Guido Zolezzi

Hydropower operations result in sharp water level and temperature fluctuations downstream the river section where water is released intermittently according to the pattern of hydropower generation. It has been widely recognized that these peaking flows cause severe degradation of the affected river reaches, but their biological effects and hydraulic behaviour have been studied mainly referring to the main channel. Field evidence (Sawyer et al., 2009, Loheide & Lundqvist, 2009) demonstrate that surface water level oscillations are associated with significant mass exchanges between the stream and its riparian aquifer that may have relevant, still largely unexplored, biogeochemical implications. The purpose of this study is to develop a simplified modelling approach to predict the effects of hydropeaking on subsurface flow into the riparian region. We propose a simplified model for surface - subsurface flow exchange where instream hydropeaking is assigned as boundary condition and that solves the unsteady, dimensionless 1D Boussinesq equations for the saturated zone of the riparian aquifer. This allows to quantify the lateral extent of the riparian region affected by hydropeaking oscillations. In particular, with this model we analyzed the temporal variations in the daily mass and thermal exchanges between the channel and the riparian aquifer, and identified the controlling factors. The role of longitudinal variations in channel morphology as well as of seasonal aquifer variations and land cover can also be examined through the proposed modelling framework. Sawyer, A.H., Cardenas, M.B., Bomar, A., and Mackey, M. 2009. Impact of dam operations on hyporheic exchange in the riparian zone of a regulated river. Hydrol. Process, DOI: 10.1002/hyp.7324 Loheide, S. P., II, and J. D. Lundquist (2009), Snowmelt-induced diel fluxes through the hyporheic zone, Water Resour. Res., 45, W07404, doi:10.1029/2008WR007329.

The present study provides a detailed quantification of the ‘thermopeaking’ phenomenon, which consists of sharp intermittent alterations of stream thermal regime associated with hydropeaking releases from hydroelectricity plants. The study refers to the Noce River (Northern Italy), a typical hydropower-regulated Alpine stream, where water stored in high-altitude reservoirs often has a different temperature compared with the receiving bodies. The analysis is based on a river water temperature dataset that has been continuously collected for 1 year at 30-min intervals in four different sections along the Noce River. A suitable threshold-based procedure is developed to quantify the main characteristics of thermopeaking, which is responsible for thermal alterations at different scales. The application of Wavelet Transform allows to separately investigate the thermal regime alterations at sub-daily, daily and weekly scales. Moreover, at a seasonal scale, patterns of ‘warm’ and ‘cold’ thermopeaking can be clearly detected and quantified. The study highlights the relevance of investigating a variety of short-term alterations at multiple time scales for a better quantitative understanding of the complexity that characterizes the river thermal regime. The outcomes of the analysis raise important interdisciplinary research questions concerning the effects of thermopeaking and of the related short- and medium-term effects on biological communities, which have been rather poorly investigated in ecological studies. Copyright © 2010 John Wiley & Sons, Ltd.

Release of hypolimnetic water from reservoir for hydropower generation production generates hydro- and thermo-peaking waves which propagate downstream. The resulting thermal regime alteration causes detrimental impacts on habitats and ecosystems due to both direct (e.g. spatial-temporal patterns of maximum and minimum temperature) and indirect (e.g. affecting timing of lifecycle stages, growth rates restriction or alterations) effects. In order to reduce such effects, mitigation procedure must be studied and implemented. The phenomenon is studied using the one-dimensional model governed by the Saint Venant equations coupled with an equation imposing thermal energy conservation. The difference between the propagation celerity of the hydrodynamic wave and that of the thermal wave identifies two different phases: (I) where the hydrodynamic wave strongly interacts with the thermal wave; and (II) where the hydrodynamic wave separates from the thermal wave. The diffusive approximation for the hydrodynamic model is assumed to hold while the hydropeaking initial condition is schematized as a square wave characterized by a peak value over a base flow. The resulting problem is solved assuming constant coefficients, i.e. the celerity is taken as a constant function of the two uniform states computed with the peak and base discharges. The resulting flow field is used to evaluate the propagation of an initial square thermal wave, assuming that the tail and the head travel downstream with different celerities in phase I and with the same celerity, equal to the velocity of the base flow, in phase II. Comparison between the proposed analytical solution and numerical solutions of the fully hydro-thermal problem are in good agreement, showing its applicability in predicting the thermal field occurring under hydropeaking conditions. The proposed model can be employed to study the spatial-temporal patterns of maximum and minimum temperature maximum that arise downstream of hydropower plants. Such simplified model is shown to be an effective tool for understanding the main processes and for assessing mitigation procedures.

... 183–211. Printed in the United Kingdom c 2001 Cambridge University Press 183 Downstream and upstream influence in river meandering. ... We show that, along with downstream overdeepening, an upstream overdeepening scenario is predicted in the super-resonant regime. ...

... 183–211. Printed in the United Kingdom c 2001 Cambridge University Press 183 Downstream and upstream influence in river meandering. ... We show that, along with downstream overdeepening, an upstream overdeepening scenario is predicted in the super-resonant regime. ...

