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The effects of low-magnitude flow conditions on bedload mobility in a steep mountain stream
No full textThe transport of coarse material strongly controls the stability and evolution of mountain fluvial systems but, despite this, bedload dynamics are not yet fully understood especially in mountain streams. In this sense, particular attention was paid on the bedload magnitude (volume) expressed at event-scale and on the long-term, while few studies were focused on when and how the transport of the coarse material occurs. The aim of this work was to investigate the bedload mobility in the Rio Cordon, a mountain stream characterized by cascade and step-pool morphologies with a rough streambed. Here, the critical conditions for initiation of motion, transport distance and virtual velocity expressed by the coarse streambed material were assessed and their relationships with hydraulic forcing conditions and grain size were analyzed. To this end, a monitoring program based on bedload tracing was maintained over 7 years, allowing to analyze the bedload mobility during persistent high frequency/low magnitude flow conditions. To investigate the bedload mobility, 250 tracers were released between 2011 and 2012 and their propagation along study site was monitored until 2018. Overall, 14 tracer inventories were realized, determining 1697 tracer localizations. During the study period, the bedload dispersion resulted well described by the peak discharge magnitude (Qp, qp and ωp − ωc), while no significant relationships were observed with duration of competent flow (tover) and effective runoff volume (ER). Transport distance and tracer grain size were negatively correlated, whereas virtual velocity increased with increasing particle size. In this sense, the propagation velocity seems to be affected by the high frequency flows that, on the one hand triggered mobilization of the coarser tracers only through limited and impulsive events and, on the other hand favored a slowdown on the finer particles due to bedforms disturbance. Compared to other study sites, the Rio Cordon exhibited accentuated threshold conditions with lower transport distance and virtual velocity, confirming that steep mountain streams are generally influenced by a reduced transport efficiency due to protruding bedforms and macro-roughness that cause a pronounced energy dissipation. Interestingly, such condition seems to have progressively reduced from under- to near-bankfull flows. The results were compared to the bedload mobility observed in the Rio Cordon during 1993–1998, enabling to quantitatively assess how the bedload dispersion varied between a setting of stable armouring layer with protruding bedforms (2012–2018) and a partial alteration of these (1993–1998). An evident difference was observed in terms of transport distance, while critical conditions did not significantly change. Lastly, the long-term bedload tracing investigation highlighted that a certain legacy on the transport efficiency was produced by the persistent high frequency flow conditions. Over the study period, a general decrease of transport distance was observed that could be explained by a progressive stabilization of streambed material. Previous studies suggested that such condition can be interrupted by high magnitude/low frequency flood
Surprising suspended sediment dynamics of an alpine basin affected by a large infrequent disturbance
No full textIn many environments, climate change causes an increase in the frequency and magnitude of Large Infrequent Disturbances (LIDs). LIDs make fragile areas, as mountain basins, even more vulnerable, along with local communities that could be severely affected by extreme events. Among all LIDs, windthrows are one of the most relevant and yet rather unpredictable disturbances affecting the Alpine region. Windthrows can affect the forest cover and morphological settings at the basin scale, changing the supply of sediments to river networks and affecting the sediment cascading processes. This work aims at (i) identifying the contribution of Suspended Sediment Load (SSL) from a windthrow-managed-affected area induced in an Alpine basin by a recent LID (Vaia rainstorm, 2018), (ii) assessing the annual, seasonal and monthly variation in the SSL, and (iii) analysing the changes in SSL fluxes and dynamics in the 2nd and 3rd year after the LID in comparison to those detected after a similar high magnitude event occurred in 1994. To do so, two multiparameter sondes measuring the water level and the turbidity were installed upstream and downstream the windthrow-affected area. Discharge measurements and water samples were collected to obtain the rating curves and calibrate the turbidity meters in order to derive suspended sediment loads (SSL). The results show that the windthrow-affected area was significantly contributing suspended sediment during events occurring two years after Vaia (2020) but less intensively in those occurred three years after the event itself (2021). Both the events average intensity rainfalls and the total precipitation appear to be the best predictors of both the peak of suspended sediment concentration (SSCp), the total suspended sediment (SSL) and the suspended sediment percentage variation among the two cross-sections. The seasonal and monthly analysis revealed to be in line with those detected prior the disturbance. The analyzed LID affected the transport efficiency for near-bankfull events but not the annual sediment yield as it was found after the previous high magnitude flood event recorded in 1994. Unexpectedly, the mean rate of SSL (42 t km -2 yr−1) is indeed comparable with what monitored during the decade before Vaia (2004–2014) (40 t km -2 yr−1). Such surprising results may be explained by the fact that the Rio Cordon basin have had an alluvial response rather than colluvial during the Vaia rainstorm event and that the basin's resilience may be in a different stage as compared to the one of 1994, when the catchment featured different cascading processes that completely overturn the suspended sediment dynamics and fluxes for about a decade
