The timing and extent of the morphological changes that occurred in the last 30 years in a gravel... more The timing and extent of the morphological changes that occurred in the last 30 years in a gravelbed river (the Brenta River, eastern Italian Alps) have been analysed using eight sets of aerial photos, repeated topographic measurements and morphological-vegetational surveys. Human activities have produced modifications in the natural sediment regime and the cessation of gravel extraction in the late 1990s seems to have caused vegetation erosion and channel widening. Alteration of sediment regime has played a major role in the medium and short-term channel evolution. However, only relevant flood events (RI > 10 years) appear to determine substantial islands erosion. The analysis at smaller scale (sub-reach level) proved to be more effective in describing morphological responses and its relationships with the sediment dynamics within the study reach (20 km). The understanding of sediment transfer at the sub-reach level will provide helpful guidelines for the discussion of channel recovery potential.
Large pieces of in-channel wood can exert an important role on the ecological and morphological p... more Large pieces of in-channel wood can exert an important role on the ecological and morphological properties of gravel-bed rivers. On the other side, when transported during flood events, large wood can become a source of risk for sensitive structures such as bridges. However, wood displacement and velocity in river systems are still poorly understood, especially in large gravel-bed rivers. This study focuses on log transport in a valley reach of Tagliamento River (Italy). Log displacement during flood events of different magnitudes recorded from June 2010 to October 2011 has been analysed thanks to the installation of 113 radio frequency identification (RFID) tags and 42 GPS tracker devices in logs of different dimensions. Recovery rates of logs equipped with RFID and GPS trackers were about 43% and 42%, respectively. The GPS devices allowed us to analyse in details the log displacement and transport overtime, indicating a higher log entrainment during rising limb of hydrographs. The threshold for the entrainment of logs from low bars is around 40% of bankfull water stage. No clear relationship was found between the peak of flood and log displacement length and velocity. However, log displacement length and velocity appear significantly correlated to the ratio between the peak of flow and the water stage exceeding the flow duration curve for 25% of time (i.e. the ratio h max /h 25 ratio). Log deposition was observed to occur at the peak flow, and logs transported during ordinary events are preferably deposited on low bars. This study reveals the potentials of GPS tracker devices to monitor the entrainment and movements of logs in large gravel-bed rivers during floods. These observations could be useful for better planning of river management practices and strategies involving the use of large wood pieces and could help for calibrating wood budgets at the reach scale.
1] The purpose of this study is to investigate the connection between bed load, channel processes... more 1] The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high-magnitude, low-recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity-duration curves are also reported, which emphasize differences between short-and long-duration events. The relationship flow-bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre-1994 and post-1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long-term impact of major floods on mountain streams.
Dead wood pieces, especially when organized in jams, play an important geomorphic role in streams... more Dead wood pieces, especially when organized in jams, play an important geomorphic role in streams because of the effects on flow hydraulics, pool formation and sediments storage. The increase in stream morphological diversity and complexity also exerts an important ecological role. This work reports on geomorphic role of large wood (LW) pieces and jams in a third-order mountain stream located in the Southern Tierra del Fuego (Argentina), and draining an old-growth Nothofagus-forested basin not influenced by the beavers damming activity. Even if the in-stream number of wood pieces (length >1 m; diameter >0.1 m) is comparable to that observed in other climatic areas, the slow growth of the Nothofagus forest causes a lower wood abundance in terms of volumetric load. Because of the relatively small dimensions of the surveyed LW pieces, almost 70% of them demonstrated to have been fluvial transported and also the wood jams reflect the apparent dynamic nature of wood in the channel. Wood jams exert a significant influence on the channel morphology, representing almost half of the drop caused by steps and being responsible for the creation of 30% of the pools. LW-forced pool volume is strongly and positively correlated to the height of the LW jam. The geomorphic influence of LW jams is also exerted by a considerable sediment storing capacity. , and the periodic windthrows governing the forest dynamics in Tierra del Fuego (Rebertus et al., 1997), produce a forest cover with mature (>200 year) trees having diameters at breast height generally <0.5 m. Only small patches were logged during the 1940s.
