In recent decades, the use of artificial nocturnal illumination has rapidly increased worldwide, imposing an increase of nocturnal light levels and a disruption of natural cycles of light and dark that have been stable over geological and evolutionary time scales. This wide-spread alteration of the natural light regime by artificial light at night (ALAN) is contributing to global environmental change and raises concerns about the potentially adverse effects on organisms and processes in illuminated ecosystems. Simultaneously, a global shift in outdoor lighting technologies from yellow high-pressure sodium (HPS) to white light-emitting diode (LED) light is taking place, changing the spectral composition of nocturnal illumination. Mounting evidence suggests that ALAN affects microorganisms, plants and animals in both aquatic and terrestrial ecosystems. Light is a major source of energy and an important environmental cue for primary producers that influences and to a large extent drives their growth, production and community structure. Freshwaters are increasingly illuminated at night, as they are often located near the human population centers. Despite this, the impacts of artificial nocturnal illumination in freshwater ecosystems are still largely unknown. In particular, effects on aquatic primary producers in urban and sub-urban rivers and streams have hardly been addressed. This thesis aimed to investigate effects of artificial nocturnal illumination on biomass and community composition of communities of benthic primary producers in freshwaters, the periphyton. The presented work is based on manipulative field studies performed in two contrasting freshwater systems whose periphyton communities are characterized by different species. The first study was performed in a stream-side flume system on a sub-alpine stream and the second in a lowland agricultural ditch. I found that two to 13 weeks of exposure to LED light at night decreased the biomass of periphyton in both aquatic systems. In stream periphy-ton, the decrease in biomass was observed for periphyton in early developmental stages (up to three weeks), but not that in the later developmental stages (four to six weeks). The effects of LED on community composition were found only in stream periphyton, where it increased the proportion of the dominant autotroph group, the diatoms and decreased the proportion of cya-nobacteria in early developmental stages, but indicated a decreased proportion of diatoms and an increased proportion of cyanobacteria in the later developmental stages. I found that LED light at night altered pigment composition and quantitative taxonomic composition in stream periphyton in later developmental stages and that several diatom and chrysophyte taxa, both autotrophic and heterotrophic, responded to ALAN by increasing or decreasing in abundance in a taxon-specific manner. LED did not affect periphyton community composition in lowland agricultural ditch, likely because periphyton was composed of different species. All effects of LED light were different between the seasons presumably due to seasonal differences in community composition and environmental variables. I did not find any evidence that HPS-light affects either biomass or community composition of periphyton. Differential effects of the two light sources are likely a result of differences in their spectral composition, in particu-lar the high proportion of blue light emitted by LED but not by HPS. This thesis provides, for the first time, evidence that LED light at night can profoundly affect benthic primary producers and periphyton communities in freshwater systems by reduc-ing their biomass and altering community composition. Systems dominated by periphyton in its early developmental stages, such as streams prone to physical disturbances, are likely to be more sensitive to ALAN compared to systems with stable flow conditions based on the results presented. Periphyton plays a fundamental role in productivity, nutrient and carbon cycling and food supply for higher trophic levels in small, clear waters; its position in the base of aquatic ecosystems suggests that the alterations induced by ALAN may have important con-sequences for ecosystem functions. This should be considered when developing lighting strat-egies for areas close to freshwaters in order to mitigate potentially adverse effects of nocturnal artificial illumination on aquatic ecosystems.

