Sediment transport in a pre-alpine stream : bed-load monitoring and flood history effects

Sediment transport along gravel-bed mountain river is a complex phenomenon governed by several factors including geomorphology aspects, duration and intensity of flood events and characteristics of the transported grains. The importance of monitoring sediment processes is indisputable: in the past century sediment balance has suffered considerable alterations affecting energy production, navigation, habitat and ecosystems, even representing a risk for the population. Depending on its magnitude and duration, each flood leaves the channel bed in different conditions, which influence the bedload transport of the next event. This work represents the continuation of the sediment monitoring which was started in 2016 by the ‘Politecnico di Milano’, under commission from the Lecco Municipality. After a proper survey of the sediment transport phenomenon, the history-dependence of bedload along the Italian pre-Alpine stream Caldone (Lecco), has been monitored using data collected in three years of RFID (Radio Frequency Identification) tracing application techniques and the new installation of direct bedload samplers in the study case (between October 2018 and June 2019). In fact, after an accurate investigation of the main available literature direct techniques, ‘Bunte’ portable bedload samplers (Bunte, 2017) and the fixed support ‘Slot’ trap instrumentations have been selected for the bedload monitoring of the study case. Both of the methods allowed us to directly collect samples of sediment material and monitor bedload dynamics in the stream. The investigations conducted permitted to individualise on field some flood history effects in accordance to literature and laboratory observations highlighted in other studies (e.g. Mao, 2018). A general decrease of sediment transport has been observed in correspondence with events preceded by high volume floods, probably induced by the history memory of the bedload built by antecedent events. Completely different behaviours have been registered when less important flood volumes preceded bigger events and the sediment transport resulted in being much less affected than the previous combination.

Studio del trasporto solido del torrente Caldone.