The aim of this paper is to explore possibilities and limitations of restoring physical processes... more The aim of this paper is to explore possibilities and limitations of restoring physical processes in five gravel-bed rivers (Brenta, Piave, Cellina, Tagliamento and Torre Rivers) in north-eastern Italy. The selected rivers were analysed through a range of techniques, specifically analysis of historical maps and aerial photographs with geographical information systems (GIS), comparison of topographic surveys and geomorphological surveys. After illustrating channel adjustments and sediment fluxes, we discuss how the understanding of physical processes can be used for channel restoration.
Many gravel bed rivers in the European Alpine area suffered different ranges and types of human p... more Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30 years along the middle portion of the river (20 km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos () have been used to assess the medium and short-term morphological changes of the floodplains and the active channel area. As to the medium-term modification, the recent changes in in-channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8-10 years appear to be able to determine substantial erosion of floodplain and island margins. Figure 4. Evolution of historical cross sections 2, 5 and 7 for the years 1932, 1997, 2010 and 2011. The horizontal line represents the bankfull stage for the sections measured in 2010 Figure 5. Temporal variation with error bars of the surface of the active channel, floodplain and islands in the analysed reach of the Brenta River CHANNEL ADJUSTMENT AND ISLAND DYNAMICS 725
We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08-0.21) of a predomina... more We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08-0.21) of a predominantly step-pool channel, the Rio Cordon, Italy, over a range of discharges (3-80% of the bankfull discharge). The resulting data were used to compute flow resistance. At-a-station hydraulic geometry relations indicate that in most reaches, the exponent describing the rate of velocity increases with discharge was between 0.48 and 0.6, which is within the range of published values for pool-riffle channels. The Rio Cordon data are also combined with published hydraulics data from step-pool streams to explore non-dimensional relationships between velocity and flow resistance and factors including unit discharge, channel gradient, and step geometry. Multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge (q * ) is the most important independent variable overall in explaining variations in velocity and flow resistance, followed by channel slope and the ratio of step height to step length. Empirical equations are provided both for dimensionless velocity and flow resistance, but prediction of the former variable appears more reliable. ª
Coarse sediment transport processes in steep headwater streams may vary between bedload and debri... more Coarse sediment transport processes in steep headwater streams may vary between bedload and debris flows events, depending on basin geomorphology and sediment supply. This paper compares two small catchments (drainage area 4-5 km 2 ) located in the Eastern Italian Alps where the dominant sediment transport processes differ substantially, i.e., debris flows in the Moscardo Torrent and bedload events in the Rio Cordon. The two basins were selected because they provided the unique opportunity to analyze long-term data of debris flow and bedload volumes, respectively. The two basins are compared in terms of peak discharge, event duration, and magnitude-frequency characteristics of transported sediment volumes. Additional bedload data from two more Alpine experimental basins are also considered. The results show that, for comparable recurrence intervals, debris flow volumes are 2-3 orders of magnitude larger than bedload volumes. This contrasting sediment transfer activity can be attributed to different basin and channel morphologies, which are analyzed in terms of sediment supply conditions, longitudinal profiles and slope-area curves.
The mobility conditions of bedload transport in an alpine high-gradient step-pool stream (Rio Cor... more The mobility conditions of bedload transport in an alpine high-gradient step-pool stream (Rio Cordon) are analysed. Since 1986, a device system at the downstream end section of the stream has been operating in order to monitor the water discharge, suspended sediment and bedload transport. Sediment distribution of bedload transported by various floods has been analysed, and equal-mobility evidence is recognized only for the high-magnitude flows ever recorded (RI >50 years). The thresholds for size-selective and equal-mobility transport conditions are identified and quantified by using both data provided by the fractional transport rate and by length displacements of marked particles. Size-selective bedload transport seems to dominate when the critical shear stress of the size fractions ci considered is exceeded, whereas the equal-mobility condition is approached as levels of excess shear stress become higher ( eqi D 1Ð45 ci ).
The paper presents the results of field measurements of critical conditions for bedload motion co... more The paper presents the results of field measurements of critical conditions for bedload motion conducted in the Rio Cordon, a steep boulder-bed stream in the Italian Alps, under conditions of high Reynolds numbers and low relative submergence poorly explored before. Two ...