Most estuaries of the world are influenced by tides. The tidal action is a fundamental mechanism for mixing river and estuarine waters, resuspending and transporting sediments and creating bedforms. The planform of tide-dominated estuaries is characterized by a funnel-shaped geometry with a high width-to-depth ratio. The width of the estuarine section tends to decrease rapidly upstream, following an exponential law and the bottom slopes are generally non-significant. In this thesis the long-term morphological evolution of tide-dominated estuaries is investigated through a relatively simple one-dimensional numerical model. When a reflective barrier condition is assigned at the landward boundary, the morphological evolution of an initially horizontal tidal channel is characterized by the formation of a sediment wave that migrates slowly landward until it leads to the emergence of a beach. The bottom profile reaches a dynamical equilibrium configuration which has been defined as the condition in which the tidally averaged sediment flux vanishes or, alternatively, the bottom elevation attains a constant value. For relatively short and weakly convergent estuaries, the beach is formed at the landward end of the channel, due to the reflective barrier chosen as boundary condition, and the equilibrium length coincides with the distance of such boundary from the mouth. When the above distance exceeds a threshold value, which decreases for increasing values of the degree of convergence of the channel, the beach forms within an internal section of the estuary and the final equilibrium length is much shorter and mainly governed by the convergence length. The final equilibrium length of the channel is found to depend mainly on two parameters, namely the physical length of the channel and the degree of convergence. Moreover, if a condition of vanishing sediment flux from the outer sea during the flood phase is imposed, a larger scour in the seaward part of the estuary is observed, though the overall longitudinal bottom profile does not differ much from that corresponding to a sediment influx equal to the equilibrium transport capacity at the mouth. This fixed banks model is not able to explain this typical funnel shape; furthermore, the bottom slopes obtained with this models are quite large if compared with the mild slopes of real tidedominated estuaries. For these reasons, the problem has been analysed to understand the reason why tidal channels are convergent and to define the conditions under which the exponential law for width variation, which is so often observed in nature, is reproduced. The long-term evolution of the channel cross-section is investigated allowing the width to vary with time. The one-dimensional model is expanded by considering a simple way to take the banks erosion into account. A strongly simplified approach is adopted, whereby only the effects related to flow and sediment transport processes within the tidal channel are retained and further ingredients, like the control exerted by tidal flats or the direct effect of sea currents in the outer part of the estuary, are discarded. The lateral erosion is taken into account and computed as a function of bed shear stress, provided it exceeds a threshold value within the cross section. The effect of different initial and boundary conditions on the widening process of the channel is tested, along with the role of the incoming river discharge. Another problem, which is somehow analogous to that of the cross-section evolution, is tackled: a part of a tidal flat dissected by a series of parallel channels is considered and the response of the system induced by a modification of the depth of the channels is studied. In particular, the aim is to assess whether the increase of the depth of one channel, starting from an equilibrium configuration, causes deposition in the others inducing their closure. The evaluation of the morphological effect of a depth variation in one of channel upon the other channels is quite a relevant task, because the stability of salt marshes and lagoons is intrinsically related to the stability of the hydrodynamic functionality of the channels. A result of this analysis is the determination of a characteristic distance between the channels to have mutual influence. It is found that this distance scales with the root of the longitudinal length of the flat; thus, a scaledependent spacing is expected in tidal networks.

River reaches worldwide historically experience morphological regulations, as channelization, as well as flow regime alterations, which often lead to degradation of freshwater ecosystems. In last seven decades a large number of dams have been designed and built worldwide contributing to such river hydromorphological alterations. In alpine and piedmont regions river reaches often experience anthropogenic flow regime alteration due to hydropower production. The fluctuating flow regime typical of river reaches downstream hydropower plant releases (hydropeaking) is known to produce several adverse ecological effects, strongly linked to morphological characteristics of the downstream channel. Hydropeaking can also alter the thermal regime of the receiving water body (thermopeaking) if released hypolimnetic water has different temperature from surface water; also thermopeaking can have adverse consequences on river environment. In a changing world with respect to renewable energy production but also to global warming, the understanding of hydropeaking and thermopeaking ecological impacts represent a lively research challenge. The first part of the present thesis is dedicated to the characterization and quantification of hydropeaking and thermopeaking alterations. Adopting a statistical approach on an extended dataset of Alpine and Norwegian rivers, a suite of indicators have been designed and exploited to identify the degree of alteration of both hydrological and thermal regime. The study provides two screening tools that can be exploited by environmental managers in the identification of critically altered river reaches. The second part is dedicated to the development of a two-dimensional numerical shallow-water model able to simulate surface water passive tracer transport over complex morphologies, exploitable in the numerical investigation of river thermal transport dynamics. In the third part the interaction between hydropeaking waves and receiving reach morphology has been investigated via numerical modeling. The work consists on a first quantitative attempt to investigate the eco-hydraulic response of river reaches with different channel morphologies to hydropeaking waves of different intensities. Such general approach can be applied to a specific case to support the choice of the most effective river restoration strategy leading to the optimal specific eco-hydraulic conditions. Finally, the last part reports an application of the designed approaches and tools to Lundesokna River, a Norwegian river affected by hydropeaking.