Interaction between climatic conditions, water- and sediment-fluxes in an alpine basin: Long-term monthly and seasonal analysis
Full text linkMountain basins are sites where complex interactions among climatic, geological, geomorphic, and ecological conditions can unfold. These interplays can be reflected in the erosion processes, and in the water and sediment f luxes released along the stream network, which can be highly variable in time and space. To investigate the relationships between temperature, precipitation, runoff, and sediment fluxes at monthly and seasonal scales in an Alpine basin, this work analyzed a long-term (January 1987–September 2018) field dataset produced in the Rio Cordon basin (eastern Italian Alps). The monthly variability revealed that higher precipitation occurred in summer (i.e., June–August). However, the runoff peaked in May generated by the snowmelt process. The total sediment load exhibited two peaks in May and September, somewhat reflecting the pattern of the monthly peak of water discharge. The monthly correlation analysis proved that temperature and precipitation weakly influenced the variables investigated, while the peak of water discharge was a good predictor of sediment fluxes observed. At the seasonal scale, temperature influenced snowmelt and summer dynamics, while precipitation showed the highest correlations in autumn. The hysteresis patterns obtained by the relationships between the averaged peak of water discharge and sediment loads (suspended load and bedload) highlighted the succession of high transport efficiency months and months characterized by supply-limited conditions. Such dynamics appeared strongly influenced by a single month, i.e., September 1994, when a large and infrequent flood affected the study basin. September 1994 resulted in a breakpoint in the mass curve built by relating cumulated runoff and cumulated total load, inducing a change in the long-term trend observed until then. Specifically, about 8 years of intensified sediment load were noted, a period during which the Rio Cordon experienced frequent streamed reworking and release of fine material. These findings demonstrated how alpine basins can respond to large and infrequent events, which can be expected more frequently due to climate change, providing important insights into areas such as soil erosion control, river basin management and hazard assessment
Bedload monitoring in a steep alpine stream: Results from the 2014 measurement campaign
No full textIn 2014, three high-frequency flood events (RI=1.1–1.7 yr) were investigated both in terms of sediment entrainment and bedload magnitude. The particle displacement was monitored using 250 PITtags installed in the streambed, while the Rio Cordon monitoring station enabled to characterize the bedload transport rate. In terms of displacement a clear difference was observed between the floods investigated. The near-bankfull events showed equal mobility conditions, with mean travel distance one order of magnitude higher respect to what detected in the under-bankfull event. In terms of bedload magnitude, only the near-bankfull events caused transport of coarse material to the monitoring station, depositing 113.0 t and 4.6 t. Both events peaked to 2.06m3s−1 but the bedload differs by more than one order of magnitude, proving that, currently, in the Rio Cordon the bedload appears more related to the sediment supply than to the magnitude of hydrological features
Characteristics of fluvial islands along three gravel-bed rivers of North-Eastern Italy
No full textRiver islands are defined as discrete areas of woodland vegetation located in the riverbed and surrounded by either water-filled channels or exposed gravels, exhibiting some stability and remaining exposed during bank-full flows. Islands are very important from both morphological and ecological points of view, representing the most natural condition of a fluvial system and are strongly influenced by human impacts. This study aims at analyzing the morphological and vegetation characteristics of three different types of islands (pioneer, building and established) in three distinct rivers in the NE of Italy, affected by different intensities of human pressure. The study was conducted on several sub-reaches of the Piave, Brenta and Tagliamento rivers. The first is a gravel-bed river, which suffered intense and multiple human impacts, especially due to dam building and in-channel gravel mining. The same alterations can also be observed in the Brenta River, which also presents bank protections, hydropower schemes and water diversions. On the other hand, the Tagliamento River is a gravel-bed river characterized by a high level of naturality and very low human pressures. The analyses were conducted using aerial photographs and LiDAR data acquired in 2010 in order to define and distinguish the three different island types and to obtain a characterization of ground and vegetation features. The