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Dike erosion is a crucial issue in coastal and fluvial flood risk management. These defense structures appear vulnerable to extreme hydrological events, whose potential occurrence risk seems recently increased due to climate change. Their design and reinforcement is, however, a complex task and, although numerical models are very powerful nowadays, some physical processes cannot be predicted accurately and, therefore, physical models constitute a useful tool to investigate different features under controlled conditions. This paper discusses an experimental study of erosion of a sand dike produced by the impact of a sudden dambreak wave. The experiments have been conducted in the Water Engineering Laboratory at University of Cassino and Southern Lazio, Cassino (FR), Italy, in a small-scale rectangular channel, where a gate constitutes the barrage creating an upstream reservoir, while downstream a dike of almost-uniform non-cohesive material is positioned. Two sands with different diameter size have been used for the dike structure. The experimental evidence proves that the whole erosion process is strongly unsteady and it is significantly different from a gradual overtopping one. Moreover, the results highlight the importance of apparent cohesion for the fine-sand dike.

River morphodynamics and sediment transport Mechanics of sediment transport

River hydrodynamics Turbulent open channel flow and transport phenomena

River hydrodynamics Unsteady open channel flow and dam break

Nella memoria è presentato un metodo per la localizzazione della sorgente di una contaminazione accidentale in un sistema di distribuzione idrica. La procedura, basata su alcune ipotesi semplificative, è di semplice utilizzo ed ha un basso costo computazionale. In particolare, nel lavoro è presentata un’applicazione del metodo proposto al caso studio del sistema di adduzione primaria dell’acquedotto Vesuviano, in provincia di Napoli. Il buon risultato ottenuto conferma l’applicabilità del metodo a reti reali complesse, che non necessariamente raggiungono condizioni stazionarie.

The quality of hydraulic model calibration depends on sensor measurements. More field information gives a better calibration, but data collection is a costly process. For this reason, ineffective measurements should be avoided and it is important to use a method for designing data collection that will give the most useful information. Some methods for sampling design of water distribution networks were proposed. The simplest method assumes the nodal sensitivity of the model outputs versus the unknown calibration parameters as a means to decide where to make measurements. Comparing this method with more sophisticated procedures, it seems effective and less costly. Its major problem is that computation of sensitivity coefficients requires some values to be assigned to the unknown calibration parameters. In this paper the dependence of the sensitivities upon the roughness values and the loading condition used for their computation is investigated. Considering two sample networks the results show the independence of nodal sensitivity distribution from both the assigned loading condition and pipe roughness. This implies that nodal sensor location can be obtained also by using a fairly inaccurate estimate of calibration parameters. The above observations suggest that nodal sensitivity distribution could represent a sort of network signature for design and management purposes.

Una sempre crescente attenzione è rivolta alla necessità di avere una gestione efficace delle reti di distribuzione idrica per un uso più oculato della risorsa. In quest’ottica, è estremamente utile poter disporre di modelli in grado di simulare adeguatamente il comportamento delle reti, in modo da effettuare le scelte gestionali più opportune. Affinché un modello di simulazione fornisca risultati prossimi a quelli osservati è necessario prevedere una sua calibrazione. Per la calibrazione dei modelli di simulazione, ma anche per risolvere altri problemi, ugualmente importanti, come ad esempio l’individuazione delle perdite, è necessario un’adeguata conoscenza del funzionamento reale della rete. Questa è rappresentata unicamente dai dati forniti dai sensori collocati nei nodi o sui lati, che misurano rispettivamente le pressioni e le portate circolanti. In genere, soprattutto nelle reti di grosse dimensioni, il numero dei sensori disponibili è molto ridotto per i costi che essi comportano; pertanto è fondamentale che le misure siano realizzate nella maniera più efficiente possibile. Per questo motivo è particolarmente importante pianificare adeguatamente le misure, problema generalmente indicato come “sampling design”. In particolare, stabiliti il numero e il tipo di misure da effettuare, un punto cruciale riguarda la scelta della disposizione dei sensori, che è lo scopo dell’operazione di progettazione della localizzazione dei punti di misura, o “sensor location”. Nella seguente nota sono presentate e confrontate alcune delle procedure maggiormente utilizzate per la localizzazione ottimale dei punti di misura con riferimento ad una rete di distribuzione precedentemente utilizzata dagli autori. L’esempio riportato evidenzia bene l’importanza di un buon posizionamento dei sensori al fine di una migliore calibrazione.

