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Preliminary evaluations of the impact of a pumped storage plant on the hypolimnetic circulation of a deep subalpine italian lake
No full textLake Iseo is an eutrophic lake located in northern Italy, having a surface of 61 km2 and a volume of about 8 billion m3. One of the most important hydrodynamic processes characterizing this deep subalpine lake is the water vertical overturn at the end of the limnological winter. This process has important physical, chemical and biological effects on the whole lake ecosystem. The gradual increase of average winter temperatures over the last decades has considerably hindered this process and, as a consequence, an increasing oxygen depletion has affected the hypolimnion of lake Iseo to the point that an evolution toward meromixis has been postulated. Although at different extent, the same problem has been observed in different lakes in the world and, when their dimension is not too large, forced circulation projects have sometimes been implemented as a tentative counteracting measure. Although the considerable dimension of the hypolimnion of lake Iseo poses an economic issue to this type of intervention, a recent debate regarding the possible construction of a pumped storage plant suggests the opportunity to use the withdrawal-discharge cycle of this plant as a way to force hypolimnetic circulation. This opens the way to a wide set of environmental questions, as a consequence of different possible plant layouts and production strategies. In this contribution we present some preliminary evaluations on the effectiveness of the operation of such a plant on lake circulation by using a 3-D model
Le onde interne del lago d’Iseo: osservazione sperimentale e modellazione numerica.
No full textA partire dal 2009 il lago d’Iseo è stato oggetto di un’intensa campagna sperimentale volta a monitorare le principali forzanti agenti sul sistema e a quantificarne la risposta. Si voleva in primo luogo verificare se anche in questo lago fossero presenti oscillazioni interne indotte dal vento, come già documentato per altri laghi dell’area alpina e prealpina. Tale attività, mirata a comprendere gli aspetti fondamentali dell’idrodinamica di questo bacino, ha costituito un primo passo preliminare finalizzato ad affrontare le criticità che riguardano lo stato di qualità delle sue acque.
La misura ad elevata frequenza della temperatura in diversi punti e profondità del lago, in particolare, ha messo per la prima volta in luce intensi fenomeni oscillatori (onde interne) che si sviluppano a scala di bacino nei primi 50 m di colonna d’acqua e che sono in primo luogo il risultato dell’azione del vento che agisce periodicamente sulla superficie del lago. Gli effetti di questo moto oscillatorio sono fondamentali per la miscelazione delle acque del lago e ne determinano il campo delle correnti, come verificato con misure di tipo lagrangiano.
Nel presente lavoro viene descritta la struttura spazio-temporale delle onde interne osservate sul lago d’Iseo in condizioni stratificate, sulla base dell’interpretazione dei dati sperimentali acquisiti e dei risultati numerici ottenuti utilizzando un modello numerico modale a layer. La risposta del bacino all’azione del vento appare caratterizzata da una struttura modale dominante del primo tipo (V1H1), alla quale si sovrappongono solo occasionalmente dei modi di ordine superiore. Il verificarsi di tali movimenti è stato quindi interpretato ponendo in relazione la struttura spazio-temporale del vento e quella delle oscillazioni naturali del bacino, ed evidenziando lo svilupparsi di condizioni di risonanza
Data set for hydrodynamic lake model calibration: A deep prealpine case
No full textHydrodynamic modeling of lakes requires an extensive amount of data that must often
be measured far from the shore and throughout the water column. Accordingly, long-term
comprehensive data sets for lakes are relatively rare. Lake Iseo is a deep prealpine lake,
which we have monitored since 1995 on a monthly basis, collecting high-resolution data
since 2011. These measurements were complemented with additional time series from
surrounding meteorological stations and a comprehensive data set has been gathered for the
period 1995–2012. This can be used, among other things, to test the capabilities of models
to investigate the dynamics of a deep lake
Steady-state distributed modeling of dissolved oxygen in data-poor, sewage dominated river systems using drainage networks
No full textThe application of environmental flow regulations to small hydropower plants in alpine areas
Full text linkwithin a framework of conflicting needs and under water scarcity scenarios, environmental issues are assuming more and more relevance in water cycle management. An important role in this context is played by the minimum discharge release downstream of diversion structures, the so-called “environmental flow”, aiming at assuring acceptable ecological conditions along natural watercourses. In this paper the regulation criteria adopted in several European alpine countries will be briefly discussed. The implementation of environmental flow regulations to small hydropower plants in the province of Brescia (Northern Italy), which is one of the most relevant Italian district in terms of number of hydropower plants, installed capacity and energy production, will be presented. In particular, the prevailing environmental flow release devices and their main technical characteristics, operating features and design criteria are briefly discussed. The introduction of environmental flow legislation entails both benefits and disadvantages which are often difficult to quantify. This paper gives a contribution to this costs-benefits analysis, by estimating the loss of energy producibility for a small alpine hydropower plant, related to the enforcement of environmental flow regulations and to the design criteria of the release devices
Contribution of Chemical Stratification to the Extent of Water Renewal in a Deep Lake
