Morphodynamic influence and antropic effects in river meanders
(G. Zolezzi, M. Tubino)

Understanding and quantifying the behaviour of natural meanders has traditionally been motivated by the need of guidelines to support traditional river engineering measures and it is becoming increasingly important for supporting river restoration practices which aim at giving “more room to the river” and sometimes at re-meandering rectified reaches. The planimetric evolution of meandering rivers has been subject of a vast body of research for decades. Many qualitative roperties of meander morphology have been replicated by simulation models: the instability of long bends, direction of migration, skewing of meander loops and variations in migration speed during different evolution stages. Mathematical odels for planimetric evolution are interesting also because they can reveal a lot about the dynamics of intriguing systems like natural meandering streams. Two examples are the phenomenon of morphodynamic influence and the convective nature of meander instability: these and other theoretical predictions still need to be verified in the field.
Aim of this research project is to gain a better understanding of how these processes control the evolution of natural eandering rivers. The research will integrate mathematical models with data and information from the field. The PhD candidate is expected to further develop existing meander morphodynamic models in particular to account for anthropic ffects, such alteration of flow and sediment regimes, gravel mining, canalization, artificial constrictions. Moreover he or he will need to look for significant field data which can allow closer comparisons between theory and reality.
Collaboration with researchers working on the subject at a national and international level is strongly envisaged.

Modelling ecohydraulic transport processes in natural and regulated alpine river systems
(G. Zolezzi, M. Toffolon, A. Siviglia)

There is an increasing recognition of the key role played by the transport of vegetation pieces, by heat as well as by organic matter in the physical-biological feedbacks that shape riverine landscapes, create a variety of aquatic and terrestrial habitats and are thus essential to sustain the services provided by freshwater ecosystems. The main purpose of this research project is to understand and model some of these hydrodynamic transport processes ith recognized ecological significance that have received little attention so far within fluvial hydraulics. The research will ave a main focus on alpine gravel-bed rivers and can preferentially focus on the transport of river water temperature or ieces vegetation particles or fine sediments (of mineral and organic nature). Both near pristine and highly regulated river conditions, typical of most of the alpine environment, will be considered.
The project will be essentially based on a mathematical modelling approach. The models will need to account for exchanges between the surface and the sub-surface regions, that are often an important ingredient for these transport processes. Models will be tested against already existing field data and additional field campaigns might be part of the project as well if needed. The case studies will be the nearly-pristine braided Tagliamento River (Friuli Region, NE Italy) end the strongly regulated Noce River (Trento Province, NE Italy) which is channelized and strongly affected by hydropeaking.

Morphodynamic and organic matter dispersion in braided rivers
(M. Tubino, W. Bertoldi)
The objective of the proposed research activity is to develop understanding of the physical processes governing wood (or more generally, buoyant particle) dispersal and deposition in braided rivers. This research will lead to cost-effective river management which takes advantage of natural biogeomorphic processes to minimise engineering interventions in river restoration.
Although large wood deposition and retention has been demonstrated to be one of the key processes controlling vegetation dynamics in braided rivers as well as large scale river pattern succession, few studies have investigated large wood dispersion processes in such river systems. A multi-channel (braided) pattern is characterised by a highly variable longitudinal and transverse morphology that drives a complex flow field and peculiar inundation dynamics, with increases in width playing a primary role and mean values of depth and velocity being more conservative. Local processes such as bifurcations, confluences, vegetated patches, and sub-surface flow strongly impact the dispersion and deposition of organic matter (wood, leaves, seeds), nutrients and fine sediments. These complex three-dimensional interactions form the focus of this proposal that will address three component objectives: (i) explanation of wood deposition dynamics, in relation to bed topography; (ii) investigation of patterns and quantities of wood transport as a consequence of flow and
flood pulses; (iii) assessment of the role of size and shape of wood pieces on the quantity, location and
structure/orientation of wood retention.
The research integrates different approaches (field observations, laboratory modelling, theoretical analysis) to address its bjectives and will be conducted in collaboration with international partners (King’s College – London; IGB – Berlin).