results suggest that the fluvial islands lie at different elevations and this fact implies a different resistance capacity during flood events. Pioneer islands and building islands lie at lower elevations than established islands causing a lower capacity to survive during considerable flood events, in fact in most cases those islands types were removed by ordinary floods. Established islands lie at higher elevations and only intense and infrequent flood events (Reccurence Interval> 10-15 years) are able to determine considerable erosions. Regarding the characteristics of vegetation, we can observe a strong distinction between the three types. Established islands always exhibit the greatest vegetation height and the presence of these plants, sometimes higher than 30 m, contributes to increase the resistance and the stability of these components of fluvial systems
Analisis of Bedload Mobility in an Andean Stream
No full textHigh gradient streams form the majority of the mountain drainage network and the sediment dynamics that occur here influence the features of sediment delivered to downstream channels, particularly by controlling the timing of sediment fluxes and the amounts and size of material released to the lowland rivers. In this sense, the bedload is the transport process that regards the coarser particles, which are mobilized by rolling, sliding and saltation on the channel bed. In mountain streams, bedload can be the main sediment transport process, particularly, during flood events. Therefore, its analysis and quantification is crucial for several aspects, including hazard assessment, understanding the morphodynamics of higher order channels and managing reservoir sedimentation. However, the importance of bedload contrasts with the fact that it is difficult and impractical to monitor, especially in steep mountain streams, due to its impulsive nature. To cover this gap, different direct and indirect methods were used to investigate the bedload. One of these is the bedload tracing, which starting from the assumption that bedload transport may be understood as the result of random individual particle displacements, permitted to obtain precious data concerning sediment dynamic in mountain streams during the last decades. In this work, bedload was analyzed in an Andean stream by the use of tracing method. The site is the Estero Morales, a high-gradient stream located in the Metropolitan Region (central Chile). The streambed exhibits boulder-cascade, step-pool and plane bed morphologies with an average slope of about 9.5% and a D50 = 59 mm. The basin (27 km2) extends between 3815 and 1850 m a.s.l., hosting the San Francisco glacier (1.8 km2) that strongly affects the hydrological regime. In particular, during the melt period (December-March) the glacier ensures daily discharge fluctuations with highly variable associated bedload transport rates. In January 2016, 197 clasts equipped with Passive Integrated Transponders were seeded along the Estero Morales and their mobility monitored along a reach 745 m long. Specifically, 11 surveys were realized between January and March 2016 using a mobile antenna in conjunction with a laser rangefinder, obtaining a mean recovery rate of 25.9 %. During the study period, the tracers experienced a maximum and mean water discharge equal to 4.28 and 2.59 m3 s-1, respectively. This hydraulic forcing conditions induced the transport up to a maximum diameter of 280.0 mm, while the average diameter mobilized and average transport distance were equal to 95.0 mm and 12.0 m. The mobility observed resulted not clearly related to the hydraulic forcing, stressing the complex transport dynamics of mountain stream
Test of methodology for developing a large wood budget: A 1-year example from a regulated gravel bed river following ordinary floods
No full textWood budgeting is a current approach for quantifying the spatial and temporal variations of in-channel large
wood (LW). Despite its importance for a proper LW management, the assessment of LW budget is still a challenge
because of the complexity of LW dynamics. Focusing on a 3.7 km-long gravel-bed reach of the Piave River
(North-eastern Italy), this study proposes an attempt to assess a short-term (1 year) LW budget as a consequence
of ordinary floods (RI < 1 yr). Lateral recruitment, mobilization and deposition of LW were investigated by two
field surveys conducted at the beginning and end of the study period. The lateral input was evaluated by
measuring, positioning and tagging all standing trees (diameter ≥ 0.1 m) within a 20 m-wide buffer strip along
the floodplain and island perimeters. Bank erosion was checked by measuring the riverbanks with a differential
Global Positioning System (dGPS), during the first and second survey. Contemporarily, the position and characteristics
of each fluvial wood element (diameter ≥ 0.1 m and length ≥ 1 m) within the active channel was
determined. These short-term analyses revealed that ordinary floods are not able to induce relevant variations in
the LW abundance and redistribution of in-channel wood. Nevertheless, the results highlighted the role of riverbank
erosion in supplying LW to the active channel. Episodic inputs of LW from bank erosion were also
observed during ordinary flood events, introducing 33% (0.21 m3
·km−1
) of the total LW input amount. Results
on LW transport showed that, in the Piave River, both deposition and mobilization of LW may also occur during
low flow conditions (i.e. 14% of bankfull stage) without causing important variations in the wood quantity.
