Browsing by Author "Pellegrini, Giacomo"

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    Surprising suspended sediment dynamics of an alpine basin affected by a large infrequent disturbance
    (2023) Pellegrini, Giacomo; Mao, Luca; Rainato, Riccardo; Picco, Lorenzo
    In 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.
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    The response of a mountain drainage basin to extreme events: the impact of Vaia storm on Rio Cordon (Dolomites, Italy)
    (2024) Pellegrini, Giacomo; Mao, Luca; Picco, Lorenzo
    Current literature is limited in understanding the responses of mountain basins to large disturbances, especially when it comes to integrating analyses of sediment transport, geomorphic changes, and topographical conditions, as these aspects are often examined at varying spatial scales. This manuscript explores the impact of the Vaia storm (27-30 October 2018) on the Rio Cordon Basin (Dolomites, Italy). Integrating data from three studies conducted at different spatial scales, it examines hydrological, geomorphological, and sedimentological responses of the basin to the large infrequent event. The findings encompass changes in channel network, step-pool morphology evolution, and sediment flux alterations. Streambed remobilization, boulder mobility, incision, and lateral erosion were observed. The step-pool morphology experienced disruption but recovered to stability within two years. Surprisingly, suspended sediment fluxes and yields after two years form the event (42 t km-2 years-1 ) were consistent with pre-Vaia conditions (40 t km-2 years-1 ). The integration of pre- and post-event data proved valuable for understanding the basin's responses to extreme events. Based on current conditions, Rio Cordon appears to be returning to a pre-event state, establishing a new dynamic equilibrium, as the Vaia event induced morphological changes while sediment fluxes and step-pool stability remained in line with pre- event conditions.