Theoretical-experimental analysis of the structure of turbulent free surface flow in low submergence conditions (gravel bed).
(M. Righetti)
Theoretical part: the heterogeneities of the flow field will be faced by means of the application of the Double Averaging Method (spatial and temporal) on the momentum balance equations (Nikora, 2002; Righetti & Armanini, 2003).
Experimental part: the main features of the flow field will be reproduced on laboratory scale and analysed by means of ast Particle image velocimetry techniques. Also the transport and deposition of fine sediments on fixed gravel bed will be analyzed both experimentally and theoretically.

Theoretical-experimental analysis of log transport in streams
(M. Righetti)
The presence of floating logs in streams can assume great importance on the efficiency of check dams and on the stability of bridge piers during flood events. The problem gets a particular importance in torrents with strongly vegetated banks, where the wood supply can be very high and thus the risk of obstruction of these works is higher. The optimisation of works for preventing logs clogging is strongly related to the hydraulic behaviour of transported logs and, in particular, to the orientation assumed by the logs varying the wood congestion degree and the hydraulic characteristics of the flow. The study is addressed to the analysis of the hydrodynamic behaviour of floating logs in torrents, and its interaction with hydraulic structures such as bridge pipers and check dams. A series of flume experiments at laboratory scale will be performed and image analysis techniques will be used for the motion analysis.

Modelling channel – riparian morphodynamics in river corridors
(M. Tubino, G. Zolezzi)
Alluvial river corridors are dominated by the planform dynamics of single- or multiplethread channels depending on their morphology being meandering or braided. Several analytical and numerical models have been developed so far to predict the morphodynamic evolution of single thread meandering channels. While the evolution of bedforms and of the channel axis of meandering rivers can be presently predicted with reasonable accuracy, less is understood about the coupling between the morphodynamics within the channel and the processes governing the banks evolution. Bank dynamics is the result of a rather complex array of physical processes, including biotic components of the riverine landscape such as vegetation. Based on a reasonable tradeoff between model simplicity and completeness, the goal of the research project is to set up a modelling tool able to predict the time evolution of the planform (width and axis) of sinuous channels being either single-thread meanders or individual branches of braided streams. The model shall account for channel width variations in space and time as well as for flow unsteadiness and for the mutual interaction between vegetation and morphodynamics. The approach to be taken will be based also on the attitudes and preferences of the candidate, and may place more emphasis on analytical – perturbation methods or on 2-D / 3-D numerical modelling. Part of the research will take advantage from the availability of a morphodynamic dataset including field data mainly from the braided Tagliamento River (NE Italy) as well as laboratory observations for model validation. Participation to additional field and/or laboratory activities is envisaged, although it won’t be the main component of the research project.

Morphodynamics of braided ecosystems
(M. Tubino, W. Bertoldi)
The proposed research activity is focused on the field, experimental and theoretical investigation of the linkages between morphodynamics, ecology and hydrology of natural braided networks. The attention will be primarily focussed on the Tagliamento River, intensely monitored in the last years, and that can be considered a Reference River Ecosystem for both geomorphological and ecological point of view. Typical features of braided rivers, such as the unsteadiness of water and sediment supply, together with sediment heterogeneity, will be investigated. The research activity is ultimately aimed at providing scientific support to predictive models of braiding dynamics. In particular, the relevant temporal scales of hydro-morphodynamic processes will be assessed and related to vegetation dynamics and ecological issues. Laboratory physical modelling will be used to highlight the effect of single unit processes and to simulate the impact of human management. Research is part of several basic and applied research projects with national and international partners. The activity will integrate basic and applied research. Relevant outcomes of the research is also the scientific support tools for the implementation of the EU Water Framework Directive 2000/60 and namely the definition of reference conditions in alpine and pre-alpine areas.

Analysis of the nutrient exchanges at the sedimentwater interfaces in water bodies
(M. Righetti)
Theoretical and experimental research, carried out using laboratory experiments and Particle Image Velocimetry techniques and laser induced Fluorescence.