In the last fifty years, new engineering problems, connected with gravel extraction, installation of dams and disposal of mine waste into rivers, have risen to the extent that sediment transport modeling is assuming a key role in realistic river hydraulic simulations. Floods, meandering, sediment load computation, river bed aggradation or degradation, channel design and navigation are some of the problems concerning the sediment transport in rivers. The description of the river dynamics and morphodynamics is then relevant and a clearer analysis of the existing models is necessary. Standard mathematical models for sediment transport in rivers employ simplified continuity equations. These equations, essentially mono-phase model, are only valid for low sediment concentrations and Froude number. A rather complete approach, quasi-two phase model, is presented and thanks to the coupling between morphodynamics and hydrodynamics it can be applied also to higher concentration regimes and Froude number. Finally, numerical simulation of river dynamics has to consider the non-uniformity of sediments. In this work, a more traditional scheme, in which the grain size distribution curve is divided into a discrete number of classes and the transport is calculated for each class, is compared with a novel formulation. This method is based on the series expansion of sediment transport formulas. In conclusion, the algorithm can be integrated with a description of grain sorting and non-uniform sediment transport in order to better describe the river dynamics and morphology.

Bottom of the mountainous rivers is generally composed of natural gravels. Flow depth in such rivers is generally shallow, with the ratio of water depth to size of bed materials (known as relative submergence) rarely higher than 20. In this type of flow, gravels intrusion induces significant spatial variation of the flow characteristics near bed region, which is known as roughness layer. The simultaneous effects of natural gravels and water surface cause formation of complicated flow structure which is to some extent different from the flow with high relative submergence (flow with relative submergence higher than 40). Despite abundance of studies in shallow flows, there are only a limited number of studies concerning spatial organization of near bed flow field for such type of flow, with also contradictory results. The spatial organization of near bed turbulent flow characteristics is also important for transport of fine sediment. Transport of fine sediments is generally correlated to the asymmetry of vertical velocity. Asymmetry of vertical velocity also arises from a quasi-cyclic process of upward motion of low-velocity fluid parcels (ejection) and downward motion of high-velocity parcels (sweep), together known as bursting process. Spatial organization of bursting process and asymmetry of vertical velocity in near bed and respect to bed topography has not been inscribed properly. In heterogeneous flows, the use of spatially averaged turbulent transport equations, known as Double Averaged Navier-Stikes equations (DANS), is common. In DANS equations viscous drag, form drag and correlation of spatial fluctuation of time averaged velocities (known as form induced stresses) are explicitly expressed. Despite prevailing usage of DANS equations in study of gravel bed flow, examination of vertical velocity has not been performed appropriately by applying double averaging method. Also, the role of form induced stresses in vertical momentum flux has not been highlighted. In present thesis, Stereoscopic Particle Image Velocimetry at near bed horizontal layer and Digital Particle Image Velocimetry in vertical planes are employed together with laser scanning of bed elevations to study flow field and turbulence structure over a coarse immobile gravel bed in submergence conditions ranges from 5 to 10. Spatial organization of flow characteristics at the near bed region is analyzed respect to bed topography. This analysis is also composed of spatial distribution of bursting process and vertical momentum flux. Moreover, vertical profiles of double averaged turbulent flow characteristics and form induced stresses with different relative submergences are compared. Results show that near bed flow field is characterized by a strip structure induced by secondary currents. Such structure tends to be disrupted by the effect of gravel protrusions. To better analyze the interaction between the flow field and gravel bed protrusions, cross-correlations of different velocity components and bed elevations in a horizontal layer just above gravel crests are computed. These results show that upward and downward flows occur not randomly on the bed, but in correspondence to upstream and downstream side of gravels. Also, turbulent momentum flux is directed downward in the downstream side of gravel crests and it is directed upward in upstream side of gravel crests. This is due to prevalence of ejection and sweep events respectively in upstream and downstream sides of gravel crests. These results are in agreement with formation of separation and reattachment zones around gravel crests. Moreover, spatial distribution of sweep and ejection events are organized in streamwise elongated strips with high and low values which are consistent with presence of secondary currents cells. Results obtained by double averaging method show that relative submergence affects the normalwise double averaged turbulence intensity profiles all along the flow depth, while only a weak effect, limited to the near bed region, is noticed on streamwise double averaged turbulence intensity profiles. Logarithmic law parameterization of double averaged velocity profiles shows that parameters change considerably with relative submergence and, in some cases, no clear log-law region was found. These results challenge application of log-law in such type of flow. Analysis of the vertical velocity shows that far from the bed, vertical turbulence momentum flux is upward, while below gravel crests it is downward. This behavior is resulted by prevalence of ejection events far from the bed and sweep events below gravel crests. Results show that vertical momentum flux resulted by form induced component is not significant, except below gravel crests which are upward in to the water column. A limited number of qualitative observations in the real case of fine sediments presence in the matrix of rough bed is in agreement with the results of turbulent flow characteristics. Sand ribbons are clearly formed due to secondary currents. Also, fine materials are mostly deposited and eroded respectively in downstream and upstream sides of gravel crests. The results of present study show that in general some regions actively participate in transport, while the other regions do not participate in the transport. From this basis, Rouse criterion has been developed by considering spatial variation of vertical momentum flux.

