of the dynamics between precipitation and suspended sediment load even though discharge volumes were not measured directly. [ABSTRACT FROM AUTHOR]

• Sediment dynamics of a small watershed in the Northern Apennines was assessed. • The watershed is affected by rill-interrill and calanchi erosion. • A morphometrical characterisation of the basin was performed using a DEM. • A suspended sediments were assessed using a laser diffraction technique. • SSC and SMD were related to the available rainfall data. The volumes and dynamics of suspended sediments drained from a watershed are often unknown, especially in small creeks and tributaries where continuous discharge measurements are seldom available. The objective of this study was to assess in a qualitative and quantitative way the sediment dynamics of a small ungauged watershed in the Northern Apennines, Italy. We analysed and correlated the sediment volume concentration and grain-size distribution of the suspended sediments with the precipitation pattern. The study area is a small ungauged watershed (0.15 km2) that is dominated by intensive soil erosion processes and related landforms. The basin is oriented East-West with the south-facing slopes characterized by badland erosion processes. The north-facing slopes are cultivated and dominated by rill-interrill erosion phenomena. An morphometrical characterization of the basin was performed using a high-resolution DTM with a 1 × 1 m resolution. Subsequently, the physical characteristics of the topsoil were investigated based on grain size laboratory analysis. A detailed NDVI analysis of the vegetation was performed using Sentinel-2A images. Finally, we assessed the suspended sediments at the outlet of the basin using a laser diffraction technique. The Suspended Sediment Volume Concentration (SSC) and the Sauter Mean Diameter (SMD) of the eroded sediments, provide insights into the morphogenetic processes and the sediment transport dynamics of the basin. The measurements were conducted in autumn 2018 after an intense precipitation period and in spring 2019 after a dry phase. The results show a direct relationship between precipitation and SSC with a delay of about 2 h after the most intense precipitation events. Moreover, we reveal that the SMD values are inversely related to the precipitation due to turbidity effects. The SSC and SMD measurements allow for a detailed assessment of the dynamics between precipitation and suspended sediment load even though discharge volumes were not measured directly. [ABSTRACT FROM AUTHOR]

• A high magnitude/low frequency event in an alpine basin was comprehensive analyzed. • The flood exhibited ω = 3865 W m−2 and an unit peak discharge = 3.3 m3 s−1 km−2. • A moderate response was shown by hillslopes, while the main stream was deeply altered. • Boulder mobility and massive bedload volume proved the outstanding flood magnitude. • Two large and infrequent floods occurred in the study basin were compared. To achieve a reliable analysis of the impacts induced in mountain basins by large and infrequent floods, all their main components, from the spatial-temporal distribution of meteorological agents to the hydrological, geomorphic and sedimentological response should be considered. Comprehensive study of the hydro-geomorphic responses is extremely valuable to increase the awareness of large floods, especially, in highly populated mountain areas. Such type of investigation requires a solid and wide dataset, which is why only few studies had the chance to describe the response in such a holistic way. This work comprehensively analyzed the high magnitude/low frequency Vaia event, a severe storm that affected northeastern Italy in October 2018 and thus the Rio Cordon study basin. The 80 h precipitation registered in the basin showed a total rainfall equal to 29.8% of the mean annual precipitation. The temporal distribution of rainfall presented two phases, i.e., a first characterized by moderate but persistent precipitation and a second more intense, exhibiting recurrence intervals over 50 years. A combination of indirect methods permitted the reconstruction of the hydraulic forcing acted in the Rio Cordon. Despite the implicit uncertainty, these methods clearly highlighted the high magnitude expressed by October 2018 flood, which generated a unit peak discharge equal to 3.3 m3 s−1 km−2 and a peak of unit stream power of 3865 W m−2, i.e., the highest hydraulic forcing conditions ever observed in 34 years. In terms of geomorphic changes, the use of pre- and post-event LiDAR data stressed out a moderate response of the hillslopes, where the (re)activation of the sediment sources was limited. Only few of these acted as sediment supplier to the main channel. The channel network instead, exhibited an evident response, with the Rio Cordon severely altered by wide lateral widening, deep streambed incision and armouring removal. The hydraulic and geomorphic forcing generated by October 2018 flood caused extensive streambed remobilization and boulder mobility that, in turn, induced the transport of a massive sediment volume. In this sense, the match between bedload observed and bedload predicted suggested the outstanding hydraulic forcing occurred and the quasi-unlimited supply conditions acted during the flood. The long-lasting monitoring program maintained in the Rio Cordon basin provided the rare opportunity to compare two high magnitude/low frequency floods and their induced effects. The September 1994 and October 2018 events were caused by different rainfall conditions, which resulted in different hydrological- and, especially, geomorphic- and sedimentological-responses. Particularly, the October 2018 flood induced an unprecedented alteration on the fluvial system, the effects of which could persist over the long-term. [ABSTRACT FROM AUTHOR]