An analytical modelling framework is proposed to reproduce the frequently observed but poorly studied occurrence of mid-channel bars in meandering channels. Mid-channel bars occur in meanders and may characterize transitional morphologies between pure meandering and braided rivers. Based on existing field and experimental observations, we propose that two different mechanisms can generate central topographical patterns in meanders. A former mechanism (‘width-forced’) is related to spatial width oscillations which determine a laterally symmetrical bed shear stress pattern that promotes mid-channel bars. A second mechanism (‘curvature-forced’) can take place also in curvilinear equiwidth streams since also longitudinal variations of channel curvature can produce laterally symmetrical alterations of the sediment transport capacity. A perturbation approach is employed to model both mechanisms within a common framework, allowing reproduction, at least qualitatively, of several observed features. While width-forced mid-channel bars are a symmetric linear altimetric response, to reproduce curvature-forced mid-channel bars requires modelling nonlinear flow-bed topography interactions at the second order of the perturbation expansion. Hypotheses on how these mechanisms operate are further discussed through an application to field cases. The amplitude of the nonlinear response can be relevant compared to that of the point bar in equiwidth meanders and the location of mid-channel bars seldom coincides with bend apexes, mainly depending upon the intrinsic meander wavelength. Central bars tend to symmetrically divert the flow against the two banks, a process which is proposed as a possible cause of cross-sectional overwidening, along with the asymmetry between the rates of bank erosion and of the opposite bank accretion. The outcomes of this first modelling step on the subject allow discussion of the mutual feedback processes that characterize interactions between mid-channel bars and width variations in river meanders. Copyright © 2010 John Wiley & Sons, Ltd.

Hydrological alteration is one of the major threats to the integrity and functioning of riverine ecosystems. We propose to analyze such alterations by means of three approaches: the well-established range of variability approach (RVA), wavelet transform analysis (WT), and a combination of the two. This allows us to separate the most relevant scales of variability and to detect their alteration independently; such information cannot be provided by purely statistical methods like RVA. We show the advantages of this multiple approach through ...

We study the effect of spatial variations of river width on bed aggradation and degradation processes, making use of a one-dimensional numerical model of channel morphodynamics. We refer to a peculiar case, the downstream reach of the Kugart River (Kyrgyzstan). The river has been partly channelized in the recent past with the aim of reducing the flooding risk for the surrounding villages; the consequent reduction of channel width in some reaches was also expected to improve channel conveyance with respect to the high sediment load produced in the upper river basin. The resulting longitudinal sequence of relatively sharp channel expansions and contractions has, however, triggered rapid siltation rates, especially in the narrowest reaches. This motivated the application of a 1-D numerical model of river morphodynamics.Abrupt channel expansions are found to be the main driving forces for aggrading processes, which may extend for long distances from where they are generated. In order to obtain a thorough understanding of the morphodynamics of channel expansions, we first apply the model to simple test cases. This allows us to characterize the basic features of the problem and the dependence of bed evolution on the upstream Froude number Fr and on the expansion ratio rb, which are the most relevant controlling parameters. We invariably find that deposition occurs in expansion regions with bed aggradation observed both upstream and downstream. The deposition prism progressively increases its height and lengthens both in the upstream and downstream directions. The deposition process is particularly intense, in terms of deposition prism height, in super-critical conditions. Moreover, it is found that higher values of Fr strongly reduce the time scale of morphological processes and faster deposition rates are further facilitated by abrupt expansions.The present outcomes are relevant for assessing the expected altimetric response of river bed to the implementation of localized channelization works and to local river widening, a practise which is increasingly being employed within river restoration projects, with the aim of enhancing habitat diversity. Copyright © 2008 John Wiley & Sons, Ltd.

Nearly three decades ago the first bend theory has been proposed to explain the origin of meanders as inherent instability of the river planform. The bend theory has allowed decisive advances in understanding the dynamics of meandering river patterns, including the prediction of typical spatial scales, the reproduction of typical meander loop shapes and migration rates, the role of intrinsic flow and geometric nonlinearities. Almost always river meandering has been modelled by assuming constant channel width, although in many meandering rivers channel width oscillates along meander loops. Spatial variations of channel width and curvature give rise to a rich variety of morphological patterns and different meandering behaviours, whose dynamics has received relatively little consideration from a modelling perspective. Evidence exists that "wider at bends" meanders are widespread and have been observed to be morphologically more active than their equiwidth counterparts. The present work revisits the classical, uniform-width bend theory with the aim to theoretically understand whether and how spatial width oscillations can affect the process of linear bend stability that initiates meander planform evolution. The theory accounts for width variations as a geometrical forcing in a depth averaged model of meander morphodynamics, and assuming the potential interaction with the curvature forcing effect. Little quantification of spatial variations in natural meandering rivers are however available at present, therefore an attempt is made to preliminarily quantify their characteristic wavelength, magnitude and spatial lag relative to the curvature distribution. The analysis suggests that the dimensionless amplitude of width variations is a "small" parameter with comparable magnitude to that of curvature variations, which suggests the suitability of a two-parameter perturbation expansion to solve the governing differential problem, thus correcting the classical bend theory. Moreover channel width often oscillates in space with a double frequency relative to curvature, a key observation that implies that one nonlinear interaction between the two forcing effects is enough to reproduce the correction on bend stability due to spatial width variations. Results indicate that width variations consistently promote the instability of relatively short bends that are stable when the channel width is uniform: in the average, this predicted tendency is supported by field data from a relatively large number of natural meander bends. The effect on meander wavelength selection depends on the location of the widest section relative to the bend apex. For larger values of the channel aspect ratio two distinct most unstable longitudinal modes develop, a behaviour that is absent when the width is uniform, and which suggests possible options to look at the dynamics of chute cutoffs, often observed in some meandering rivers but not others, and still awaiting sound mechanistic explanations.