Braided rivers are dynamic and complex environments shaped by the balance of the flow and sedimen... more Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes and by the influence of the riparian vegetation and disturbances such as floods. In particular, the balance between sediment supply and transport capacity can determine the morphological evolution of a river. For instance, aggrading and widening trends are distinctive of reaches where sediment supply is higher than transport capacity. In contrast, incising and narrowing tendencies are dominant. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a small reach of a relatively unimpacted gravel-bed braided river (Tagliamento River, northeast Italy) using a terrestrial laser scanner (TLS). The study area is around 23 ha and has been surveyed before after two periods with relevant flood events, two of which were higher than the bankfull level and occurred between September 2010 and September 2011. The very high point clouds density allowed us to derive three high resolution digital elevation models (DEMs) with 0.125 × 0.125 m pixel size. Scan data cloud merging was achieved with an overall high degree of accuracy and resolution (subcentimeter). Topographic data were more accurate for exposed surfaces than those collected in wet areas. A detailed net of dGPS control points allowed us to verify the high quality of the DEMs derived from the surveys (RMSE of about 5 cm). Two DEMs of difference (DoD) were computed, revealing different and consistent episodes of erosion and deposition within the analyzed area, and changes in morphology of channel and bars could also be detected, such as bar edge accretion and bank erosion demonstrating a strong dynamicity of the Tagliamento River. Moreover, a very detailed estimation of the surface roughness in the study area has been carried out, permitting a large-scale analysis of the roughness values distribution. The results of the analysis on the TLS collected data show that along a river with a high natural character (i.e., Tagliamento River), the dynamic processes are also common during low magnitude events.
The timing and extent of the morphological changes that occurred in the last 200 years in a large... more The timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river (the Piave River, eastern Italian Alps) that was heavily impacted by human activities (training structures, hydropower schemes, and gravel mining) have been analyzed by historical maps, aerial photos, repeated topographic measurements, and geomorphological surveys. Results show that the channel underwent a strong narrowing during the twentieth century, but with a faster pace during the 1970s-1990s and with an associated shift from a dominant braided pattern to a wandering morphology. Bed incision up to 2 mmostly from gravel mininghas been documented for this period. Large areas of the former active channel were colonized by riparian forests, both as islands and as marginal woodlands. The ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetation erosion/channel widening even though a significant aggradation phase is not present. We conclude that alteration of sediment regime has played a major role on the long-term channel evolution. However, only relevant flood events (RI N 10-15 years) appear to determine substantial island erosion, and therefore the proportion of island vs. channel area fluctuates depending on flood history.
This paper presents an analysis of critical thresholds for bedload transport based on field measu... more This paper presents an analysis of critical thresholds for bedload transport based on field measurements conducted in two small, high gradient streams: the Rio Cordon (Italian Alps) and the Tres Arroyos (Chilean Andes). The threshold of incipient motion was identified by using marked particles displacement and both flood and flow competence approaches. The findings are expressed in terms of Shields
Geografiska Annaler: Series A, Physical Geography, 2014
River islands are defined as discrete areas of woodland vegetation surrounded by either water-fil... more River islands are defined as discrete areas of woodland vegetation surrounded by either water-filled channels or exposed gravel. They exhibit some stability and are not submerged during bank-full flows. The aim of the study is to analyze the dynamics of established, building, and pioneer islands in a 30-km-long reach of the gravel-bed Piave River, which has suffered from intense and multiple human impacts. Plan-form changes of river features since 1960 were analyzed using aerial photographs, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces, and maximum and mean height of their vegetation. The results suggest that established islands lie at a higher elevation than building and pioneer islands, and have a thicker layer of fine sediments deposited on their surface after big floods. After the exceptional flood in 1966 (RI > 200 years) there was a moderate increase in island numbers and extension, followed by a further increase from 1991, due to a succession of flood events in 1993 and 2002 with RI > 10 years, as well as a change in the human management relating to the control of gravel-mining activities. The narrowing trend of the morphological plan form certainly enhanced the chance of islands becoming established and this explains the reduction of the active channel, the increase in established islands and reduction of pioneer islands.
