1,721,038 research outputs found
The role of vegetation and large wood on the topographic characteristics of braided river systems
No full textRiparian vegetation and large wood play a crucial role in shaping rivers. On the one hand, vegetation tends to increase bank stability, reduce channel width and reduce the braiding index in gravel-bed rivers. Conversely, large wood tends to increase channel dynamics and promote avulsions. The effects of vegetation and large wood have been rarely simulated together in flume experiments. In this paper we present a series of experimental runs conducted in a large flume facility, using cylindrical wooden dowels and alfalfa seedlings to represent logs and vegetation, respectively, in order to investigate their role on determining the topographical nature of braided river systems. A terrestrial laser scanner was used to measure in detail the topography of the channels and bars, and distribution of the elevations and second-order structure functions were used to explore the topography that large wood and vegetation imposed on the braided pattern in the flumes. Results show that vegetation reduced the braiding index and produced wider and deeper channels. The standard deviation of the bed elevations was higher than in flumes without vegetation. Large wood alone was not effective in changing the braiding index or the standard deviation of the elevations. When vegetation and large wood acted together in the flume, the braiding index reached a minimum value, and the skewness of the distribution of elevation increased, revealing shallower pools if compared with runs conducted with only vegetation
Rock glaciers and paraglacial features influence stream invertebrates in a deglaciating Alpine area
Full text linkThis is the pre-peer reviewed version of the following article: Brighenti, S, Tolotti, M, Bertoldi, W, Wharton, G, Bruno, MC. Rock glaciers and paraglacial features influence stream invertebrates in a deglaciating Alpine area. Freshwater Biology. 2020; 00: 1– 14. https://doi.org/10.1111/fwb.13658, which has been published in final form at https://doi.org/10.1111/fwb.13658. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions
Thermal patterns in a proglacial pond create Windows of Opportunity for periphyton growth (Cevedale glacier, Italy)
Full text linkIn high mountain areas, deglaciation is the most evident effect of anthropogenic climatic changes. Glacier retreat is inducing worldwide an increase of both number and size of proglacial lakes and ponds, i.e., lentic water bodies located in the proglacial area and directly linked to the glacier activity: the depressions carved in the land surface allow meltwater impoundment and accumulation of glacier sediment. Over the past decades, glacier-fed lakes have become an increasingly represented ecosystem in the Alpine landscape. However, their ecological characteristics are only partially known. Glacial runoff determines cascade effects in glacier-fed standing waters. It influences both water temperature, by delivering cold meltwater to the system, and water transparency, because of the high amount of inorganic suspended solids (so called “glacial flour”) that determine high water turbidity. Therefore, proglacial lakes are highly selective habitats, where planktonic communities are quantitatively scarce and taxonomically simplified. On the other hand, given the low input of allochthonous organic matter from bare proglacial forefields, benthic primary producers (periphyton) are the major autochthonous carbon source sustaining food webs in glacially fed water bodies. Studies on glacial streams show that periphyton growth is concentrated in “Windows of Opportunity” (WOs), mainly occurring in periods of reduced glacial runoff, i.e., autumn. In the euphotic zone along the littoral area of proglacial lakes, local conditions can allow algal growth (e.g., cyanobacteria), and the abrasive impact of glacial flour is low due to scarce water turbulence. Furthermore, in lentic proglacial ecosystems, periphyton ecological niches appear to be influenced also by water thermal stratification dynamics. Previous studies observed different mixing patterns in proglacial and clear mountain lakes (i.e., without glacial influence); high-altitude ponds (surface area < 2 ha) are expected to show even different patterns of response to physical environmental setting. To better understand the link between thermal dynamics and ecology of proglacial lentic systems, we investigated temperature dynamics in the water column of a proglacial pond located in the Eastern Italian Alps (South Tyrol, Italy) and compared them with density and taxonomic composition of PPNW 2024. Universitat de Girona (Spain) benthic diatom communities, which are key components of littoral periphyton in lakes and are useful bioindicators of environmental changes. Diatoms are eukaryotic photosynthetic microalgae, characterised by a cell wall composed of silica (frustule), whose morphological characteristics are used for taxonomical identification. Specific aims of the study were: (i) to characterise the temperature dynamics in a shallow proglacial pond; (ii) to investigate how thermal dynamics can influence the presence of WOs for periphyton growth. The proglacial pond is located at 2850 m a.s.l. in the Martell valley (Stelvio National Park, CE Italian Alps). It is moraine-dammed and originated from the retreat of the Cevedale glacier about 10 years ago. Its surface area is about 4270.5 m2 with a maximum depth of around 3 m. In the ice-free seasons 2022 and 2023, we installed water level sensors and performed water discharge measurements at the pond outlet with the salt dilution method, to build a flow rating curve and estimate outlet and inlet discharge time series. In summer 2023, we installed two buoys, one in the upstream part of the pond and one in the downstream part. Each buoy was equipped with 5 temperature sensors, located at 0, 0.2, 0.4, 0.8, 1.6 m depth, recording water temperature at 5-minute intervals. We