Search

The investigation of coarse sediment transport in wide mountain streams is very challenging because of limitations that affect the adoption of direct and indirect field monitoring techniques. Such river type represents then a stream typology for which quantitative knowledge of coarse sediment mobility, dynamics and fluxes is poor. This study uses tracer particles and a virtual velocity approach to gain insights about sediment mobility and coarse bed material transport in a wide mountain stream. Monitoring at the study area was conducted by tagging two types of tracer clasts, installing scour chains, and collecting data on water levels, transport processes, particle travel distances, and active layers over a period of 19 months and five transporting floods in the Tegnas Torrent (the Dolomites, Italy). Bed sediment was mobilized during a limited number of hydrological events (i.e. 6 to 9 per year), taking place in 50 to 400 h and inducing a total coarse sediment flux of about 1200 to 1400 m3 yr−1. The mobilization of coarse particles forming the streambed begins in response to a dimensionless shear stress (τ* c = 0.018) that is lower than thresholds commonly reported in the literature for mountain rivers and the estimated unit bed material transport was remarkably higher compared with those obtained in other mountain streams or gravel-bed rivers. The high mobility of streambed material can be related to the availability of fresh and unstructured sediment transported and deposited in the active channel by a high-magnitude event that occurred a few months before monitoring was conducted in this work, suggesting that sediment fluxes in a stream recently affected by a large flood could be considerably higher than those normally expected in a specific context. Finally, despite the existence of some application constraints, the experiment generated satisfactory results as regards the applicability of the virtual velocity approach in investigations of this stream type. This new quantitative knowledge of sediment fluxes and dynamics is also crucial for management of wide mountain streams. • Quantification of coarse sediment fluxes in wide mountain streams is very challenging. • The virtual velocity approach provides new knowledge of sediment fluxes and dynamics. • High bed material mobility was observed in the Tegnas Torrent after the Vaia storm. [ABSTRACT FROM AUTHOR]

Abstract: A new index has been developed for the hydromorphological assessment of Italian rivers. The method was designed to comply with the EU Water Framework Directive requirements, but its use can be extended to other applications in river management. The evaluation of stream morphological quality is preceded by a phase of river segmentation, consisting of an initial division of the network into river reaches with homogeneous morphological characteristics. The evaluation procedure consists of a set of 28 indicators, which were defined to assess longitudinal and lateral continuity, channel pattern, cross section configuration, bed structure and substrate, and vegetation in the riparian corridor. These characteristics are analyzed in terms of geomorphological functionality, artificiality, and channel adjustments. Indicators, classes, and the scoring system were defined based on expert judgement. The scoring system leads to the definition of the Morphological Quality Index (MQI). Application of the method to 102 river reaches covering a wide range of physical conditions and human pressures of Italian streams enabled the testing of the overall methodology and the refinement of the indicators and scores. Limitations, strengths, and the applicability of the method are also discussed in the paper. [Copyright &y& Elsevier]

The alteration of sediment transport regime represents one of the key factors controlling the response of fluvial systems to human impacts. Additional research efforts are required to define sound cause-effect relationships between the human pressures and the morphodynamic responses of sediment-starved gravel-bed rivers. This work deals with the evolution of the Parma River, a large Italian gravel-bed river affected over the last 160 year by multiple anthropogenic disturbances (i.e., land-use changes, in-channel mining and, more recently, the construction of a detention basin), which strongly limited the sediment availability to the high-plain portion of its course. The data collected allowed us to reconstruct the history of the impacts, the river morphological evolutionary trajectory, the sedimentological characteristics of the bed, and to estimate, leveraging a morphological approach, the variation in coarse sediment regime over the most recent period. After the cessation of intense in-channel mining in 1990s, the study sector located upstream from the detention basin shifted from a degrading (i.e., narrowing and incising) to a recovery phase, characterized by a mean coarse transport of 15,000–31,000 m3yr−1. These results demonstrate that the in-channel mining was the most impactful historical pressure acting on the river, and that a channel preserving some natural characteristics and passing from a condition of sediment-starvation to one of increasing sediment availability can recover an active morphodynamics within a short time period. The downstream sector was greatly affected by the reduction of sediment transfer that occurred in the 2000s caused by the construction of a detention basin. Its degradation continued over the most recent period and the coarse transport is currently very low (up to about 3000 m3yr−1 4 km downstream from the basin). This sector is characterized at the present by a morphodynamics defined as "slack", a condition induced by the complete evacuation of the mobile channel sediments, leading to extremely low bed material mobility and transport, and, consequently, to morphological degradation up to the achievement of a new quasi-equilibrium. We hypothesized that such an extreme state of alteration is relatively limited in space, affecting a gravel-bed river only at the reach scale in response to multiple overlapped impacts or because of an extremely severe perturbation (i.e., reduction) of the sediment availability and dynamics. • Human-induced sediment starvation causes alteration of gravel-bed river morphodynamics. • High armouring and low bed material load characterize a sediment-starved channel. • Increasing sediment availability can lead to a recovery of channel morphodynamics. • Multiple overlapped impacts can induce extreme morphodynamic alterations. [ABSTRACT FROM AUTHOR]