The study of the interaction of mud-flows with obstacles is important to define inundation zones in urban areas and to design the possible structural countermeasures. The paper numerically investigates the impact of a mud-flow on rigid obstacles to evaluate the force acting on them using two different depth-integrated theoretical models, Single-Phase Model (SPM) and Two-Phase Model (TPM), to compare their performance and limits. In the first one the water-sediment mixture is represented as a homogeneous continuum described by a shear-thinning power-law rheology. Alternatively, the two-phase model proposed by Di Cristo et al in 2016 is used, which separately accounts for the liquid and solid phases. The considered test cases are represented by a 1D landslide flowing on a steep slope impacting on a rigid wall and a 2D mud dam-break flowing on a horizontal bottom in presence of single and multiple rigid obstacles. In the 1D test case, characterized by a very steep slope, the Two-Phase Model predicts the separation between the two phases with a significant longitudinal variation of the solid concentration. In this case the results indicate appreciable differences between the two models in the estimation of both the wave celerity and the magnitude of the impact, with an overestimation of the peak force when using the Single-Phase Model. In the 2D test-cases, where the liquid and solid phases remain mixed, even if the flow fields predicted by the two models present some differences, the essential features of the interaction with the obstacles, along with the maximum impact force, are comparable.

The evaluation of the impact force on structures due to a flood wave is of utmost importance for estimating physical damage and designing adequate countermeasures. The present study investigates, using 2D shallow-water approximation, the morphodynamics and forces caused by a dam-break wave against a rigid obstacle in the presence of an erodible bed. A widely used coupled equilibrium model, based on the two-dimensional Saint&ndash, Venant hydrodynamic equations combined with the sediment continuity Exner equation (SVEM), is compared with a more complex two-phase model (TPM). Considering an experimental set-up presented in the literature with a single rigid obstacle in a channel, two series of tests were performed, assuming sand or light sediments on the bottom. The former test is representative of a typical laboratory experiment, and the latter may be scaled up to a field case. For each test, two different particle diameters were considered. Independently from the particle size, it was found that in the sand tests, SVEM performs similarly to TPM. In the case of light sediment, larger differences are observed, and the SVEM predicts a higher force of about 26% for both considered diameters. The analysis of the flow fields and the morphodynamics shows these differences can be essentially ascribed to the role of inertia of the solid particles.

Assessment of river embankments security is an important task for land use planning. In the MISALVA Project (financed by the Italian Government), the case study of the Volturno River in Southern Italy is analyzed. A two-dimensional numerical model (2D) of the about 30 km long reach from Ponte Annibale Dam to the sea is built and used to assess levee vulnerability for different hydrographs. In this paper, a simplified analysis procedure will be shown for the evaluation of the levees critical issues. Among the various types of bank failure, only overtopping and piping will be taken into account, considering two types of levees: erodible and unerodible levees. The different criticality scenarios of the levees are assessed by comparing three fundamental parameters. These three parameters are the maximum water level obtained from the hydraulic simulation, the elevation of the top of the levee and the elevation of the land on the countryside. The aim of the work is to define a rapid procedure for the determination of possible embankment criticalities.

Because of climate change, flood-prone areas are more and more frequently exposed to potential casualties and damage. The capability of the flow to carry relevant quantities of sediments during these critical events adds to the further complexity of the resulting scenarios. The interaction between the flow and the obstacles in flood-inundated areas contributes to an increase in the hazard level and constitutes a relevant concern in the framework of risk analysis. Despite this relevance, the existing literature on the topic is relatively scarce, especially for the estimation of the forces acting on rigid obstacles in the presence of a mobile bed. In the present paper, a recent two-phase shallow-water morphodynamical model, particularly suited for the analysis of fast geomorphic transients, is applied for the numerical simulation of the propagation of a dam-break wave over an erodible floodplain in the presence of a rigid obstacle. The geometry of the test-case is inspired by a recent fixed-bed study reported in the literature, for which extensive experimental and numerical data concerning the flow field and the dynamic loading against the obstacle are available. Results of the numerical simulations contribute to highlight the effects of the obstacle on the changes in the bottom topography.