Dinamica dello scambio di soluti tra acqua e sedimenti degli alvei, e conseguenti implicazioni per la qualità delle acque fluviali e lacustri
(M. Righetti, P. Bertola, M. Ragazzi, M. Tubino)
Questa ricerca è dedicata all’analisi sperimentale e teorica dello scambio di soluti tra l’acqua e i sedimenti degli alvei. Saranno analizzati dapprima l’effetto idrodinamico del flusso idrico sul rilascio di nutrienti dei sedimenti e successivamente il trasporto e la trasformazione dei soluti nel corpo idrico. La ricerca sarà condotta per mezzo di studi teorici, numerici e sperimentali. Saranno effettuati degli esperimenti in una canaletta di laboratorio impiegando tecniche di analisi delle immagini per le misure di concentramento e le tecniche LDA, PIV, PTV per la caratterizzazione su campo della velocità e la stima sperimentale del coefficiente di diffusione-dispersione, anche sulla base della teoria di Taylor. Il PdE che sovrintende alle equazioni sarà definito (anche in caso di traccianti reattivi scalari, come l’ossigeno dissolto) e risolto a differenti livelli (concentrazioni, ecc.) e secondo le condizioni iniziali, stimate sulla base delle misure su campo sulla colonna d’acqua e sul materiale del fondo.

Ecohydraulic modelling of benthic organisms in Alpine streams
(M. Tubino, G. Zolezzi)
Human alteration of the natural flow and thermal regime of rivers as well as regulation measures that constrained the natural morphodynamics of most Alpine streams have reduced the integrity and functionalities of freshwater and related terrestrial ecosystems. Hydropower releases and channelization are examples of some of these impacts. Only few quantitative tools exist at present to reproduce the observed alteration in the ecosystem structure of alpine rivers and to support decision-making through predictive ability within river basin restoration and management. Several measures already in place, such as minimum ecological flows imposed downstream of artificial reservoirs, also lack a quantitative assessment of their ecological effectiveness. Hydraulic habitat models have been recently developed to address the problem but still miss several important ingredients. The aim of this research project is to develop a quantitative model able to predict the quality and quantity of riverine habitat under different flow and morphodynamic conditions. Benthic invertebrates will be chosen as target taxa. Ecohydraulic processes relevant for these organisms, such as stream temperature and dissolved oxygen dynamics, flow unsteadiness, catastrophic drift, transport of fine sediments as well as substrate mobility are planned to be incorporated into the hydraulic-based simulation model. Model development and testing will be a significant part of the research, which will be integrated with experiments in existing streamside ecohydraulic flumes and field campaigns in representative Alpine catchments. The research will be of interdisciplinary nature and will find support by a working team of researchers in hydraulics, hydrology and freshwater biology. More emphasis on field and/or experimental work with respect to modelling can be chosen also based on the candidate’s preferences and attitudes.

In-stream contaminant dynamics
(A. Bellin, M. Tubino, D. Tonina)
What is the role of hyphoreic and riparian flows on the fate of in-stream nutrients and contaminants? Hyporheic and riparian zones are extremely rich ecotones where solute uptake and transformations take place with kinetics mainly controlled by the solute residence time within the streambed sediment. While the hyporheic zone is the saturated band of sediment that surrounds the river and forms a linkage between rivers and shallow aquifers, riparian areas act as buffer zones between hillslope and river dynamics affecting runoff generation and solute transport at the reach and larger scales. Solute residence time within the hyporheic and riparian zones is primarily controlled by a complex interplay among in-stream and subsurface water flows and channel morphology. Additionally, vegetation introduces further complexity in the riparian zones because of its feedback on soil moisture. Understanding the dynamics of these processes and subsequently modeling them are the first steps toward an improved management of rivers and theirs watersheds. The main objective of this project is to improve our knowledge on hyporheic and riparian flows, to highlight their differences, and most importantly, to model the impact of chemical and biological processes controlling solute (for example contaminants and nutrients) uptake.