applied the CE-QUAL-W2 model, a 2-D hydrodynamic laterally averaged model, to reconstruct the temperature time series in the water column for both ice-free seasons 2022 and 2023. We calibrated the parameters of the model based on field buoy data. Meteorological data and discharge time series were used as boundary conditions for the model. We developed a simplified numerical model to estimate the inflow water temperature (i.e., glacial runoff running on debriscovered ice) as a function of air temperature and solar radiation. Preliminary results show good agreement between the observed and modelled temperature data (RMSE < 1.5°C). During the Alpine glacial summer, we observed periods of pronounced daily thermal stratification. In these periods, shallow layers showed daily fluctuations, while deeper layers were colder. Total mixing and cooling of the water column followed intense precipitation events, with lower air temperature and solar radiation. In 2022 and 2023, we analysed the benthic diatom communities collected from a known area of colonised substrata (stones covered by a layer of consolidated sediment). In the laboratory, we eliminated the organic matter in the samples by chemical oxidation, to allow the morphological observation of diatom frustules. We equalised the sample volumes at 6 ml and added an aliquot (1 ml) of solution containing a known concentration of divinylbenzene microspheres, which served as reference to compare diatom densities in the different samples. Permanent diatom mounts were prepared, and diatom frustules and microspheres were counted under the optical microscopy. Diatom communities in the Cevedale proglacial pond reached higher density values (1-43 and 2.8-404.9 × 103 N valves/cm2, respectively) than in glacier-fed streams investigated in the same geographical area (Vulcano 2020, unpublished data). Moreover, we observed a density peak in August 2022 (404.9 × 103 N valves/cm2), and not in autumn as expected. In all samples, the community was numerically dominated by the pioneer species Achnanthidium minutissimum s.l. The observed diatom density patterns suggest that periphyton PPNW 2024. Universitat de Girona (Spain) growth in the proglacial pond can be sustained also in periods of high glacial runoff. Accordingly, the model results suggest the presence of additional temperature-driven WOs for periphyton growth during the Alpine summer, with respect to the ones described in glacier-fed streams. The presence and temporal extension of the WOs in the proglacial pond depend on meteorological conditions, as thermal gradients form during dry and warm periods. In a climate change perspective, this implies that colonisation processes in the periphyton of newly formed proglacial ponds may be accelerated by prolonged periods of drought, high air temperatures and increased glacial runoff. Consequently, the natural ecological evolution of proglacial ponds may be accelerated by global warming
Interaction between migrating bars and bifurcations in gravel-bed rivers
No full textIn the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations. Laboratory experiments are carried out to study a
Y-shaped bifurcation with fixed banks and erodible bed composed of well-sorted sand. The problem is also analyzed by developing a theoretical, one-dimensional model. Results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node, which are strictly related to bar migration. The effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration
speed. Four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers. The theoretical predictions are in good qualitative agreement with the experimental observation
Morphometric properties of alternate bars and water discharge: A laboratory investigation
Full text linkThe formation of alternate bars in straightened river reaches represents a fundamental process of river morphodynamics that has received great attention in the last decades. It is well-established that migrating alternate bars arise from an autogenic instability mechanism occurring when the channel width-to-depth ratio is sufficiently large. While several empirical and theoretical relations are available for predicting how bar height and length depend on the key dimensionless parameters, there is a lack of direct, quantitative information about the dependence of bar properties on flow discharge. We performed a series of experiments in a long, mobile-bed flume with fixed and straight banks at different discharges. The self-formed bed topography was surveyed, different metrics were analyzed to obtain quantitative information about bar height and shape, and results were interpreted in the light of existing theoretical models. The analysis reveals that the shape of alternate bars highly depends on their formative discharge, with remarkable variations in the harmonic composition and a strong decreasing trend of the skewness of the bed elevation. Similarly, the height of alternate bars clearly decreases with the water discharge, in quantitative agreement with theoretical predictions. However, the disappearance of bars when discharge exceeds a critical threshold is not as sharp as expected due to the formation of so-called "diagonal bars". This work provides basic information for modeling and interpreting short-term morphological variations during individual flood events and long-term trajectories due to alterations of the hydrological regime
Analysis of the large scale dynamics of the Tagliamento River (Italy)
No full textIn the last century human activities have strongly affected the natural river behaviour often constraining complex
patterns into more manageable single-thread channels with the aim to enhance conveyance of water flow and
floods. In much of the developed world, the remarkable degree of spatio-temporal heterogeneity characterising
riverine landscapes has been masked by a long history of river engineering. Floodplain reaches, which exhibit
the highest heterogeneity in their natural state, have been the most severely altered.