The Kinematic Wave Model (KWM), representing a simplification of the Full Wave Model (FWM) under the shallow-water approximation, is often used in the one-dimensional formulation for describing the dynamics of debris/mud flood. The present work investigates the applicability conditions of the KWM to analyze mud floods, characterized by a rheological behaviour expressed through a power-law model. The study is carried out through the numerical solution of both the Full and Kinematic Wave models changing the characteristic time of hydrograph, i.e. the wave duration, imposed at the channel inlet. The predictions of the two models are compared in terms of maximum flow depth, maximum discharge and peak discharge at the downstream channel end. The study is performed for several values of the Froude (F) and the Kinematic Wave (K) numbers. Similarly to the clear-water case, present results show that higher errors in applying KWM pertain to shorter flood wave durations, but the performance of KWM appears to strongly depend on the value of the rheological index, becoming worse as the fluid rheology becomes more shear-thinning. The study furnishes applicability criteria representing a guideline for practical applications in terms of minimum wave duration above which the KWM error in reproducing the considered flood characteristics (the maximum flow depth, the maximum discharge and the peak discharge at the downstream channel end) is less or equal than 5%. Finally, it has been shown that the wave period criterion, obtained considering linearized wave dynamics, may be safely applied at least for fluid with moderate shear-thinning behavior and for moderate values of the dimensionless number KF2.

This paper investigates the spatial evolution of a disturbance in an open-channel flow of a power-law fluid at non-uniform accelerated and decelerated initial profiles, up to the occurrence of roll-waves in mild and steep slope channels. Both theoretical and numerical analyses are applied to the depth-averaged continuity and momentum conservation equations, deduced from the von Kármán's integral method. For the theoretical investigation, the nonlinear near-front expansion technique was applied. Then, the full nonlinear problem in its conservative formulation was numerically solved. Independently of the rheology of the flowing medium, non-uniform initial conditions strongly influence the perturbation celerity, the disturbance evolution and the roll-wave development. For mild slope channels, an initially decelerated profile of shear-thinning fluids has a stabilizing effect, while the opposite is found for accelerated profiles. For shear-thickening fluids, only the stabilizing effect caused by a decelerated profile is observed. In steep slope channels, independently of the fluid rheology, decelerated initial conditions promote roll-wave occurrence, while accelerated conditions inhibit the perturbation growth. Although experimental verifications are needed, the present results have to be properly accounted for in defining roll-wave prediction methods and in assigning appropriate boundary conditions to enhance or to reduce their formation.

The present work proposes a methodology to model the diffusion of pollutants along urban drainage networks (UDNs) based on a topological analysis. The strategy is based on both the complex network theory and backtracking algorithms. The goals are to highlight the role of topology in the diffusion of pollutants and to simplify the computational effort compared with other methodologies without the need to solve optimization problems and to resort to hydraulic simulations. In particular, a new index called the node contamination index IC is proposed to evaluate the effect that the contaminant spill in each node has on the entire system. The strategy is presented as a support tool for both technicians and water utilities who must face the management of increasingly complex and unpredictable systems.

The study investigates the dynamical characteristics of the impact of mud and lava flows against a rigid wall. The flowing medium is modeled as a non-Newtonian homogeneous continuum fluid and a power-law model with a shear-thickening behavior is chosen as the constitutive equation. The flow occurs as a dam-break wave and it is simulated adopting the 1-D shallow water equations. The analysis focuses on the evaluation of the peak force under different conditions. Results suggest that both the fluid rheology and the wall distance have a strong influence on the dynamics of the impact.

Mud flows represent one of the major causes of natural hazards in mountain regions. Similarly to debris flows, they consist of a hyper-concentrated mixture of water and sediments flowing down a slope and may cause serious damages to people and structures. The present paper investigates the force produced by a dam-break wave of mud impacting against a rigid wall. A power-law shear-thinning model is used to describe the rheology of the hyper-concentrated mixture. A one-dimensional shallow water model is adopted and a second-order Finite Volume scheme is employed to numerically solve the governing equations. The results indicate that depending on the fluid rheological parameters and on the bottom slope, there exists a minimum value of the wall distance above which the peak force does not exceed the asymptotic value of the hydrostatic final condition. For two different values of the channel slope, the dimensionless value of this lower bound is individuated for several values of the power-law exponent and of a dimensionless Basal Drag coefficient. An estimation of the maximum peak force for wall distance smaller than the minimum value is also provided.

Water distribution networks are expected to fulfill the water demand by all consumers and at all times, even during critical scenarios, such as pipe failures. In this work, a methodology is proposed to maximize the quality of service during pipe failures by operating valves. The selection of the valves to operate is done by solving an optimization problem using Gondwana, a generic optimization tool for drinking water distribution networks. Different objective functions and different failure scenarios are investigated, considering a real-life water distribution network. The analysis is performed considering the peak demand condition. The proposed methodology is useful for water companies in managing the operation of their networks during critical scenarios.

