Beta-diversity and stressor specific index reveal patterns of macroinvertebrate community response to sediment flushing

Abstract Anthropogenic increase of fine sediment loading is one of the main pressures for rivers worldwide. Particularly, Alpine streams are increasingly facing this issue due to sediment flushing operations from hydropower reservoirs, aimed at recovering storage for preserving electricity generation. Although Controlled Sediment Flushing Operations (CSFOs) are becoming increasingly frequent, ecological indicators to adequately assess and monitor their impact on the stream ecosystem have been poorly developed. In this work, we aimed to perform a screening of currently available biomonitoring tools to evaluate the CSFO effects on the riverine biota and adequately assess its recovery, starting from the recognition of the main ecological mechanisms triggered by the mentioned activities on benthic macroinvertebrate communities. We used two independent datasets concerning two reservoirs in the central Italian Alps to investigate the temporal effects of CSFOs repeated for four consecutive years (case-study I), and the impact of a single CSFO at a seasonal scale through a before/after-control/impact approach (case-study II). Initially, we quantified the CSFO impact on the richness and beta-diversity of macroinvertebrate communities by combining multivariate and univariate statistical techniques. Then, we compared the performance of the Siltation Index for LoTic EcoSystems (SILTES), recently developed for detecting siltation impact in Alpine streams, with that of the generic index currently adopted to assess the ecological status (sensu Water Framework Directive) of the Italian rivers, and of another sediment-specific index, but developed for a different bio-geographical area. The analysis of the two case-studies demonstrated that the nestedness (i.e. taxa loss) is the primary source of biological impairment caused by CSFOs. Moreover, we found that SILTES was more effective than the other indices because of its strong correlation with the nestedness, and since it properly discriminated impaired and pristine conditions, at both multi-annual and seasonal scale. In the first case-study, a threshold in the temporal trend of this index was detected, indicating a recovery within three months. In the second one, SILTES showed a recovery to pre-event seasonal values after nine months from the CSFO, due to larger and more persistent sediment deposition. This study demonstrates that SILTES could be adopted as a benchmark to improve the management of CSFOs from an ecological viewpoint. Our findings can be extended to the management of other sediment-related activities affecting mountainous streams worldwide, and, more generally, the adopted approach can be replicated for developing new ecological tools to manage other disturbances to river environments

Monitoring and Management of Inland Waters: Insights from the Most Inhabited Italian Region

Monitoring of freshwaters allows the detection of the impacts of multiple anthropic uses and activities on aquatic ecosystems, and an eco-sustainable management of natural resources could limit these impacts. In this work, we highlighted two main issues affecting inland waters, referring to findings from the most inhabited Italian region (Lombardy, approximately 10 M inhabitants): the first issue is lake pollution by old generation pesticides, the second is river development for hydropower. In both cases, some management strategies reducing the anthropic impacts on freshwaters were discussed: organic farming and biocontrol as an alternative to diffuse pollution by agrochemicals; environmental flows and controlled sediment flushing operations to limit the hydropower impact on rivers. Although the two mentioned issues were discussed separately in this paper, the management of water resources should be carried out in a comprehensive way, accounting for the multiple impacts affecting freshwater ecosystems, including those related to the climate changes

Towards ecological flows: status of the benthic macroinvertebrate community during summer low-flow periods in a regulated lowland river

Climate change along with the increasing exploitation of water resources exacerbates low-flow periods, causing detrimental effects on riverine communities. The main mitigation measure currently adopted to counteract hydrological alterations induced by off-stream diversion is the release of minimum flows (MFs), even if within the European Union Water Framework Directive an upgrade towards ecological flows is urgently required to achieve good ecological status (GES). In this study, we investigated the temporal evolution of the benthic macroinvertebrate community in an Italian-regulated lowland river (Ticino River) to clarify the ecological effects of summer low flows, and we evaluated the current MFs in the perspective of meeting GES standard. Biomonitoring was carried out for four consecutive years (2019-2022), in a river site immediately below a large off-stream diversion. The four study years were characterized by different streamflow patterns, thus allowing us to compare the temporal trajectories of the community under different flow conditions. Moreover, the interruption of the low-flow periods due to overflow spilled by the upstream dam gave us the opportunity to assess the effects of experimental flow peaks. Contrary to the expectation, the macroinvertebrate assemblage kept almost unvaried across the years, showing great resistance and resilience to hydrological changes. Even in extraordinarily dry 2022, the community composition varied only slightly, with a reduction of mayflies and an increase of mollusks. However, a deterioration of the ecological status below GES standard was recorded that summer, indicating the need for an upgrading of the current MFs. This upgrade would include experimental flow peaks in critical periods, which act as intermediate disturbances, enhancing community richness, diversity, and overall quality, as well as compliance with a threshold of an index specifically developed for the hydrological pressure

Experimental investigation of 3D separation in a cylinder rough flat plate junction