In the paper we review some recent work on the mechanics of formation and development of river bars. The emphasis is placed on the instability process which leads to the spontaneous development of bars in almost straight reaches of alluvial rivers. A three dimensional formulation of the problem is presented along with a discussion on the relevant closure relationships. Results

The Tagliamento River, Northeast Italy, represents an important Alpine to Mediterranean braided system, where interactions between river flows, sediment dynamics and vegetated landforms can be investigated within a relatively unconfined setting.We analysed data from contemporary and historical sources, including stage records, photographs and topographic surveys. From these we identified river stages at which thresholds in surface hydrological connectivity and biogeomorphological adjustment appeared to occur, contributing to a shifting habitat mosaic.Significant adjustments in landscape elements within the active tract commence at river stages well below bankfull with return periods of a few months. Flow pulse events with return periods from a few months to 2 years support a dynamic inundation pattern, ranging from a patchwork of isolated water bodies within a predominantly terrestrial landscape at low river stages to isolated vegetated islands within a fully connected aquatic landscape as the river approaches bankfull. Across this range, interactions between flow, sediment and vegetation lead to gradual and abrupt transitions in persistence, form and connectedness of different landscape elements. Bankfull flows (return period over 2.5 years) topple and disperse significant numbers of large trees, seeding the next generation of vegetated patches, and larger floods (return period around 10 years) induce significant turnover of established islands and floodplain surfaces.The results reported in this paper illustrate how extensive interdisciplinary research on a single river system can provide useful insights concerning the time scales and thresholds that characterize water–sediment–vegetation interactions in piedmont reaches of Alpine to Mediterranean braided systems. Anthropogenic effects on river systems are ubiquitous throughout Europe. However, systems such as the Tagliamento River that retain significant process dynamism and morphological integrity, provide a laboratory within which reference processes and process–form interactions can be investigated, understood and then incorporated into innovative restoration design on more impacted systems. Copyright © 2009 John Wiley & Sons, Ltd.

Most simulation models of meandering dynamics have considered meanders as constant - width systems, with channel curvature being the major planform effect. However, field evidence indicates the presence of significant spatial variations of the river width even in the most active meanders. Geomorphic classification of alluvial meandering streams suggests that the intensity of width variations may discriminate between different meandering behaviours: regular, equiwidth sinuous forms and more irregular, wider at bends meanders, with chutes common. Moreover spatial width oscillations present strong correlations with mid-channel bars, whose occurrence in meandering rivers has also been scarcely modelled so far. The present work reviews recent advances concerning the interactions between width variations, channel curvature and the associated bed topography patterns in meandering streams, based on a mathematical modelling approach integrated with data from natural rivers. A first ingredient are mid-channel bars, which can develop spontaneously in equiwidth meanders due to the nonlinearities in the flow-bed topography dynamics and can be forced by spatial width variations. Modelling mid-channel bars, together with field observations on a freely evolving meandering river, suggests that spatial width variations in meanders may be due to a topographically-driven and to a bank-driven mechanism. The presence of width variations may in turn produce a significant effect on meander growth, consistently with field observations. The overall modelling framework allows to formulate hypotheses on the possible processes which cause width variations to be much more intense in some meandering rivers compared to others.

The recent (25 years) morphodynamics of a proglacial reach of the Ridanna Creek, North-East Italy, evolving in the absence of human constraints, has been investigated by means of an intensive field activity and of the analysis of aerial photographs. The study reach mostly displays a braided morphology, with sharp downstream variations of valley gradient, sediment size and formative conditions within the main channel. These discontinuities are associated with different processes of channel adjustment at different timescales, which have been quantified by coupling hydrological with morphological information.Several processes of channel change and variations in braiding intensity have been documented along the whole reach and highlight how a regular, weakly meandering main channel may significantly affect the morphodynamics of the braided network. A first attempt to predict the morphological instability of this main channel at the observed spatial scales through existing linear theories of curved river channels shows a good agreement with field observations. Finally, the complete hydro-morphodynamical characterization of such an undisturbed alpine river reach can provide a relevant contribution to the definition of reference conditions for Alpine rivers required by the EU Water Framework Directive. Copyright © 2007 John Wiley & Sons, Ltd.