The present work is focused on braided rivers whose multiple-thread pattern is related to a highly complex
eco-morphodynamism. The Tagliamento River (Italy), even though it has undergone morphological changes
due to human activities, still displays semi-natural morphodynamic behaviour that would have characterised
the pristine state of the lower valley sections of several alpine rivers. It, therefore, offers the rare opportunity to
investigate the intrinsic dynamics of braided streams.
This study is motivated by the need to recognize and maintain this eco-morphodynamism which prevents
habitat degradation and is therefore crucial to the maintenance of ecological integrity.
An analysis of island and active corridor dynamics is presented for a 16 km island-braided reach of the
gravel-bed Tagliamento River based upon information extracted from three map and 9 aerial photograph
sources, encompassing the period from 1803-present.
The active corridor width showed a general decline over the study period but with some recent widening.
Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50%
per annum. Very few island surfaces were found to persist for more than 24 years.
Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island
area to active corridor area remained relatively constant at around 0.08 and supported a consistently high
bankfull shoreline to downstream length ratio of around 6 km·km-1
Stepping stones or shelter areas? The role of rock glaciers in context of Alpine deglaciation
No full textDeglaciation is causing significant consequences on the hydrology of the Alpine watersheds and their
lowland areas, which will be characterized by an increasing variability and hydrological stochasticity due to
the loss of glaciers and the reduction of snow cover. On the other hand, given the greater climatic resilience
of permafrost, this component of the cryosphere will play an increasingly important role in the hydroecology
of high altitude basins. Rock glaciers are the most evident form of alpine permafrost and are the source
of streams whose baseflow derives from the melting of interstitial ice. While streams fed by glacial melt
(kryal), snow-melt and precipitation (rhithral) and groundwater (krenal) have been extensively investigated,
little is known about the ecology of rock-glacier fed streams, despite their peculiar hydromorphological,
physical and chemical characteristics . We investigated a high altitude Alpine basin in the South Tyrolean
Alps (Solda Valley, Ortles -Cevadale Massif) to illustrate the ecological peculiarities of the streams fed by
rock glacier in comparison with high altitude streams of different origins: glacial, spring and mixed. We
sampled streams of different origin but with comparable geology and altitude within two sub-basins, for a
total of 13 stations sampled in the three key moments of the summer season: peak of snow melting, ablation
and recession. Preliminary community analysis of invertebrates, albeit with low taxonomic detail (genus
/ family), shows how the rock glacier streams, which share the same altitude of glacial-fed streams, are
similar in composition, abundance and diversity to the streams of the valley floor. Therefore, a fundamental
question arises, inherent the biogeographic role of rock glaciers in the context of deglaciation: are rockglacier
fed streams stepping stones used by invertebrates to colonize high altitude areas, or rather hotspots
of diversity in which cold-stenotherm taxa will they shelter after glaciers will melt and disappear
Implications of channel processes for juvenile fish habitats in Alpine rivers
No full textWe adopt a multidisciplinary approach
toward the quantitative assessment of juvenile fish
habitats in Alpine rivers using analytical modeling.
The study focuses on braided and single-thread
channel configurations together with their associated
hydrodynamic patterns.Adistinct difference between
flows in these channels is the number and spatial
arrangement of recirculation zones. These are due to
the separation of flow from the river banks and result
in a higher retention of flow in braided channels. Braided channels were also shown to provide more
favourable shelter and nursing conditions for fish
larvae and juveniles by mitigating high velocities
during floods, by maintaining relatively shallow areas
of flow, and by significant adjustments in the thermal
regime. A historical analysis revealed a significant
reduction of braided reaches along Alpine rivers that
have most likely led to a significant degradation of the
fish fauna
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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