No full textAbstract Turnover events in deep lakes are crucial for water quality and trophic balance. If on one side there is wide consensus that climate change will strengthen the thermal stability of the epilimnion, on the other a smaller number of studies have investigated the stabilization operated by the chemical stratification, and the conditions under which it may occasionally fail. In this paper we investigate the role of thermal and trophic‐induced stratification on the deep mixing of Lake Iseo, a pre‐alpine lake that used to be monomictic in the past. The coupling of a site–specific density equation to 9 years of high‐resolution temperature and wind data made it possible to compute the time series of the resistance to upwelling of deep waters and to quantify the role of the chemical stability. We show that the presence of calcium, bicarbonate and sulfate ions in the bottom layers, and the related increase in stability of about 103 Jm−2, has hindered the occurrence of a deep overturn, even in presence of a thermally uniform water column. A full upwelling was thus inhibited in 3 of the observed winters, when it would otherwise have occurred. On the other hand, whereas our computations show that a destabilizing effect of possible future stronger winds is unlikely, the progressive deep‐water warming that followed the isolation of the monimolimnion has strongly decreased the lake's thermal stability, counteracting the chemical stratification and progressively increasing the probability of a future deep overturn
Dam-break modeling in alpine valleys
No full textDam-break analysis is of great importance in mountain environment, especially where reservoirs are located upstream of densely populated areas and
hydraulic hazard should be assessed for land planning purposes. Accordingly, there is a need to identify suitable operative tools which may differ from the ones used in flat flood-prone areas. This paper shows the results provided by a 1D and a 2D model based on the Shallow Water Equations (SWE) for dam-break wave propagation in alpine regions. The 1D model takes advantage of a topographic toolkit that includes an algorithm for pre-processing the Digital Elevation Model (DEM) and of a novel criterion for the automatic cross-section space refinement. The 2D model is FLO-2D, a commercial software widely used for flood routing in mountain areas. In order to verify the predictive effectiveness of these numerical models, the test case of the Cancano dam-break has been recovered from the historical study of De Marchi (1945), which provides a unique laboratory data set concerning the consequences of the potential collapse of the former Cancano dam (Northern Italy). The measured discharge hydrograph at the dam also provides the data to test a simplified method recently proposed for the characterization of the hydrograph following a sudden dam-break
Study of tributary inflows in Lake Iseo with a rotating physical model
No full textThe influence of Coriolis force on the currents of large lakes is well acknowledged; very few contributions, however, investigate this
aspect in medium-size lakes where its relevance could be questionable. In order to study the area of influence of the two major tributary
rivers in Lake Iseo, a rotating vertically distorted physical model of the northern part of this lake was prepared and used, respecting both
Froude and Rossby similarity. The model has a horizontal length scale factor of 8000 and a vertical scale factor of 500 and was used both
in homogeneous and in thermally stratified conditions. We explored the pattern of water circulation in front of the entrance mouth for dif-
ferent hydrologic scenarios at the beginning of spring and in summer. We neglected the influence of winds. The primary purposes of the
model were twofold: i) to increase our level of knowledge of the hydrodynamics of Lake Iseo by verifying the occurrence of dynamical
effects related to the Earth’s rotation on the plume of the two tributaries that enter the northern part of the lake and ii) to identify the areas
of the lake that can be directly influenced by the tributaries’ waters, in order to provide guidance on water quality monitoring in zones of
relevant environmental and touristic value. The results of the physical model confirm the relevant role played by the Coriolis force in the
northern part of the lake. Under ordinary flow conditions, the model shows a systematic deflection of the inflowing waters towards the
western shore of the lake. The inflow triggers a clockwise gyre within the Lovere bay, to the West of the inflow, and a slow counter-clockwise
gyre, to the East of the inflow, that returns water towards the river mouth along the eastern shore. For discharges with higher return period,
when only the contribution by Oglio River is relevant, the effect of the Earth’s rotation weakens in the entrance zone and the plume has a
more rectilinear pattern, whilst in the far field the current driven by the inflows keeps moving along the western shore. On the basis of
these results one could expect that the north-western part of the lake between Castro and Lovere, although not aligned with the tributaries’
axes, is more sensitive to accumulation effects related to river-borne pollution. The results obtained with the physical model are critically
compared with data obtained from different sources: the trajectory of a lagrangian drogue; a map of reflectivity data from the lake floor;
a map of water turbidity at the intrusion depth. The findings are also confirmed by the results of a 3D numerical model of the lake
Modellazione numerica e sperimentale dei primi istanti di moto conseguenti alla rapida rimozione di uno sbarramento
No full textNumerical simulations of mountain winds in an alpine valley
Full text linkThe meteorological model WRF is used to investigate the wind circulation in Valle Camonica, Italy, an alpine valley that includes a large subalpine lake. The aim was to obtain the information necessary to evaluate the wind potential of this area and, from a methodological point of view, to suggest how numerical modeling can be used to locate the most interesting spots for wind exploitation. Two simulations are carried out in order to analyze typical scenarios occurring in the valley. In the first one, the diurnal cycle of thermally-induced winds generated by the heating-cooling of the mountain range encircling the valley is analyzed. The results show that the mountain slopes strongly affect the low-level winds during both daytime and nighttime, and that the correct setting of the lake temperature improves the quality of the meteorological fields provided by WRF significantly. The second simulation deals with an event of strong downslope winds caused by the passage of a cold front. Comparisons between simulated and measured wind speed, direction and air temperature are also shown
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