Displacement length and velocity of tagged logs in the tagliamento river

Large wood enhance the dynamics of geomorphic processes in river systems, increases the morphological complexity of the channel bed, and provides habitats for fish and invertebrates. On the other side, if transported during high-magnitude events, large wood pieces can increase flood risks in sensitive places such as bridges and narrow cross sections prone to outbank flows. However, the dynamics and mobility of logs in rivers is poorly understood, especially in wide gravel-bed rivers. Recent studies have employed fixed video cameras to assess logs velocity, but little evidence is still available about travel length during flood events of different magnitude. This study was conducted in a valley reach of the Tagliamento river, located in the North East of Italy. The Tagliamento river is approximately 800 m wide in the study area, and is characterized by relatively high natural conditions and complex fluvial dynamics. Log mobility have been studied from June 2010 to October 2011, a period characterized by a relatively high magnitude flood in November 2010. Log mobility and displacement during floods have been measured by implanting active radio transmitters (RFID) in 113 logs and GPS track devices in 42 logs. The first devices allow to recover the log after flood events by using a portable antenna, and to derive the displacement length over the monitoring period, whereas the second devices allows to calculate instantaneous (1 sec) and average log velocity of moving logs. Recovery rate of logs equipped with RFID and GPS was about 50% and 60%, respectively. A preliminary analysis of the data collected indicates that there is a positive relationship between displacement length and the peak of flood events, as well as a positive relationship between log velocity and the flood magnitude. Also, a critical flow rate over which logs stranded on active bars can be transported has been identified. The ability to predict wood mobility in gravel-bed rivers could allow to define better strategies of river management and restoration, by improving the ability to understand wood transport processes and calibrate budgets of wood in rivers

Volume and travel distance of wood pieces in the Tagliamento River (Northeastern Italy)

Wood enhances the dynamics of geomorphic processes in river systems, increases the morphological complexity of the channel bed, and provides habitats for fish and invertebrates. On the other side, if it is transported during floods, wood can increase the risks in sensitive places such as bridges. However, dynamics and mobility of logs in rivers is poorly understood, especially in large rivers. This study was conducted in a sub-reach of the Tagliamento river, located in the north-east of Italy. In the study area the river is approximately 800 m wide. Six cross sections were selected and surveyed with DGPS on the study river. On transects spanning approximately 200 m around the cross-sections, both single wood pieces and wood accumulations were surveyed. The volume, tree species, orientation to flow, state of decay, delivery mechanism and position in the channel were collected for each piece and jam. Log mobility and displacement during floods have been measured by inserting in logs active radio transmitters (RFID) and GPS track devices. A preliminary analysis of the data collected indicates a positive but weak relationship between displacement length and the peak of flood events, as well as a positive relationship between log velocity and flood magnitude. The ability to predict wood mobility in gravel-bed rivers could allow to define better river management and restoration, by improving the ability to understand wood transport processes and calibrate budgets of wood in rivers

Displacement length and velocity of tagged logs in the Tagliamento river

Large wood enhance the dynamics of geomorphic processes in river systems, increases the morphological complexity of the channel bed, and provides habitats for fish and invertebrates. On the other side, if transported during high-magnitude events, large wood pieces can increase flood risks in sensitive places such as bridges and narrow cross sections prone to outbank flows. However, the dynamics and mobility of logs in rivers is poorly understood, especially in wide gravel-bed rivers. Recent studies have employed fixed video cameras to assess logs velocity, but little evidence is still available about travel length during flood events of different magnitude. This study was conducted in a valley reach of the Tagliamento river, located in the North East of Italy. The Tagliamento river is approximately 800 m wide in the study area, and is characterized by relatively high natural conditions and complex fluvial dynamics. Log mobility have been studied from June 2010 to October 2011, a period characterized by a relatively high magnitude flood in November 2010. Log mobility and displacement during floods have been measured by implanting active radio transmitters (RFID) in 113 logs and GPS track devices in 42 logs. The first devices allow to recover the log after flood events by using a portable antenna, and to derive the displacement length over the monitoring period, whereas the second devices allows to calculate instantaneous (1 sec) and average log velocity of moving logs. Recovery rate of logs equipped with RFID and GPS was about 50% and 60%, respectively. A preliminary analysis of the data collected indicates that there is a positive relationship between displacement length and the peak of flood events, as well as a positive relationship between log velocity and the flood magnitude. Also, a critical flow rate over which logs stranded on active bars can be transported has been identified. The ability to predict wood mobility in gravel-bed rivers could allow to define better strategies of river management and restoration, by improving the ability to understand wood transport processes and calibrate budgets of wood in rivers

Il trasporto parziale in un fiume a canali intrecciati e a fondo ghiaioso (Fiume Tagliamento).

Il trasporto di sedimenti è uno dei fattori più importanti che influenzano l’ambiente fisico di un ecosistema fluviale. Il suo studio e la sua comprensione sono essenziali per poter meglio comprendere le dinamiche morfologiche e le variazioni che si succedono lungo i corsi d’acqua. Sfortunatamente, però, è estremamente difficile misurare il trasporto solido attraverso campionamenti diretti e l’impiego di trappole per sedimenti, soprattutto lungo i grandi e complessi fiumi a canali intrecciati. L’obbiettivo del presente lavoro è di presentare un tentativo di valutazione della mobilità dei sedimenti lungo il corso di un fiume a canali intrecciati e a fondo ghiaioso (Fiume Tagliamento, Nord-Est d’Italia), come primo passo per la quantificazione del tasso di trasporto di sedimento utilizzando l’approccio della velocità virtuale. Le analisi sono state condotte lungo tre sezioni trasversali del Fiume Tagliamento dove sono state approntate varie aree colorate per il monitoraggio degli spostamenti dei sedimenti a seguito di eventi di piena. Il monitoraggio è consistito nella misurazione delle dimensioni di ogni singolo elemento trasportato e del relativo spostamento orizzontale. I risultati mostrano come il trasporto parziale si sia verificato per intervalli dello sforzo di taglio adimensionale compresi tra 0,015 e 0,039, mentre con valori maggiori a 0,045 la mobilità del sedimento è stata completa. L’analisi della distanza di trasporto dei diversi sedimenti ha altresì rivelato come in condizioni di trasporto parziale la distanza percorsa da una particella di dimensione data aumenta con lo sforzo di taglio e, inoltre, può esser identificata una soglia di mobilità per le diverse dimensioni dei sedimenti