Deciphering sediment dynamics in a debris-flow catchment: insights from instrumental monitoring and high-resolution topography

In mountainous catchments, the quantification of sediment yield is of paramount relevance for land-use planning and design of sediment control structures. However, deciphering the contribution of the different sediment transport processes (debris flows, debris floods and bedload transport) is often challenging as they are strongly controlled by basin morphometry, hydrological regime, and sediment supply. Therefore, long-term instrumental monitoring through catchment-scale sensor networks can provide precious information, especially if coupled with high-resolution topographical surveys. The Gadria catchment, located in the eastern Italian Alps, offers the possibility to perform a systematic monitoring of sediment transport processes. This catchment typically features several low-magnitude flood episodes and a few debris-flow events per year, from late spring to early fall. Starting from 2011, various instruments mainly devoted to debris-flow detection (geophones, video cameras, flow stage sensors) have been installed along the main channel, just upstream of a retention basin. High-resolution topographical surveys of the retention basin are carried out each year, at the beginning and at the end of the summer season and after debris-flow events. Rainfall is measured in the intermediate part of the catchment and in the headwaters, while PIT-tracing of bedload was performed in the main channel. In this work, we present the reconstruction of the sediment dynamics at the catchment scale during the 2014 and 2015 monitoring seasons. Instrumental monitoring was used to estimate the contribution of the different flow processes, and data from topographical surveys to quantify the transported volumes. Results show that (i) coarse sediment yield is driven by sporadic debris flows while flood events allow the continuous fine-sediment migration along the channel network; (ii) volume estimations may be significantly different - up to 30% lower - if performed through a DEM of Difference (DoD) analysis of the retention basin or by analyzing monitoring data; (iii) a multi-parametric monitoring is needed to decipher sediment dynamics at catchment scale.