Lake Baikal (Southern Siberia) is the world's oldest, deepest and largest freshwater body by volume. In spite of its enormous depth, episodically (i.e. almost twice a year) large volumes of surface, cold and oxygenated water sink towards the bottom of the lake. This phenomenon is known as deep ventilation and determines the periodical, partial renewal of deep water, playing a key role in the ecology of the whole lacustrine system. Despite deep ventilation has been widely observed, still significant uncertainties exist about the detailed characterization of deep downwellings. In order to tackle this issue, a simplified, one-dimensional numerical model has been developed, which allows for a suitable simulation of deep ventilation in profound lakes. Three main algorithms are at the basis of the model: a reaction-diffusion equation for temperature and other tracers (e.g. dissolved oxygen), and two Lagrangian algorithms, the first to handle buoyancy-driven convection due to density instability (including thermobaric effects) and the other to reproduce the deep downwelling mechanism. Thanks to its simple structure, such a model ensures a considerable computational speed that makes it suitable to perform long-term simulations (i.e. decades, centuries). At the same time, it has been shown to be appropriate for quantitatively and qualitatively simulating deep ventilation, well capturing the relative contribution of the different processes involved. The model has been applied to investigate deep ventilation in the South Basin of Lake Baikal. The numerical results have been shown to be in good agreement with observed data (concerning temperature, CFC-12 and dissolved oxygen profiles), indicating a proper performance of the core algorithms. The analysis of results allowed for a detailed description of the major mixing and thermal dynamics of the lake, and for an in-depth characterization of deep water renewal (e.g. typical downwelling temperatures and volumes, vertical distribution of sinking water, energy balance). Numerical simulations have been performed under current conditions and climate change scenarios, thus permitting to assess the future behavior of the lake and the possible impact on deep ventilation, in response to the expected evolution of climate. In addition to the main results discussed above, this study provided some additional outcomes: a simplified lumped model to convert air temperature into surface water temperature of lakes, and a novel downscaling procedure to transform meteorological data (i.e. wind speed and air temperature) from the global scale to the lake scale. In the light of the proven performance of the deep ventilation model, further improvements of the model could bring to the development of a suitable module to simulate biogeochemical processes in the lake, thus providing valuable information to assess the role of deep ventilation in affecting the lake ecosystem.

The thesis tackles the mechanics of submerged granular flows driven by gravity, focusing on the rheological formulations and on the numerical solutions of the equations that govern this type of flow. In particular, a two-phase approach is assumed. The liquid phase, usually water, is described with a Newtonian rheology. The rheology of the granular phase depends on the type of contacts among the particles. Two opposite conditions are identified: if the contacts among particles are instantaneous the regime is named collisional, while, when the contacts become long lasting and involved more particles at the same time the regime is called frictional. In the thesis a proper model for the rheology of the granular phase, able to account for both the regimes, is presented. This model is based on the fundamental evidence that the granular phase is characterized by the coexistence of the collisional regime, which dominates near the free surface, and of the frictional regime, which becomes relevant approaching the loose static bed Armanini et al. [5]. The kinetic theories of dense gases Jenkins and Savage [48] are adopted to describe the collisional regime, while for the frictional regime a new rheological formulation, dependent on the Savage number, which comes from the analysis of the force involved, is given. In addition, the model, named heuristic model [11], introduces a specific equation of state also for the frictional regime. The model is based only on a single parameter, which presumably depends on the properties of the contact forces of the material. A numerical code able to integrate the equations of the mass, momentum and energy of the two-phase, in uniform flow conditions, was developed by Armanini et al. [6] and the results are compared with the experimental data. In the applications to hyperconcentrated channel flows the effect of the side walls and of the internal stresses of the liquid phase are neglected in the momentum balance equations, therefore the drag force is balanced by the weight of the liquid phase. The heuristic model is able to predict in a satisfactory way the distributions across the flow depth of the velocity, concentration, granular temperature and stresses and in particular, it allows to discriminate between the collisional and the frictional components of the shear and of the normal stresses. Another important issue addressed in the thesis concerns the balances of the energy of the granular phase. The model is able to describe the mechanisms of production, diffusion and dissipation of energy, relevant to both the mean component of the flow and the fluctuating component (i.e., the collisional component). In uniform flow