In this paper the results of an experimental study concerning the separated flow region which develops upstream from a cylindrical obstacle vertically placed on a rough surface and subjected to a fully developed turbulent boundary layer, will be analyzed. This region is characterized by a strongly time-variant vortex system, whose main features were described by a great number of authors through velocity and pressure measurements and flow visualizations, for a wide range of Reynolds number. The experimental results were interpreted using the topological approach by Hunt et al. (1978), Baker (1980), Dargahai (1989). However for a turbulent inlet boundary layer the number of vortices observed is still a controversial issue. This type of flow occurs in several technical applications, e.g. local scouring at bridge piers. To this avail bed roughness could play an important role in defining the vortex system structure and stability. In this paper, the instantaneous characteristics of the vortical structure were highlighted by the flow visualizations. LDV measurements of the longitudinal and vertical velocity components were carried out for three different Reynolds numbers. In one of these conditions, the crosswise mean velocity component was computed and a statistical analysis of the instantaneous velocity signals was performed. Furthermore by applying the topological concepts of critical point and sectional streamline the main features of the separated flow were identified

Experimental study of the scour regimes downstream of an apron for intermediate tailwater depth conditions

The local scour of a non-cohesive bed due to a 2-D submerged horizontal jet is investigated experimentally in presence of a protection apron. Previous researches conducted without protection apron demonstrate that, when the tailwater depth is either deep or shallow, the equilibrium state characteristics of the scoured bed profile are mainly a function of the densimetric Froude number. However, when the submergence is between these two extremes, at fixed Froude number, three different scour regimes are possible. For relatively shallow tailwater depths, the jet mainstream directs towards the free surface (surface jet scouring regime), determining shallow and elongated scour profiles. For relatively large tailwater depths, the jet remains attached to the channel bottom (bed jet scouring regime), leading to deeper and shorter scour profiles. For intermediate conditions, the flicking of the jet between the erodible bed and the water free-surface is possible. When this instability occurs, the shape of the scour hole rapidly changes as a response of the jet mainstream position (bed-surface jet scouring regime). This paper aims to give an experimental description of the three mentioned regimes when a protection apron partly reduces the action of the flow on the loose bed. Scour hole profile evolution and velocity profile measurements obtained by LDA and ADV velocimetry are discussed

Experimental study of the scour regimes downstream of an apron for intermediate tailwater depths

Local scour, Non-cohesive bed, Protection apron, Submerged jet, Open channel flow, LDA mesurements, ADV measurements, Visualization.Local scour due to 2-D submerged horizontal jets is a well-known topic of hydraulic research. When the jet is directly injected on the erodible bed and the tailwater depth is either \u201cdeep\u201d or \u201cshallow\u201d, the equilibrium state characteristics of the scoured bed profile are mainly a function of the densimetric Froude number. If the submergence is between these two extremes, for fixed Froude number, three different scour re-gimes are possible, depending on the jet mainstream position. This paper integrates previous work by the Au-thors concerning the experimental description of the three mentioned regimes, in presence of a protection apron. The results of the investigation are also intended as a dataset to validate numerical models able to simulate the interaction between free-surface flows and non-cohesive granular beds

Flusso turbolento indotto da un getto verticale piano immesso dal fondo di un recipiente a pelo libero di modesta profondità

L'interazione di un getto verticale piano immesso dal fondo di una vasca con la superficie libera ed il campo di moto che conseguentemente si sviluppa, dipendono dalla velocità del getto in rapporto al tirante liquido nel recipiente. Fissati il tirante e la larghezza della luce da cui il getto viene immesso, se la velocità del getto è sufficientemente elevata, esso determina un rigonfiamento della superficie libera. Per velocità inferiori ad una certa soglia, questo rigonfiamento rimane stabile, mentre per velocità superiori esso oscilla con regolarità in direzione longitudinale. Flussi di questo tipo, caratterizzati da forte interazione tra getto e superficie libera, sono stati simulati dagli autori utilizzando la tecnica Lagrangiana SPH (Smoothed Particle Hydrodynamics), che permette di calcolare flussi rapidamente variati con superficie libera mobile, difficilmente trattabili per altra via. Allo scopo di incrementare le prestazioni del codice di calcolo SPH di-sponibile, è stato implementato un modello di turbolenza, ispirato ai modelli di similarità di scala sviluppati per la Large Eddy Simulation e basato sulla funzione interpolante SPH impiegata come operatore di media per le velocità. Per un caso di getto formante rigonfiamento stabile della superficie libera, i risultati ottenuti con questo modello vengono messi a confronto sia con misure LDA di velocità, che con risultati numerici ri-cavati con la tecnica Euleriana dei volumi finiti

Experimental study of the scour regimes downstream of an apron for intermediate tailwater depths

ABSTRACT: Local scour due to 2-D submerged horizontal jets is a well-known topic of hydraulic research. When the jet is directly injected on the erodible bed and the tailwater depth is either “deep” or “shallow”, the equilibrium state characteristics of the scoured bed profile are mainly a function of the densimetric Froude number. If the submergence is between these two extremes, for fixed Froude number, three different scour regimes are possible, depending on the jet mainstream position. This paper integrates previous work by the Authors concerning the experimental description of the three mentioned regimes, in presence of a protection apron. The results of the investigation are also intended as a dataset to validate numerical models able to simulate the interaction between free-surface flows and non-cohesive granular beds