ABSTRACT The extension and volume of erosion/deposition processes affecting gravel-bed rivers aft... more ABSTRACT The extension and volume of erosion/deposition processes affecting gravel-bed rivers after floods can be assessed in a very accurate way on the basis of digital terrain models derived from LiDAR surveys developed before and after flood events. When LiDAR surveys are acquired without using the bathymetric option (red laser), the uncertainty related to the estimation of the bed surface underwater may increase substantially. A relatively novel method to develop more precise and detailed DTMs is to integrate LiDAR-derived elevations of dry surfaces with water depth of wetted areas derived from aerial photos and a predictive depth - color relationships. This approach has been applied to eight different sub-reaches of three gravel bed rivers featuring in North-Eastern Italy (Brenta, Piave, and Tagliamento), before and after the flood events occurred in November and December 2010 (recurrence intervals &gt; 10 years). The key of this methodology consists in the calibration of a regression model between depth values of Differential Global Positioning System (DGPS) points and Red, Green and Blue (RGB) bands values of aerial photos, contemporary to the LiDAR flight. The regression model calibrated for each river and each year, estimates the quota (Z coordinate) in respect to the reference plane, for each RGB pixel of the channel. Final DTMs (uncertainty was estimated about ± 25 cm) include detailed representations of flood-plains and active channels, that have been used to evaluate the morphological evolution of the rivers as effect of floods through the difference of DTMs (DoD). The erosion-deposition volumetric results derived from the two-years DoD differencing models have highlighted the impact of the two flood events. In all the sub-reaches, erosion processes have dominated. The quantitative results of morphological changes at sub-reach level show different intensity of the processes, as result of the diverse site conditions and the historical channel-adjustments.
Braided rivers are dynamic and complex environments shaped by the balance of the flow and sedimen... more Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes, and the influence of the riparian vegetation and disturbances such as floods. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a relatively un-impacted gravel-bed braided river (Tagliamento river, Northeast Italy) using a Terrestrial Laser Scanner (TLS). The device used in this study (Leica Scan Station 2) is a ...
The timing and extent of the morphological changes that occurred in the last 30 years in a gravel... more The timing and extent of the morphological changes that occurred in the last 30 years in a gravelbed river (the Brenta River, eastern Italian Alps) have been analysed using eight sets of aerial photos, repeated topographic measurements and morphological-vegetational surveys. Human activities have produced modifications in the natural sediment regime and the cessation of gravel extraction in the late 1990s seems to have caused vegetation erosion and channel widening. Alteration of sediment regime has played a major role in the medium and short-term channel evolution. However, only relevant flood events (RI > 10 years) appear to determine substantial islands erosion. The analysis at smaller scale (sub-reach level) proved to be more effective in describing morphological responses and its relationships with the sediment dynamics within the study reach (20 km). The understanding of sediment transfer at the sub-reach level will provide helpful guidelines for the discussion of channel recovery potential.
Large pieces of in-channel wood can exert an important role on the ecological and morphological p... more Large pieces of in-channel wood can exert an important role on the ecological and morphological properties of gravel-bed rivers. On the other side, when transported during flood events, large wood can become a source of risk for sensitive structures such as bridges. However, wood displacement and velocity in river systems are still poorly understood, especially in large gravel-bed rivers. This study focuses on log transport in a valley reach of Tagliamento River (Italy). Log displacement during flood events of different magnitudes recorded from June 2010 to October 2011 has been analysed thanks to the installation of 113 radio frequency identification (RFID) tags and 42 GPS tracker devices in logs of different dimensions. Recovery rates of logs equipped with RFID and GPS trackers were about 43% and 42%, respectively. The GPS devices allowed us to analyse in details the log displacement and transport overtime, indicating a higher log entrainment during rising limb of hydrographs. The threshold for the entrainment of logs from low bars is around 40% of bankfull water stage. No clear relationship was found between the peak of flood and log displacement length and velocity. However, log displacement length and velocity appear significantly correlated to the ratio between the peak of flow and the water stage exceeding the flow duration curve for 25% of time (i.e. the ratio h max /h 25 ratio). Log deposition was observed to occur at the peak flow, and logs transported during ordinary events are preferably deposited on low bars. This study reveals the potentials of GPS tracker devices to monitor the entrainment and movements of logs in large gravel-bed rivers during floods. These observations could be useful for better planning of river management practices and strategies involving the use of large wood pieces and could help for calibrating wood budgets at the reach scale.