conditions, near the static loose bed, the model predicts that the flux of the diffused fluctuating energy exceeds an order of magnitude the locally dissipated flux of fluctuating energy. This suggests that the motion of the grains, even at concentrations close to that of packing, is always accompanied by a certain degree of granular temperature as already observed by Armanini et al. [10]. Furthermore, the description of the mechanisms of exchange among the terms of the total energy balance and of the kinetic energy balance, and between the two energy balances is given. In the thesis, the role of the interaction between the liquid and the solid phase in the kinetic energy balance is analysed [59]. A specific experimental investigation to understand the difference between the drag averaged over time and the drag calculated with respect the average velocities and concentration is carried out. This difference between the two drags represents the contribution to the drag due to the correlations between the fluctuating components of the concentration and of the velocities. By integrating the heuristic model across the flow depth, it is possible, in principle, to derive a set of shallow water equations that are able to describe the behaviour of debris flows and wet avalanches.

Zoning of avalanche risk areas is one important task of land-use planning in alpine areas. The lack of records, due to the low frequency of these events, makes it di cult to implement a statistical analysis. Simulations made with physical and mathematical models can improve the knowledge of the dynamics of these events. In this thesis three di different mathematical and numerical models, based on the rheological theory of Savage and Hutter for granular flows, are introduced. A one dimensional model with variable width, written in a curvilinear coordinate system, has been developed. New rheological hypotheses have been introduced in order to describe the frictional interaction with vertical sides. The model has been tested against experimental data, relevant to cases with constant width, collected in the literature. The results give a first confirmation of the rheology, but a campaign of laboratory experiments is necessary in order to verify the model in cases with variable width. The two-dimensional model, implemented at the Department of Civil and Environmental Engineering, has been developed. It was originally written in an horizontal coordinate system, assuming a linear stress tensor distribution and the constancy of the velocity along the vertical instead of along the normal to the bottom direction. The model has been improved by defining a rotated absolute coordinate system, which best fits the sliding surface. This modification corrected the initial delay shown in the simulations of the original model, with respect to the experimental data reported by Gray et al.(1993) in literature. An experimental campaign has been carried out in order to calibrate the two-dimensional model. A two-axial moving system, carrying a laser distance sensor, has been added to the double slope chute of the Hydraulic Laboratory of the University of Trento, in order to automatically scan the final deposits. Alternative choices of the structure of the stress tensor have been tested against experimental data. The two-dimensional model cannot still be applied to real cases. The severe distortion of the mesh, due to strong planimetric gradients of velocity, causes the crash of the model in presence of complex topographies. We focused our attention on meshless methods in order to solve this type of problems. In meshless methods, the computational nodes, which discretize the fluid domain, are not bound by topological relations, as in finite difference and nite element methods. This make them suitable to problems characterized by mass separations, strong deformations and discontinuities. The Moving Least Square Particle Hydrodynamics (MLSPH) method, created by Dilts (1999, 2000), has been applied to the one-dimensional model developed at the Department of Civil and Environmental Engineering of Trento. In the MLPSH model the equations of motion are written in weak form and the field variables of the problem are approximated by means of the Moving Least Square Approximants (MLSA) (Lancaster and Salkauskas (19)). The one-dimensional MLSPH model showed a strong increase of the computational time with respect to its original version, not compensated by a significant improve of performances. Nevertheless, the extension to the two-dimensional case, where more strict constraints are imposed by the mesh, looks promising. The avalanche site of Lavina Granda, in Vigolana range, has been equipped in order to measure the dynamic parameters of real avalanches. We set up three sections for velocity measurements, one section for flowing depth measurement, two graduated poles to estimate snow cover depth in the initiation zone. Wooden tracers were spread over the catchment in order to evaluate avalanche activity. During the winter season 2002-2003 no avalanche activity affected the channel. On summer 2003 a debris flow ran down along the channel. Its flowing velocity was measured and its paths were identified by means of the wooden tracers. During the winter 2003-2004 three granular avalanches and one powder avalanche took place in the channel. The final deposit of the first granular avalanche has been mapped through a detailed topographical survey. A quick topographical survey has been tested in order to evaluate the geometrical modifications of the sliding surface, due to the passage of the avalanches.

