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.

The gravitational granular flows (e.g. debris flows or snow avalanches) are catastrophic and destructive phenomena affecting many areas in the world, and especially the mountain areas of Europe. Proper design criteria are required in order to improve protection structures and prevention strategies. Due to their complex nature, these phenomena present many aspects still unsolved in the research field. This research addresses some aspects of the mechanics of dry granular flows: a 1D depth integrated model has been developed, based on a two phase approach. The system of equations consists of three partial differential equations, derived from the mass balances for the solid and fluid phase and from the momentum balance for the solid phase, and two rheological relations determined through experimental tests and particle numerical simulations. The experimental investigation hes been conducted in a laboratory channel, by recording through high speed cameras the motion of polystyrene spherical particles. Within this research, it has been developed an ad hoc optical method to analyze and process the images recorded, with the aim of defining the main flow characteristics. From a numerical point of view, a path conservative finite volume scheme has been adopted to solve the system of equations previously described: the numerical solution is compared to the experimental results for different configurations, in order to verify the effectiveness of the mode

The purpose of the Mountain Reno River Basin assessments is to analyse the trend of inflows and outflows and suspended solids from the 1920s to present. The Mountain Reno River Basin is situated in North-Central Apennines,(Emilia Romagna Region, Italy) and its area is 1061 km2. The data set consisted of the monthly and annual rainfall data (91 years), the flow data (90 years) and suspended sediment transport data (36 years) (S.I.M.I stations, ARPA-SMR). On annual basis, the rainfall showed a decreasing trend -16%. Seasonal precipitation showed a negative trend in winter and spring, and a positive one 17.1%, in summer during the period 1960-2006 compared to the 1926-1959 period. Time series for rainfall intensity indicated a significant increasing trend in the last years, where the extreme value were maximum in February (1947) and in November (2004) respectively. In terms of flow, the significative negative trend was equal to -49%, -41m3/s per 100 years of maximum discharge and -58%, -15 m3/s per 100 years of average discharges.. Total suspended solids showed an average reduction of 46% in 100 years, differently from the summer season, which an increase of 56% SSY have been observed. Moreover, were described the Reno riverbed pattern, the morphological features of the floods river flow and minimum river flow from 1863 to 2008. The decreasing trend in streamflow observation data, particularly strong since the 1960s, may be mainly due to climate variability and human factors even if difficult to separate them in different role and weight.

Il modello afflussi-deflussi e di erosione Kineros2, fisicamente basato, distribuito e a scala di evento, è stato applicato a due bacini idrografici montani della provincia di Bologna (Italia) al fine di testare e valutare il suo funzionamento in ambiente appenninico. Dopo la parametrizzazione dei due bacini, Kineros2 è stato calibrato e validato utilizzando dati sperimentali di portata e di concentrazione dei solidi sospesi, collezionati alla chiusura dei bacini grazie alla presenza di due stazioni di monitoraggio idrotorbidimetrico. La modellazione ha consentito di valutare la capacità del modello di riprodurre correttamente le dinamiche idrologiche osservate, nonchè di trarre conclusioni sulle sue potenzialità e limitazioni. / The event scale runoff and erosion model Kineros2, distributed and physically based, was applied to two mountain catchments in the province of Bologna (Italy) in order to test and evaluate it in an Apennine environment. Following basins parameterization, Kineros2 was calibrated and validated by means of experimental discharge and suspended solids concentration data, which had been previously collected at the catchments’ outlets by two hydro-turbidimetric monitoring stations. The simulation activity allowed to evaluate the ability of the model to correctly reproduce the observed hydrological dynamics and, in addition, to gain some knowledge of both its potentialities and limitations.