1] The purpose of this study is to investigate the connection between bed load, channel processes... more 1] The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high-magnitude, low-recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity-duration curves are also reported, which emphasize differences between short-and long-duration events. The relationship flow-bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre-1994 and post-1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long-term impact of major floods on mountain streams.
Dead wood pieces, especially when organized in jams, play an important geomorphic role in streams... more Dead wood pieces, especially when organized in jams, play an important geomorphic role in streams because of the effects on flow hydraulics, pool formation and sediments storage. The increase in stream morphological diversity and complexity also exerts an important ecological role. This work reports on geomorphic role of large wood (LW) pieces and jams in a third-order mountain stream located in the Southern Tierra del Fuego (Argentina), and draining an old-growth Nothofagus-forested basin not influenced by the beavers damming activity. Even if the in-stream number of wood pieces (length >1 m; diameter >0.1 m) is comparable to that observed in other climatic areas, the slow growth of the Nothofagus forest causes a lower wood abundance in terms of volumetric load. Because of the relatively small dimensions of the surveyed LW pieces, almost 70% of them demonstrated to have been fluvial transported and also the wood jams reflect the apparent dynamic nature of wood in the channel. Wood jams exert a significant influence on the channel morphology, representing almost half of the drop caused by steps and being responsible for the creation of 30% of the pools. LW-forced pool volume is strongly and positively correlated to the height of the LW jam. The geomorphic influence of LW jams is also exerted by a considerable sediment storing capacity. , and the periodic windthrows governing the forest dynamics in Tierra del Fuego (Rebertus et al., 1997), produce a forest cover with mature (>200 year) trees having diameters at breast height generally <0.5 m. Only small patches were logged during the 1940s.
The aim of this paper is to explore possibilities and limitations of restoring physical processes... more The aim of this paper is to explore possibilities and limitations of restoring physical processes in five gravel-bed rivers (Brenta, Piave, Cellina, Tagliamento and Torre Rivers) in north-eastern Italy. The selected rivers were analysed through a range of techniques, specifically analysis of historical maps and aerial photographs with geographical information systems (GIS), comparison of topographic surveys and geomorphological surveys. After illustrating channel adjustments and sediment fluxes, we discuss how the understanding of physical processes can be used for channel restoration.
Many gravel bed rivers in the European Alpine area suffered different ranges and types of human p... more Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30 years along the middle portion of the river (20 km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos () have been used to assess the medium and short-term morphological changes of the floodplains and the active channel area. As to the medium-term modification, the recent changes in in-channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8-10 years appear to be able to determine substantial erosion of floodplain and island margins. Figure 4. Evolution of historical cross sections 2, 5 and 7 for the years 1932, 1997, 2010 and 2011. The horizontal line represents the bankfull stage for the sections measured in 2010 Figure 5. Temporal variation with error bars of the surface of the active channel, floodplain and islands in the analysed reach of the Brenta River CHANNEL ADJUSTMENT AND ISLAND DYNAMICS 725
We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08-0.21) of a predomina... more We measured velocity and channel geometry in 10 reaches (bed gradient = 0.08-0.21) of a predominantly step-pool channel, the Rio Cordon, Italy, over a range of discharges (3-80% of the bankfull discharge). The resulting data were used to compute flow resistance. At-a-station hydraulic geometry relations indicate that in most reaches, the exponent describing the rate of velocity increases with discharge was between 0.48 and 0.6, which is within the range of published values for pool-riffle channels. The Rio Cordon data are also combined with published hydraulics data from step-pool streams to explore non-dimensional relationships between velocity and flow resistance and factors including unit discharge, channel gradient, and step geometry. Multiple regression analysis of this combined field dataset indicated that dimensionless unit discharge (q * ) is the most important independent variable overall in explaining variations in velocity and flow resistance, followed by channel slope and the ratio of step height to step length. Empirical equations are provided both for dimensionless velocity and flow resistance, but prediction of the former variable appears more reliable. ª
Coarse sediment transport processes in steep headwater streams may vary between bedload and debri... more Coarse sediment transport processes in steep headwater streams may vary between bedload and debris flows events, depending on basin geomorphology and sediment supply. This paper compares two small catchments (drainage area 4-5 km 2 ) located in the Eastern Italian Alps where the dominant sediment transport processes differ substantially, i.e., debris flows in the Moscardo Torrent and bedload events in the Rio Cordon. The two basins were selected because they provided the unique opportunity to analyze long-term data of debris flow and bedload volumes, respectively. The two basins are compared in terms of peak discharge, event duration, and magnitude-frequency characteristics of transported sediment volumes. Additional bedload data from two more Alpine experimental basins are also considered. The results show that, for comparable recurrence intervals, debris flow volumes are 2-3 orders of magnitude larger than bedload volumes. This contrasting sediment transfer activity can be attributed to different basin and channel morphologies, which are analyzed in terms of sediment supply conditions, longitudinal profiles and slope-area curves.