Bifurcation is one of the fundamental building blocks of a braided network; it is the process that determines the distribution of flow and sediments along the downstream branches. Braiding is a complex and highly dynamical system, whose evolution is at present predictable only on a short time scale; in this context bifurcations are the crucial process that control the adjustment of braiding intensity, being one of the main causes of the system continuous evolution. A complete description of river bifurcations is still lacking in the literature, though their importance for the onset of braiding is clearly recognized. Moreover, the physical quantitative description of river bifurcation appears as one of the main limitation of the most effective predictive models available at present, i.e. the branches or object-based models. In the first part of the work the attention has been focused on the quantitative description of the evolution of a single laterally unconstrained channel until the occurrence of the first bifurcation. The analyses has been carried out performing four different sets of experimental runs with both uniform and graded sediments. An objective criterion for the occurrence of the bifurcation has been established, using the data provided by the Fourier analysis of the evolving bank profiles; the procedure enabled to characterise the morphodynamic sequence leading to flow and channel bifurcation and to point out the importance of the mutual interactions between the bed deformation and the planimetric configuration of the channel. Along with the characterisation of the onset of bifurcations, it is crucial to investigate their further evolution, that has been pursued starting from the theoretical findings of Bolla Pittaluga et al. (2003), concerning their possible equilibrium configurations. Two sets of experiments has been carried out on a “Y-shaped†symmetrical configuration, in which the upstream channel diverge into two branches. The experimental results show the existence of an unbalanced configuration, when the Shields stress reaches relatively low values and the width to depth ratio is large enough. This asymmetrical configuration is characterised by different values of water and sediment discharges in the downstream branches and by a different bed elevation at their inlet, the channel carrying the lowest discharge showing a higher elevation. Experimental runs characterised by the presence of migrating alternate bars displayed an oscillating behaviour, generally leading to a more unbalanced configuration and, in some cases, to the abandonment of one of the branches. Experimental findings can be interpreted in the light of the morphodynamic influence theory (Zolezzi & Seminara, 2001): the distance of the flow from the resonant value of the aspect ratio seems to be a good parameter to represent such phenomenon. The dynamics of river bifurcation were also analysed in the field. Two field campaign were performed on the Ridanna Creek, Italy and on the Sunwapta River, Canada, joining an international research group. The detailed and repeated measurements allowed to point out the common features showed by the bifurcations, namely the unbalanced water distribution, the difference in bed elevation and the lateral shift of the main flow toward the external bank of the main downstream channel. The monitoring activity on the Ridanna Creek provided also the description of the planimetric and altimetric configurations of the study reach, employing both traditional survey techniques and digital photogrammetry together with the complete characterisation of morphological and hydraulic patterns. Moreover, the analysis of the long term evolution of the network pointed out the existence of three regions in the braided reach, with different morphological features and highlighted the crucial role of bifurcations in controlling braiding evolution. Theoretical analysis, laboratory and field investigations have allowed a much deeper insight in the bifurcation process, giving a quantitative detailed description of the phenomenon. The investigation now provides a suitable description of the bifurcation process that can readily be implemented in predictive models for braiding evolution, for which the adoption of physically based nodal point conditions would be highly desirable and represent the main sought outcome of the present analysis.