However, the low magnitude of floods was reflected in the small amount of LW involved in the input
(0.4 m3
·km−1
) and output (0.7 m3
·km−1
) processes, pointing out that sites for deposition and mobilization of LW
are conditioned by flood magnitude and morphological settings. The combination of recruitment, mobilization
and deposition processes resulted in a slight decrease of LW storage (−0.07%), while including also the anthropic
removal of LW the amount of wood storage decreased to −9.7%. Despite the short-term investigation, this
work sheds further light on LW processes, providing useful results for the management of LW in wide piedmont
rivers
Analysis of Bedload Mobility in an Andean Stream
No full textHigh gradient streams form the majority of the mountain drainage network and the sediment dynamics that occur here influence the features of sediment deliv-ered downstream. In this sense, the bedload is the transport process that regards the coarser particles, which are mobilized by rolling, sliding, and saltation on the channel bed. In mountain streams, bedload can be the main sediment transport process, there-fore, its analysis and quantification are crucial. However, the importance of bedload contrasts with the fact that it is difficult and impractical to monitor due to its impul-sive nature. Different direct and indirect methods were used during the last decades to cover this gap. One of these is the bedload tracing method. In this work, bedload tracing was used to analyze the sediment dynamics in a high-gradient Andean stream, the Estero Morales, located in central Chile. The Estero Morales stream exhibits an average slope of 14.0% and a D50 = 59 mm. The basin (27 km2) extends between 1780 and 4497 m a.s.l., hosting the San Francisco glacier (1.8 km2) that strongly affects the hydrological regime. In January 2016, 197 clasts tracers were seeded along the Estero Morales stream and their mobility was monitored by 9 surveys between January and March 2016. During this study period, the tracers experienced an average transport distance equal to 12.0 m, while the average diameter mobilized was 95.0 mm. However, the mobility observed was not clearly related to the hydraulic forcing, stressing the complex transport dynamics of a mountain stream
Coarse sediment transfer and geomorphic changes in an alpine headwater stream
No full textHeadwater sediment dynamics is very important for fluvial processes in downstream reaches, but have been poorly studied until now, and existing sediment processes are not well understood. In fact, quantifying bedload transport and assessing river changes is challenging, in mountain environments, and traditional bedload formulas tend to overestimate sediment fluxes. This work aims to analyze the coarse sediment transfer processes acting in the high-altitude headwater reach of Rio Cordon (eastern Italian Alps). Pebbles provided by Passive Integrated Transponders (PITs) were deployed in the study site and their position was periodically monitored over 20 months, in order to quantify sediment mobility (i.e., travel distances and virtual velocities) and, the bedload volumes. The geomorphic changes due to flood events were analyzed through high resolution DEMs (0.05 m cell), derived with Structure from Motion (SfM) in three different periods (October 2017, September 2018 and June 2019), allowing to compute the DEMs of Difference (DoDs). The study period was characterized by two distinct flood events, a short and intense summer rainstorm in August 2018 (recurrence interval RI > 2 years) and a severe cyclonic rainfall event (i.e. Vaia storm, RI > 50 years) in October 2018. The combination of tracer and DoDs analysis shows that ordinary flood maintained the reach in an apparent equilibrium state, with balanced volumes of erosion and deposition. Differently, the high-magnitude flood was characterized by bedload fluxes an order of magnitude higher than ordinary event and an evident net erosion. However, in both events the volumes mobilized in the headwater did not contribute significantly to the sediment fluxes delivered at the Rio Cordon basin outlet. This highlighted the reduced efficiency in sediment propagation mainly due to a marked dis-connectivity associated with the presence of a hanging valley in the middle part of the basin. This study discusses the complex dynamism of headwater streams, stressing the importance of understanding sediment fluxes and connectivity conditions that can strongly affect human activities and infrastructures even in lowland reaches
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