Purpose: The sediment flushing operation performed in the Valnegra reservoir (Brembo Orientale) has resulted in the release of a huge amount of sediments, deposited along the river downstream of the dam. The following water releases (flushing flow) aimed at cleaning, at least partially, the riverbed have redistributed this deposit, mainly in the upstream of the confluence with the river Brembo Occidentale. The estimates of the deposition in the riverbed as a result of the sediment flushing and the redistribution of the deposit after the flushing flow have been used to implement and calibrate a one-dimensional sediment transport model in the monitored river stretch. Materials and Methods: The Valnegra reservoir (BG) is located in Val Brembana; the current capacity (120,000 m3) is consistently lower than the original (520,000 m3) due to the deposited sediment. The investigated river stretch is upstream limited by the dam and downstream by the confluence with the river Brembo Occidentale, with a total length of 2.124 kilometers. The monitoring of the sediment deposited in this section has been mainly carried out by measurements of the deposit thickness and volumetric sampling. The simulation of the deposit evolution in the riverbed as a result of the flushing flow was conducted through the use of SRH-1D software. The model parameters were calibrated by comparing the simulation results with those observed in the river stretch. Results: The volume of sediment deposited along the river stretch analyzed after the sediment flushing has been estimated at about 10,000 m3. This deposit, mostly sandy, was concentrated in the first km downstream of the dam. The flushing flow has generally resulted in the erosion of the deposit in the first km downstream of the dam and its prevailing relocation in the downstream reach of the investigated river stretch. Even though within the carried schematization limits, the numerical simulation has substantially reproduced this displacement. However, locally significant differences between the simulated deposit and the observed one at the end of the flushing flow were verified. The sensitivity tests have revealed the predominant incidence of the parameters that control the riverbed roughness and the active layer thickness.
Scopo: L’operazione di sediment flushing eseguita nel serbatoio di Valnegra (Brembo Orientale) ha provocato la fuoriuscita di un ingente quantitativo di sedimenti che si sono depositati lungo il corso d’acqua a valle dello sbarramento. I successivo rilasci idrici (flushing flow) finalizzati alla pulizia, almeno parziale, dell’alveo hanno ridistribuito il suddetto deposito, perlopiù nel tratto a monte della confluenza con il fiume Brembo Occidentale. Le stime della deposizione in alveo a seguito del sediment flushing e della redistribuzione del deposito dopo il flushing flow sono state utilizzate per realizzare e calibrare un modello monodimensionale di trasporto solido nel tratto fluviale monitorato. Materiali e Metodi: Il serbatoio di Valnegra (BG) si trova in Val Brembana; la capacità attuale (120.000 m3) è consistentemente inferiore a quella originaria (520.000 m3) a causa del sedimento depositato. Il tratto fluviale analizzato è limitato a monte dalla diga e a valle dalla confluenza con il fiume Brembo Occidentale, per una lunghezza totale di 2,124 km. Il monitoraggio del sedimento depositato in tale tratto è stato realizzato perlopiù mediante misure di spessore del deposito e campionamenti volumetrici. La simulazione dell’evoluzione del deposito in alveo a seguito del flushing flow è stata condotta attraverso l’utilizzo del software SRH-1D. I parametri del modello sono stati calibrati comparando i risultati della simulazione con quelli osservati sul campo. Risultati: il volume di sedimento depositato lungo il tratto fluviale analizzato dopo il sediment flushing è stato stimato in circa 10.000 m3. Tale deposito, perlopiù sabbioso, risultava concentrato nel primo km a valle della diga. Il flushing flow ha generalmente determinato l’erosione del deposito nel primo km a valle della diga e la sua prevalente ridislocazione nel tratto di valle dell’asta fluviale oggetto di indagine. Pur nei limiti delle schematizzazioni effettuate, la simulazione numerica ha sostanzialmente riprodotto questo spostamento. Tuttavia, localmente si sono riscontrate differenze significative tra il deposito simulato e quello osservato al termine del flushing flow. I test di sensitività hanno messo in luce l’incidenza preponderante dei parametri che controllano la scabrezza del fondo alveo e lo spessore dell’active layer.
Simulazione numerica monodimensionale del trasporto solido nel fiume Brembo Orientale a valle della diga di Valnegra
MORONI, SILVIA
2015/2016
Abstract
Purpose: The sediment flushing operation performed in the Valnegra reservoir (Brembo Orientale) has resulted in the release of a huge amount of sediments, deposited along the river downstream of the dam. The following water releases (flushing flow) aimed at cleaning, at least partially, the riverbed have redistributed this deposit, mainly in the upstream of the confluence with the river Brembo Occidentale. The estimates of the deposition in the riverbed as a result of the sediment flushing and the redistribution of the deposit after the flushing flow have been used to implement and calibrate a one-dimensional sediment transport model in the monitored river stretch. Materials and Methods: The Valnegra reservoir (BG) is located in Val Brembana; the current capacity (120,000 m3) is consistently lower than the original (520,000 m3) due to the deposited sediment. The investigated river stretch is upstream limited by the dam and downstream by the confluence with the river Brembo Occidentale, with a total length of 2.124 kilometers. The monitoring of the sediment deposited in this section has been mainly carried out by measurements of the deposit thickness and volumetric sampling. The simulation of the deposit evolution in the riverbed as a result of the flushing flow was conducted through the use of SRH-1D software. The model parameters were calibrated by comparing the simulation results with those observed in the river stretch. Results: The volume of sediment deposited along the river stretch analyzed after the sediment flushing has been estimated at about 10,000 m3. This deposit, mostly sandy, was concentrated in the first km downstream of the dam. The flushing flow has generally resulted in the erosion of the deposit in the first km downstream of the dam and its prevailing relocation in the downstream reach of the investigated river stretch. Even though within the carried schematization limits, the numerical simulation has substantially reproduced this displacement. However, locally significant differences between the simulated deposit and the observed one at the end of the flushing flow were verified. The sensitivity tests have revealed the predominant incidence of the parameters that control the riverbed roughness and the active layer thickness.È consentito all'utente scaricare e condividere i documenti disponibili a testo pieno in UNITESI UNIPV nel rispetto della licenza Creative Commons del tipo CC BY NC ND.
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https://hdl.handle.net/20.500.14239/25530