The abandonment of less productive fields and agro-forest activities has occured in the last decades, interesting large mountain areas in all mediterranean basin. Until the fifties, agricultural practices dealt mainly with soil surface and surface runoff control systems. However, the apparent sustainability of soil use results often in contrast with historical documents, witnessing heavy hydrogeological instability, in naturally fragile areas. The research focused on the dynamics and effects of post-coltural land abandonment in a critical mountain area of the Reno River. The Reno River rappresents a typical Tuscan-Emilian Apennines Watershed where soil erosion occurs under very different conditions depending on interactions between land use, climate, geomorphology and lithology. Landslides are largely rappresented, due to the diffusion of clay hill slopes. Recent researches suggest that climatic variability will increase as a consequence of global climate change, resulting in greater frequency and intensity of extreme weather events, which could increase rates of erosion, landslides reactivations and diffusion of calanchive basins. As far as hill slopes are concerned, instability is today basically due to intrinsic factors, as the Apennine range is a rather young formation, in geological terms, and is mainly formed by sedimentary rocks with high occurrence of clays. Therefore landslides and rockfalls are very frequent, while surface soil erosion is generally low and anyway concentrated in the low Apennine, where intensive farming is still economically worth. The study, supported by GIS use, analyses the main fisical characteristics of the area and the historical changes of land use, and focalizes the dynamics of spontaneous reafforestation. Futhermore, the research studies the results of soil bioengineering and surface water control solutions for the restablishment of landslides occured in the last period. Infact soil bioengineering has been recently used in different situations in order to consolidate slopes and hillsides and prevent erosion; when applied, it gave good results, both in terms of engineering efficiency and vegetational development, expecially if combined with a good hydraulic control, thus proving to be an actual alternative to other techniques with heavier environmental impacts. Research into the specific site features and the use of proper plant species is vital to the success of bioengineering works.

The aim of this PhD thesis, developed in the framework of the Italian Agroscenari research project, is to compare current irrigation volumes in two study area in Emilia-Romagna with the likely irrigation under climate change conditions. This comparison was carried out between the reference period 1961-1990, as defined by WMO, and the 2021-2050 period. For this period, multi-model climatic projections on the two study areas were available. So, the climatic projections were analyzed in term of their impact on irrigation demand and adaptation strategies for fruit and horticultural crops in the study area of Faenza, with a detailed analysis for kiwifruit vine, and for horticultural crops in Piacenza plan, focusing on the irrigation water needs of tomato. We produced downscaled climatic projections (based on A1B Ipcc emission scenario) for the two study areas. The climate change impacts for the period 2021-2050 on crop irrigation water needs and other agrometeorological index were assessed by means of the Criteria water balance model, in the two versions available, Criteria BdP (local) and Geo (spatial) with different levels of detail. We found in general for both the areas an irrigation demand increase of about +10% comparing the 2021-2050 period with the reference years 1961-1990, but no substantial differences with more recent years (1991-2008), mainly due to a projected increase in spring precipitation compensating the projected higher summer temperature and evapotranspiration. As a consequence, it is not forecasted a dramatic increase in the irrigation volumes with respect to the current volumes.

Hydropower plants, in particular High-head Hydropower Plants (HPPs), are an important source of energy also for their role in covering the daily peaks of energy demand. However, HPPs, especially storage power plants, have several negative effects on the ecosystems of downstream watercourses inducing unnatural changes in flow regime (hydropeaking). One way to study ecological implications induced by hydropeaking is represented by the coupling of hydrodynamic models (CFD) with habitat suitability models, in which hydrodynamic parameters are typically used to describe the physical habitat of indicator species. The research activity wanted to investigate possible differences between the use of 2D and 3D CFD approaches to determine the watercourse hydraulic characteristics and their effects on habitat evaluations, performed with CASiMiR software, in complex morphology as usually presents in hydropeaked reaches. In particular the habitat suitability for the two case studies (Valsura River and Rio Selva dei Molini), is analysed comparing different approaches for the reconstruction of the velocity field (depth-averaged velocities from 2D modelling, bottom velocity field reconstruction with log-law approach from 2D modelling and bottom velocity field from 3D modelling). The results show that the habitat suitability index (HSI) using 2D or 3D hydrodynamic models can be significantly different. Considering the entire flow range of hydropeaking events, the habitat simulations with bottom flow velocities from 3D modelling provide suitable habitats over the entire flow range representing the availability of stable suitable habitats. The results from the hydraulics and habitat analyses are used to investigate the effects of a hydropeaking mitigation project on the Valsura River (realization of a compensation bypass tunnel to decrease the peak flow rate and to remodel the up and down flow ramping rates) and on Rio Selva dei Molini (morphological measures to reduce the hydropeaking effects).

