Sediment transport has been considered an important phenomenon with numerous civil engineering implications. Whilst classic approaches for sediment transport evaluation have been based on theoretical relationships, over the past few decades many authors have put their focus on experimental work in natural streams by designing pebble-tracing experiments. This study reports a wide overview on literature concerning the tagged-particles approach, highlighting the application fields, methodologies, data analysis and results. Further, an RFID tracing experiment was carried out in a partly natural pre-alpine gravel-bed torrent (the Caldone River, Italy), combining a Lagrangian and a semi-Eulerian approach for sediment transport estimation. This work is the continuation of the study that begun in 2016 and includes the additional seeding and analysis of 514 particles in total, positioned in four different reaches of the river. Surveys were performed after each relevant rainfall event for a period of 10 months. Recovery rates of tracers present a fluctuating trend (77% max), with a dramatic decline in the last surveys (16% min). The declining recovery rate was interpreted as resulting from both the rapid dispersion, with frontrunners more than 180 m displaced after the deployment, and the permanent deposit in morphologic units such as pools. Sediment entrainment diameter was determined with painted tracers areas, while critical discharge was estimated through an iterative procedure, using as criterion the virtual velocity of pebbles. A statistical elaboration of the collected data indicated that the spatial distribution of travel distances follows log-normal (ordinary events) and Gamma (intense events) distributions. The analysis indicates a tendency of size-selective transport, but considerable scatter exists in the relationship weight-travel lengths due to the influence of many variables (e.g. particle shape and river morphology). The influence of stream power leads to quadratic or exponential relationship with the travel distances. Finally, the evolution of morphological units has been evaluated in the upstream reach, highlighting the development of aggradation areas.
Il trasporto solido è considerato un importante fenomeno con numerose implicazioni nell’ambito dell’ingegneria civile. Accanto agli approcci classici sul tema, basati su relazioni teoriche, negli ultimi decenni molti autori si sono focalizzati su attività sperimentali, sviluppando studi sul rilievo di ciottoli nei fiumi stessi. Il seguente studio riporta un’ampia panoramica sulla letteratura esistente in materia, sottolineando campi d’applicazione, metodologie, approcci elaborativi e risultati. Un’attività sperimentale di ricerca di RFID è stata condotta in un torrente prealpino a fondo ghiaioso (torrente Caldone, Lecco, Italia), combinando approccio Lagrangiano e semi-Euleriano per la stima delle dinamiche di trasporto solido. Lo studio è la continuazione di una ricerca iniziata nel 2016, essendo stati aggiunti nuovi traccianti, per un totale di 514 ciottoli soggetti ad analisi, posati in quattro diversi tratti del torrente. I rilievi sono stati eseguiti per 10 mesi dopo ogni evento piovoso rilevante. I tassi di ritrovamento dei clasti presentano un trend fluttuante (max 77%), con un brusco calo nelle ultime uscite (16% min). Tale risultato è stato interpretato come conseguenza di rapida dispersione a valle dei ciottoli, con spostamenti fino a 180 m dal punto di posa, e deposito degli stessi sul fondo in tratti profondi. Il diametro critico di moto incipiente è stato valutato tramite analisi di aree colorate sul campo, mentre la portata critica è stata stimata con procedura iterativa basata sulla velocità virtuale. Un’elaborazione statistica dei dati ha mostrato che la distribuzione spaziale delle distanze percorse è descritta da distribuzioni log-normali (eventi ordinari) e Gamma (eventi intensi). In prima analisi emerge tendenza al trasporto selettivo in base a peso e dimensioni dei clasti; tuttavia vi è molta dispersione nella relazione a cause di molte variabili (e.g. forma dei clasti, morfologia del tratto). L’energia del flusso è legata con relazioni quadratiche o esponenziali alle distanze percorse dai ciottoli. Infine, è stata valutata l’evoluzione morfologica del tratto più a monte, evidenziando la creazione di zone di accumulo.
Sediment transport in a pre-alpine stream : an experimental approach through RFID technology
CASTELNUOVO, FRANCESCO;FRANCESCONE, ALICE;BERNASCONI, SIMONE
2016/2017
Abstract
Sediment transport has been considered an important phenomenon with numerous civil engineering implications. Whilst classic approaches for sediment transport evaluation have been based on theoretical relationships, over the past few decades many authors have put their focus on experimental work in natural streams by designing pebble-tracing experiments. This study reports a wide overview on literature concerning the tagged-particles approach, highlighting the application fields, methodologies, data analysis and results. Further, an RFID tracing experiment was carried out in a partly natural pre-alpine gravel-bed torrent (the Caldone River, Italy), combining a Lagrangian and a semi-Eulerian approach for sediment transport estimation. This work is the continuation of the study that begun in 2016 and includes the additional seeding and analysis of 514 particles in total, positioned in four different reaches of the river. Surveys were performed after each relevant rainfall event for a period of 10 months. Recovery rates of tracers present a fluctuating trend (77% max), with a dramatic decline in the last surveys (16% min). The declining recovery rate was interpreted as resulting from both the rapid dispersion, with frontrunners more than 180 m displaced after the deployment, and the permanent deposit in morphologic units such as pools. Sediment entrainment diameter was determined with painted tracers areas, while critical discharge was estimated through an iterative procedure, using as criterion the virtual velocity of pebbles. A statistical elaboration of the collected data indicated that the spatial distribution of travel distances follows log-normal (ordinary events) and Gamma (intense events) distributions. The analysis indicates a tendency of size-selective transport, but considerable scatter exists in the relationship weight-travel lengths due to the influence of many variables (e.g. particle shape and river morphology). The influence of stream power leads to quadratic or exponential relationship with the travel distances. Finally, the evolution of morphological units has been evaluated in the upstream reach, highlighting the development of aggradation areas.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/138495