The mobility conditions of bedload transport in an alpine high-gradient step-pool stream (Rio Cor... more The mobility conditions of bedload transport in an alpine high-gradient step-pool stream (Rio Cordon) are analysed. Since 1986, a device system at the downstream end section of the stream has been operating in order to monitor the water discharge, suspended sediment and bedload transport. Sediment distribution of bedload transported by various floods has been analysed, and equal-mobility evidence is recognized only for the high-magnitude flows ever recorded (RI >50 years). The thresholds for size-selective and equal-mobility transport conditions are identified and quantified by using both data provided by the fractional transport rate and by length displacements of marked particles. Size-selective bedload transport seems to dominate when the critical shear stress of the size fractions ci considered is exceeded, whereas the equal-mobility condition is approached as levels of excess shear stress become higher ( eqi D 1Ð45 ci ).
The paper presents the results of field measurements of critical conditions for bedload motion co... more The paper presents the results of field measurements of critical conditions for bedload motion conducted in the Rio Cordon, a steep boulder-bed stream in the Italian Alps, under conditions of high Reynolds numbers and low relative submergence poorly explored before. Two ...
Braided rivers are dynamic and complex environments shaped by the balance of the flow and sedimen... more Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes and by the influence of the riparian vegetation and disturbances such as floods. In particular, the balance between sediment supply and transport capacity can determine the morphological evolution of a river. For instance, aggrading and widening trends are distinctive of reaches where sediment supply is higher than transport capacity. In contrast, incising and narrowing tendencies are dominant. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a small reach of a relatively unimpacted gravel-bed braided river (Tagliamento River, northeast Italy) using a terrestrial laser scanner (TLS). The study area is around 23 ha and has been surveyed before after two periods with relevant flood events, two of which were higher than the bankfull level and occurred between September 2010 and September 2011. The very high point clouds density allowed us to derive three high resolution digital elevation models (DEMs) with 0.125 × 0.125 m pixel size. Scan data cloud merging was achieved with an overall high degree of accuracy and resolution (subcentimeter). Topographic data were more accurate for exposed surfaces than those collected in wet areas. A detailed net of dGPS control points allowed us to verify the high quality of the DEMs derived from the surveys (RMSE of about 5 cm). Two DEMs of difference (DoD) were computed, revealing different and consistent episodes of erosion and deposition within the analyzed area, and changes in morphology of channel and bars could also be detected, such as bar edge accretion and bank erosion demonstrating a strong dynamicity of the Tagliamento River. Moreover, a very detailed estimation of the surface roughness in the study area has been carried out, permitting a large-scale analysis of the roughness values distribution. The results of the analysis on the TLS collected data show that along a river with a high natural character (i.e., Tagliamento River), the dynamic processes are also common during low magnitude events.
The timing and extent of the morphological changes that occurred in the last 200 years in a large... more The timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river (the Piave River, eastern Italian Alps) that was heavily impacted by human activities (training structures, hydropower schemes, and gravel mining) have been analyzed by historical maps, aerial photos, repeated topographic measurements, and geomorphological surveys. Results show that the channel underwent a strong narrowing during the twentieth century, but with a faster pace during the 1970s-1990s and with an associated shift from a dominant braided pattern to a wandering morphology. Bed incision up to 2 mmostly from gravel mininghas been documented for this period. Large areas of the former active channel were colonized by riparian forests, both as islands and as marginal woodlands. The ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetation erosion/channel widening even though a significant aggradation phase is not present. We conclude that alteration of sediment regime has played a major role on the long-term channel evolution. However, only relevant flood events (RI N 10-15 years) appear to determine substantial island erosion, and therefore the proportion of island vs. channel area fluctuates depending on flood history.