High-concentration flows are complex phenomena typical of Alpine mountain areas. Essentially, they are free-surface flows with intense sediment transport, often caused by intense rainfall events and involving large volumes of solid material. Because of the amount of sediments moved, the intense erosion and deposition processes typically observed and the quite unexpected character, these phenomena represent a serious hazard in populated mountain areas, where reliable and effective hazard-management and -protection strategies are required. In mountain-hazard management, high-concentration flows modelling represents a key factor, since it allows to evaluate impacts of possible hazard scenarios and the effectiveness of possible protection and mitigation measures. However, the intrinsic phenomenon complexity makes high-concentration flow modelling and hazard assessment quite challenging. In this thesis, some of the effects of high-concentration flow complexity on modelling are experienced directly and suitable solutions are proposed, to make the phenomenon description more reliable and straightforward. Among very different modelling approaches present in the literature, this work embraced the quasi-two phase, mobile-bed approach proposed in Armanini et al. (2009b) and in Rosatti and Begnudelli (2013a), which is implemented in the TRENT2D model. TRENT2D is a quite sophisticated model that solves a system of Partial Differential Equations over a Cartesian mesh by means of a finite-volume method with Godunov-type fluxes. By means of TRENT2D, the back-analysis of a couple of real debris-flow events occurred in Italy was first performed. These applications revealed clearly some troublesome "complexity issues", i.e. modelling issues generated by phenomenon complexity that may affect hazard assessment. Because of the public importance of the subject, four of the "complexity issues" identified were then faced directly. According to the purpose of this thesis, possible solutions to the issues were proposed, to ensure a proper description of the flow behaviour and possibly limit intricacy in the model use. The first complexity issue is "operational" and regards the use of the TRENT2D model and, more in general, the amount of work necessary to perform a complete hazard-assessment job about high-concentration flows. Because of the phenomenon complexity and the sophisticated character of the model, the operational chain necessary to assess hazard by means of TRENT2D appears quite demanding. The large efforts required in terms of handwork, computational charge and resources may divert the user attention from the physical meaning of the hazard-assessment process, possibly leading to inaccurate results. To overcome this issue, a possible solution is proposed, based on the use of a loosely-coupled Service Oriented Architecture approach. The aim is to develop a unique, user-friendly working environment able to support high-quality, cost-effective hazard assessment and, in perspective, the possible development of a Decision Support System for mountain hazard. The second complexity issue is "geometrical" and "numerical" and concerns morphology representation. Because of the strong interaction between high-concentration flows and bed morphology, these phenomena require bed morphology to be described with the right level of detail, especially where heterogeneity is outstanding. This is typically the case of urbanised mountain areas, with their characteristic terrain shapes, buildings, infrastructures, embankments and mitigation structures. A believable representation of these geometrical constraints may be fulfilled acting on the computational mesh used to solve model equations, preferably avoiding regular Cartesian meshes. In this work, a new version of the TRENT2D model is developed, based on the use of Delaunay, triangular unstructured meshes. To reach second order accuracy, a MUSCL-Hancock approach is considered, with gradient computation performed by means of the multidimensional method proposed in Barth and Jespersen (1989) for Euler equations. The effects of different gradient limiters are also evaluated, aiming at a proper description of the flow dynamics in heterogeneous morphology contexts. The third complexity issue is both "geometrical" and "mathematical". It concerns the effects of artificial structures, i.e. artificial geometrical constraints, on the flow dynamics. Among different structures aimed expressly at controlling the high-concentration flow behaviour, attention was paid to sluice gates, which can be used in channels and hydropower reservoirs to control sediment routing. In the literature, the effects of sluice gates have been studied especially with reference to clear water flows over fixed beds, while knowledge about the influence on high-concentration flows over mobile beds is still limited. Here, a rough, bread new mathematical description is proposed, in order to take into account the 3D morphodynamics effects caused by sluice gates in high-concentration flow modelling. The last complexity issue is pretty "numerical" and arises from the challenge of numerical models to comply with the phenomenon complexity. Generally speaking, reliable numerical models are expected to catch the main characteristics of the physical processes at both a general and a local spatial scale, although with a certain level of approximation, depending on the numerical scheme. Sometimes it may be hard to close the gap between the local phenomenon complexity and its numerical representation, leading to non-physical numerical results that could affect hazard assessment. In this work, a particular numerical issue is investigated, which was identified through a thorough analysis of TRENT2D model results. In particular, it was observed that the direction of the numerical mixture-mass flux is occasionally opposite to the direction of numerical solid-mass flux, despite the isokinetic approach which the model is based on. This incoherence was studied with a rigorous method, trying to fix the source of the problem. However, the question turned out to be quite tricky, due to the sophisticated character of the model. These four, deliberately heterogeneous, "complexity issues" allow to perceive clearly the size of complexity effects on high-concentration modelling. Furthermore, they give the measure of how much diffcult is reaching the right level of detail in describing and modelling high-concentration flows. The research of solutions that are accurate and as much simple as possible was not straightforward and required a quite large effort. Nonetheless, possible solutions were found in the end for three of the four "complexity issues", therefore the goal of the thesis can be considered as achieved.