The Kinematic Wave Model (KWM), representing a simplification of the Full Wave Model (FWM) under the shallow-water approximation, is still used for predicting flood propagation requiring a reduced computational effort and for being more easily coupled with high-resolution topographic data. It is as suitable approach also for predicting debris and mud-flows. In this work the applicability of the KWM for reproducing the propagation of mud-flows characterized by a power-law shear-thinning rheology is investigated. The power-law model is suitable for fluids with negligible yield stress, named mud-flows, such as fine sediment-water mixtures which are often encountered in natural estuaries or in landsides. For this reason, it is very important to individuate the hydraulic conditions in which the KWM can be safely applied for predicting mud flow propagation. In this study the power-law shallow-layer model of Ng and Mei (1994) is considered for the analysing the applicability conditions of the KWM through an unsteady analysis. Comparing the flow depth profiles predicted by KWM with those of the Full Wave Model the applicability conditions are characterized by evaluating the error on the dimensionless maximum flow depth along the channel. The preliminary results presented herein confirm a strong dependency from the rheology and indicate that criteria for clear-water flows do not apply to shearthinning fluids. Moreover, the error increases moving in the downstream direction and higher errors pertain to lower wave durations of the discharge hydrograph. Further investigations will provide specific criteria for the applicability of the KWM for predicting mud-flows propagation.

Water distribution networks (WDNs) need to guarantee that water is delivered with adequate quality. This paper compares the performance of 12 multiobjective procedures to limit water quality deterioration in a WDN through the optimal operation of valves. The first objective (ObF1) is to minimize the water age, chosen as a surrogate parameter of quality deterioration, and the second objective (ObF2) is to minimize the number of valve closures. The 12 procedures are derived from the combination of 4 different optimization algorithms and 3 formulations of ObF1, namely, to minimize the maximum, the arithmetic mean, and the demand-weighted mean water age. The optimization algorithms considered are random search (RS), Loop for Optimal Valve Status Configuration (LOC), and a combination of each of these two with the Archive-based Micro Genetic Algorithm. The procedures are tested on two networks of different complexity. Results show how LOC is able to find near-optimal solutions using a fraction of the computational time required by a brute force search. Furthermore, among the ObF1 formulations, the use of the averages (either arithmetic or demand-weighted) gives better results in terms of impact on the population served by a WDN.

A depth-averaged model with a power-law rheology, usually adopted for debris flow of fine sediment-water mixtures, is analytically studied, considering the complete (Full Dynamic Model) and the simplified momentum equations, known as Kinematic, Diffusion, and Quasi-steady Models. Applying the Laplace transform to the linearized equations, the analytical first-order expressions of the upstream and downstream spatio-temporal channel response functions are derived for the full and the approximate models. Both shear-thinning and shear-thickening fluids have been considered in hypocritical and linearly stable conditions, to investigate the influence of the rheology on the wave propagation. The effect of the upstream and downstream boundary conditions is considered assuming a finite channel length. The analytic description of the various linearized approximations allows a multi-faceted comparison of the wave propagation. About the basic wave characteristics, in the entire investigated range of Froude numbers and for all the considered rheologies, the Kinematic and Diffusive models underestimate the primary wave celerity and the Quasi-steady model underestimates the wave round-trip time. However, the examination of the response function reveals that the approximate models may provide relatively accurate reproduction of the full dynamic response. Moreover, the analytical solutions in the Laplace variables permit a systematic analysis of some relevant shape factors of the response functions. Independently on the models and the rheology, the damping of the downstream response function is several orders of magnitude larger than that of the upstream one. The results of the shape factor analysis of the upstream response function indicate that the diffusive approximation is the most accurate one. The performed comparison among the wave characteristics of the different approximations provides indications on the applicability of the simplified models for predicting debris or mud flows.

Water quality deterioration in water distribution networks can be associated with high water residence time in the network. To this end, some previous studies have proposed optimization procedures for valve management. However, these studies generally come up with operational configurations assuming deterministic user demand patterns that may never occur in reality. In consequence, the proposed solutions may not be effective for improving water quality or do not comply with pressure constraints if different demand patterns are observed. This study proposes a methodology to determine robust configurations of the valves to keep water residence time at acceptable levels regardless of the variability in demand patterns. The methodology is tested on four different distribution systems of varying topology and size. Results show the importance of executing robust – instead of deterministic, optimization to find valve configurations that guarantee the performance of the networks in terms of hydraulics and water quality.