In the past decades, hydrodynamic cavitation (HC) process was the subject of study by many researchers worldwide. This phenomenon was widely studied in order to understand the reason of its negative effects on hydraulic machinery such as pumps,turbines, valves, etc. Many efforts were made in order to better understand mechanisms of HC process with the main aim of preventing its generation and trying to avoid severe physical damage such as erosions, vibrations and noises. In recent years, in order to cope with a decrease in available water resources worldwide, an increasing demand of water by population in developing/developed countries and more restrictive environmental legislations on water quality, HC was increasingly used as a novel energy-efficient technique in the field of wastewaters treatment. The main purpose of this thesis is to investigate on the effectiveness of a modified swirling-jet device called Ecowirl reactor, patented by Econovation GmbH, Germany and produced and commercialized by Officine Parisi s.r.l., Italy. Experimental studies were carried out in order to evaluate the effects of different operative conditions and parameters such as reactor geometry, flow rate, flow velocity, pressure, medium pH, medium concentration and medium temperature on (i) the degradation of a toxic and carcinogenic pollutant dye (Rhodamine B, RhB) in waste dye aqueous solutions and on (ii) the improvement of activated sludge solubilisation and aerobic sludge biodegradability in the field of biological wastewater treatments. In order to better understand the fluid dynamics into Ecowirl reactor, it was modelled. The model based on previous experimental data was implemented in a Computational Fluid Dynamics software (ANSYS, 16.2).

Driftwood is widely recognized as a relevant component of riverine systems due to its complex interactions with flow, sediment transport and vegetation dynamics. In-channel large wood has a relevant geomorphic and ecological role as it enhances morphological diversity and creates a variety of physical habitats that sustain high biodiversity. Its presence can also increase flood risk and therefore wood is often removed from streams especially in densely populated areas. Recent river restoration policies aim to maximise the environmental benefits of driftwood and minimise risks. The study of wood dynamics can provide useful information to define guidelines for sustainable wood management. Multi-thread systems represent a particularly interesting and challenging context for the investigation of wood dynamics because of their complex geometry, the presence of vegetated islands and the frequent, intense changes in channel pattern observed even for moderate discharge fluctuations. However, comparatively few studies focus on driftwood in large braided rivers and limited quantitative information is available on wood transport, deposition and remobilisation in these systems. The goals of the present work are: a) characterising the spatial organisation of wood deposits and identifying typical retention sites and styles; b) analysing the influence of flow regime, channel morphology, wood supply and log properties (size and shape) on dispersal patterns; c) investigating wood remobilisation induced by discharge fluctuations and bed reworking; and d) analysing long-term wood storage volume and budget. A combination of field-scale direct observations, remote sensing techniques and physical modelling was used to investigate wood and channel dynamics. Field-scale monitoring carried out on the Tagliamento River (Italy) allowed the observation of complex interactions and feedbacks between channel, vegetation and wood dynamics. Laboratory simulations – carried out in two large flumes at the University of Trento (Italy) and at the University of Hull (UK) – were employed to investigate individual wood dispersal mechanisms under controlled conditions and to explore the role of governing parameters. In large rivers, floods are the primary driver of wood recruitment through the erosion of vegetated banks and islands; field-scale observations showed that these localised wood inputs control wood storage at sub-reach scale because a large proportion of eroded trees is retained close to the input point in sparse, small jams. Physical modelling highlighted a complex relationships between flow stage and the longitudinal and vertical distribution of wood; high discharge increases the ability of the system to transfer wood, but at the same time generates complex inundation patterns where a larger number of sites are available for wood retention. No clear link between flow stage and the vertical distribution of wood is observed, probably because water surface elevation exhibits small changes with discharge in flat braided river cross-sections. Driftwood element properties also influence deposition patterns; log diameter controls travel distance as it governs flotation and therefore the likeliness of deposition. High element length and complex piece shape sustain the formation of large jams. The presence of a root bole is also associated to short travel distance and low relative elevation. At reach-scale, the spatial distribution of wood is the product of local inputs during major floods and reorganisation of deposits induced by minor events. Wood pattern exhibits a threshold behaviour with supply. High input rates determine very high spatial density and the formation of large, stable jams. Two processes govern wood reorganisation over different time scales, namely network inundation – inducing rapid changes in flow field – and bed reworking. In the first case, the persistence of deposits depends on the magnitude of discharge fluctuations as wood dispersed by small floods is easily removed by larger events. High relative elevation and large jam size enhance wood stability, while the presence of a root wad has a dual effect as it determines large accumulations at low elevation. Channel pattern reworking determines intense turnover of driftwood deposits regardless of supply rate, piece properties and jam size, save for very large accumulations. As a consequence, wood deposition occurs mostly on empty braidplain areas as opposed to pre-existing sites. These results suggests that wood (alone) has little direct effect on reach-scale bed geometry in a large braided river; however, deposited wood significantly influences local hydraulics and morphology, enhancing physical habitat diversity. Moreover, deposited wood favours the accumulation of fine sediment, nutrients and seeds and often exhibits vegetative regeneration. These processes lead to the transformation of instable driftwood pieces into vegetated islands, which in turn can trap more wood. Therefore, wood has a relevant, indirect effect on braided river morphodynamics through the establishment of vegetation, whose presence influences network complexity and evolution.