This paper presents an analysis of critical thresholds for bedload transport based on field measu... more This paper presents an analysis of critical thresholds for bedload transport based on field measurements conducted in two small, high gradient streams: the Rio Cordon (Italian Alps) and the Tres Arroyos (Chilean Andes). The threshold of incipient motion was identified by using marked particles displacement and both flood and flow competence approaches. The findings are expressed in terms of Shields
Geografiska Annaler: Series A, Physical Geography, 2014
River islands are defined as discrete areas of woodland vegetation surrounded by either water-fil... more River islands are defined as discrete areas of woodland vegetation surrounded by either water-filled channels or exposed gravel. They exhibit some stability and are not submerged during bank-full flows. The aim of the study is to analyze the dynamics of established, building, and pioneer islands in a 30-km-long reach of the gravel-bed Piave River, which has suffered from intense and multiple human impacts. Plan-form changes of river features since 1960 were analyzed using aerial photographs, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces, and maximum and mean height of their vegetation. The results suggest that established islands lie at a higher elevation than building and pioneer islands, and have a thicker layer of fine sediments deposited on their surface after big floods. After the exceptional flood in 1966 (RI > 200 years) there was a moderate increase in island numbers and extension, followed by a further increase from 1991, due to a succession of flood events in 1993 and 2002 with RI > 10 years, as well as a change in the human management relating to the control of gravel-mining activities. The narrowing trend of the morphological plan form certainly enhanced the chance of islands becoming established and this explains the reduction of the active channel, the increase in established islands and reduction of pioneer islands.
ABSTRACT The extension and volume of erosion/deposition processes affecting gravel-bed rivers aft... more ABSTRACT The extension and volume of erosion/deposition processes affecting gravel-bed rivers after floods can be assessed in a very accurate way on the basis of digital terrain models derived from LiDAR surveys developed before and after flood events. When LiDAR surveys are acquired without using the bathymetric option (red laser), the uncertainty related to the estimation of the bed surface underwater may increase substantially. A relatively novel method to develop more precise and detailed DTMs is to integrate LiDAR-derived elevations of dry surfaces with water depth of wetted areas derived from aerial photos and a predictive depth - color relationships. This approach has been applied to eight different sub-reaches of three gravel bed rivers featuring in North-Eastern Italy (Brenta, Piave, and Tagliamento), before and after the flood events occurred in November and December 2010 (recurrence intervals &gt; 10 years). The key of this methodology consists in the calibration of a regression model between depth values of Differential Global Positioning System (DGPS) points and Red, Green and Blue (RGB) bands values of aerial photos, contemporary to the LiDAR flight. The regression model calibrated for each river and each year, estimates the quota (Z coordinate) in respect to the reference plane, for each RGB pixel of the channel. Final DTMs (uncertainty was estimated about ± 25 cm) include detailed representations of flood-plains and active channels, that have been used to evaluate the morphological evolution of the rivers as effect of floods through the difference of DTMs (DoD). The erosion-deposition volumetric results derived from the two-years DoD differencing models have highlighted the impact of the two flood events. In all the sub-reaches, erosion processes have dominated. The quantitative results of morphological changes at sub-reach level show different intensity of the processes, as result of the diverse site conditions and the historical channel-adjustments.
Braided rivers are dynamic and complex environments shaped by the balance of the flow and sedimen... more Braided rivers are dynamic and complex environments shaped by the balance of the flow and sediment regimes, and the influence of the riparian vegetation and disturbances such as floods. The aim of the present study is to analyze the short-term morphological dynamics and the processes of erosion and sediment deposition along a relatively un-impacted gravel-bed braided river (Tagliamento river, Northeast Italy) using a Terrestrial Laser Scanner (TLS). The device used in this study (Leica Scan Station 2) is a ...
Uploads
Papers by L. Mao