Abstract: The timing and extent of the morphological changes that occurred in the last 200years 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 2m — mostly from gravel mining — has 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 >10–15years) appear to determine substantial island erosion, and therefore the proportion of island vs. channel area fluctuates depending on flood history. [Copyright &y& Elsevier]

Large floods may produce remarkable channel changes, which determine damages and casualties in inhabited areas. However, our knowledge of such processes remains poor, as is our capability to predict them. This study analyses the geomorphic response of the Nure River (northern Italy) and nine tributaries to a high-magnitude flood that occurred in September 2015. The adopted multi-disciplinary approach encompassed: (i) hydrological and hydraulic analysis; (ii) analysis of sediment delivery to the stream network by means of landslides mapping; (iii) assessment of morphological modifications of the channels, including both channel width and bed elevation changes. The spatial distribution of rainfall showed that the largest rainfall amounts occur in the upper portions of the catchment, with cumulative rainfall reaching 300 mm in 12 h, and recurrence intervals exceeding 100–150 years. The unit peak discharge ranged between 5.2 and 25 m 3  s −1  km −2 . Channel widening was the most evident effect. In the tributaries, the ratio between post-flood and pre-flood channel width averaged 3.3, with a maximum approaching 20. Widening was associated with channel aggradation up to 1.5 m and removal of riparian vegetation. New islands formed due to the fragmentation of the former floodplain. In the Nure River, the average width ratio was 1.7, and here widening occurred mainly at the expenses of islands. Bed level dynamics in the Nure were varied, including aggradation, incision, and overall stability. The flood geomorphic effectiveness was more pronounced in the middle-higher portions of the basin. Planimetric and elevation changes were well correlated. Regression analysis of the relationship between widening and morphological/ hydraulic controlling factors indicated that unit stream power and confinement index were the most relevant variables. The study provides useful insights for river management, especially with regard to the proportion of the valley floor subject to erosion and/or deposition during large events. [ABSTRACT FROM AUTHOR]

Our capability to accurately predict at the local scale the morphological changes in a mountain river affected by a severe flood is currently limited. Given the importance of including geomorphological hazards in flood risk evaluation, some improvements in this direction are required. In fact, on the basis of the controlling factors typically considered in the literature, that are the unit stream power (ω pk) and the confinement index (CI), a relatively broad spectrum of width ratios (WR = ratio between channel width after a flood and channel width before a flood) occurring in response to major floods is commonly observed in streams. We extended previous research focused on parameters that influence the morphological response of a channel to a severe flood by investigating the role played by the type of sediment transport process that occurs in a mountain river. In particular, we explored whether a relationship exists between debris flood (i.e. a condition characterised by extremely high bedload) and intense channel widening. The case study was the Cordevole River (the Dolomites, Italy) and four of its tributaries, which were affected by the severe flooding induced by the Vaia storm in October 2018. WR was determined at the sub-reach scale, and ω pk was calculated with consideration for the flood peak discharge ascertained via hydrological modeling. Despite the significant relationships found among WR, ω pk and CI, 21 sub-reaches (out of 144) of the studied stream network widened intensely reaching a WR higher than approximately 4, often to a substantially larger extent than predicted by statistical models. Such sub-reaches were certainly or probably affected by debris floods, leading us to infer that debris floods can induce widening processes that are more intense than those occurring in response to water flows. Having said that, debris floods do not automatically imply very intense widening. In addition to hydraulic and morphological constraints, the sediment transport processes possibly taking place at a sub-reach of a mountain river should be regarded as another driver of channel modifications during high-magnitude hydrological events. • Intense widening occurred along mountain rivers in response to a large flood. • Different types of fluid gravity flow drive various morphological channel responses. • Debris flood can induce more intense widening than that caused by water flow. [ABSTRACT FROM AUTHOR]