Rivers provide to society many important goods and benefits. Some of these ecosystem services depend on the river flow regime, which has been deeply modified by human structures and activities. These alterations have a direct influence on biodiversity, natural habitat and on the supply of river ecosystem services. The release of water from storage hydropower plants generates rapid flow and stage fluctuations (hydropeaking) in the receiving water bodies at a variety of sub-daily time-scales. In this thesis, we describe an approach to quantify such variations, which is easy to apply, requires stream flow data at a readily available resolution, and allows for the comparison of hydropeaking flow alteration amongst several gauged stations. Hydropeaking flow alteration is quantified by adopting a rigorous statistical approach and using two indicators related to flow magnitude and rate of change. We utilised a comprehensive stream-flow dataset of 105 gauging stations from Italy, Switzerland and Norway to develop and test our method. Next, we introduce a modelling approach to evaluate the spatial and temporal variations of a discharge-related ecosystem service, the rafting. The application of hydraulic and habitat models allowed to define spatially thresholds of suitability in each river reach and the application of an hydrological model allowed to assess temporally the suitability for the rafting navigability in different discharge conditions. We applied the method to the Noce River, an Alpine River in Northern Italy affected by hydropeaking. Our analysis showed that in this river, the water releases are fundamental to maintain high flow conditions required for rafting, which can be granted only by hydropower production especially in summer months. Together with present discharge conditions, our approach allows to analyse also the effects of an additional withdrawal which locally has a negative impact on river suitability. Finally, the application of the methodology was extended to include in the analysis the fish habitat and the small hydropower production, along with the rafting. The effects of hydropeaking on these ecosystem services were assessed in space and time. Hydropeaking has a strong influence on rafting navigability and less obvious consequences on the other services. Different management scenarios of the water releases from the hydropower plants were produce, with the aim to evaluate spatially the reciprocal effects of optimizing each ecosystem services. Only the scenario of rafting optimization will significantly increase rafting navigability, while the effects of other scenarios are less evident. Moreover, two additional increasing withdrawals have been simulated to evaluate their impacts on the services. The small hydropower withdrawals will have a negative impact on rafting and fish habitat, while the preservation of requirements for rafting will greatly affect the small hydropower production. This ecosystems-services based approach can be integrated in the decision-making process to evaluate river management alternatives.