Efficienza idrodinamica di ossigenatori ipolimnici e loro utilizzo nei processi di risanamento dei laghi
(M. Ragazzi, M. Righetti, M. Toffolon)
La ricerca si propone di studiare i processi di diffusione/dispersione dell’ossigeno nelle acque più profonde (ipolimniche) dei laghi. Di norma, infatti, in presenza di stratificazione termica l’ipolimnio presenta concentrazioni ridotte di ossigeno. L’ossigenazione forzata è d’altra parte un processo molto complesso e la valutazione dei meccanismi che si innescano richiede un approccio multidisciplinare che deve prendere in considerazione l’idrodinamica indotta dai sistemi di alimentazione dell’ossigeno, la chimica delle sostanze organiche e inorganiche coinvolte e l’influenza del ciclo dei nutrienti sulla componente biologica. Gli obiettivi della ricerca sono la definizione di un modello matematico in grado di simulare l’effetto di un ossigenatore e lo studio della complessa trama di interazioni tra l’idrodinamica e le reazioni biochimiche. La parte idrodinamica potrà eventualmente essere simulata anche con modello fisico con l’utilizzo di tecniche LDA e di analisi di immagine.

Mathematical modelling and numerical solution of multiphase flows
(E.F. Toro, M. Toffolon)
There are many processes in nature, in industry and in science that involve the dynamic interaction of different materials (or phases or fields). The natural environment is full of such processes; examples include: rain and hailstones falling through the gaseous atmosphere. In industrial processes there are numerous kinds of phenomena of the multiphase type, such as in the chemical industry, the petroleum industry, the nuclear industry, the aerospace industry (propulsion, aerodynamics, materials); the food industry, and the list is endless. This proposal concerns the construction of mathematical models for some of these processes, their study, their approximation via suitable numerical methods and the application of the most successful models and methods to simulate some specific problems related to water systems.
There are currently several approaches to the construction of mathematical models for multi-phase flow processes. The approach of interest to this proposal is based on averaging (time, space or statistical), whereby one first establishes the point-valued set of balance laws for each separate phase, or field. Of course in most practical situations of interest the field interfaces are very complicated and are part of the mathematical problem to solve. Averaging the equations for each phase leads to systems of averaged partial differential equations (PDEs), which in most cases are identical to the original point-valued PDEs. The challenging problem left is to account for the interfacial phenomena. It is known that apart for some special cases, the theoretical determination of the associated terms in the equations is impossible or is the subject of current research. A practical way forward is to choose closure laws based on empirical correlations; for each case of study and flow regime, these have to be appropriately chosen. Having established the averaged PDEs and closure conditions, there are then potential mathematical problems. Wellposedness is a main one. It is known that some existing models, in the absence of dissipative effects, are of mixed elliptic-hyperbolic type. This means that solutions do not depend continuously on the data. Another potential problem is the presence of non-conservative products. This problem has also numerical implications, as would prevent the straightforward application of the successful class of conservative methods.
The candidate is expected to develop mathematical models for time dependent multi-dimensional flow processes involving several phases or fields, each one with its own velocity vector. The fields will in general be assumed to be compressible, specializing in some cases to admit incompressible fluids. The basic approach will be space averaging with the aim of constructing coupled systems of non-linear partial differential equations, one system of equations for each phase. The candidate shall then study models (theoretical or empirical) for the terms that account for the interaction of the phases for specific problems, concerning water systems and the environment.

Numerical modeling of hydropeaking effects
(E. F. Toro, M. Tubino)
Deregulation of the electric power market leads to an increasing requirement for electricity on demand. This often results in the generation of 'hydropeaking' effects which induce rapid changes in river characteristics as the discharge, the water temperature, the electrical conductivity and the suspended solid transport. Such variations have important consequences on the various habitat along the river. A quantitative description of these variations is required to model effectively riverine habitat modifications due to human-induced alteration of the hydrologic regime. This will allow a more precise definition of the actual flow requirements, which is needed for environmentally-sensitive management strategies. The general aim of the project is twofold. First, the fundamental parameters which alters natural habitat must be identified. Second, appropriate numerical techniques must be developed in order to describe quantitatively the variation of the fundamental parameters. The use of advanced numerical techniques will be exploited in the project, focusing the attention on problems which are governed by either hyperbolic or 'weakly parabolic' equations. For the former class a 'robust' method able to account for the presence of the source terms would be highly desirable; for the latter class the main problem is to achieve high accuracy of the overall solution while modeling the parabolic term.