Cristiana Di Cristo

Modelling trihalomethanes formation in water supply systems

Chlorination is the most widely used method for disinfection of drinking water, but there are concerns about the formation of by-products, such as trihalomethanes (THMs), since the chronic exposure to them may pose risks to human health. For these reasons regulations fix maximum acceptable THMs levels throughout distribution networks, so it is very important to be able to correctly reproduce their formation. In the literature many models for predicting THMs formation have been developed, both based on empirical relationships and on kinetics involved during chlorine reactions. In this work the use of some of these models and their reliability in real situations is investigated through the application to the Aurunci-Valcanneto Water Supply System in Southern Lazio (Italy). The comparison of the performances of 18 selected literature empirical models furnishes interesting observations, indicating that the formula, developed using field data, results in being more suitable for reproducing THMs formation for the presented case study. Other considerations are also offered from the comparison with the results obtained using a simple first order kinetic model, calibrated using measured data.

Simulation of dam-break waves on movable beds using a multi-stage centered scheme

Abstract This paper presents the application of the multi-stage first-order centered scheme GMUSTA to solve a two-phase flow model with four equations for simulating dam-break floods without and with sediment transport. Computation of generalized Riemann invariants can be particularly complex and costly in simulating dam-break floods with sediment transport. GMUSTA numerical scheme, which does not require complete knowledge of the eigenstructure of the hyperbolic mathematical model, offers a suitable and attractive option. The quality of the dam-break flood simulations with GMUSTA scheme is verified by comparing the results against laboratory tests and some experimental data available in the literature, on fixed and mobile bed conditions, with different bed materials and flush or stepped bottoms. The numerical results reproduce quite well the experimental evidence, proving that the model is capable of predicting the temporal evolution of the free-surface and the bed. The GMUSTA scheme, which is not only simple to implement but also both accurate and computationally efficient, is proposed as an appropriate tool for integrating non-equilibrium sediment-transport models.

Green's Function of the Linearized Saint-Venant Equations in Laminar and Turbulent Flows

In the present paper, an analytical expression of the Green’s function of linearized Saint-Venant equations (LSVEs) for shallow water waves is provided and applied to analyse the propagation of a perturbation superposed to a uniform flow. Independently of the kinematic character of the base flow, i.e., subcritical or supercritical uniform flow, the effects of a non-uniform vertical velocity profile and a non-constant resistance coefficient are accounted for. The use of the Darcy-Weisbach friction law allows a unified treatment of both laminar and turbulent conditions. The influence on the wave evolution of the wall roughness and the fluid viscosity are finally discussed, showing that in turbulent regime the assumption of constant friction coefficient may lead to an underestimation of both amplification and damping factors on the wave fronts, especially at low Reynolds numbers. This conclusion has to be accounted for, particularly in describing hyper-concentrated suspensions or other kinds of Newtonian mixtures, for which the high values of the kinematic viscosity may lead to relatively low Reynolds numbers.

Influence of Relative Roughness and Reynolds Number on the Roll-Waves Spatial Evolution

The paper investigates the influence of the resistance coefficient variability onto the spatial development of roll-waves. Two models, based on time-asymptotic solutions of the linearized St. Venant equations, subject to either impulsive or oscillating perturbation, have been modified by including the dependence of the resistance coefficient on flow conditions, wall roughness, and fluid viscosity. Independently of the perturbation type, it has been shown that the hypothesis of constant resistance coefficient leads to underestimate the disturbance spatial growth. Theoretical predictions are finally compared with results of a fully nonlinear model and with literature experimental data for several combinations of Froude and Reynolds numbers and relative roughness values. The representation of variability of the resistance coefficient fundamentally improves the performance of minimum channel length criteria, whereas its neglect may lead to noncautious channel design.

Applicability of Kinematic, Diffusion, and Quasi-Steady Dynamic Wave Models to Shallow Mud Flows

Unsteady shallow-layer flows may be described through full dynamic models or using simplified momentum equations, based on kinematic, diffusion, and quasi-steady approximations, which guarantee a reduction of the computational effort. This paper aims to investigate through linear analysis the applicability range of simplified shallow-wave models with special concern to unsteady flows of mud. Considering a three-equation depth-integrated Herschel-Bulkley model, the applicability of the approximated wave models is discussed comparing the propagation characteristics of a small perturbation of an initial steady uniform flow as predicted by the simplified models with those of the full dynamic model. Based on this comparison, applicability criteria for the different wave approximations for mud flows of Herschel-Bulkley fluids, which account for the effects of the rheological parameters, are derived. The results show that accounting for the fluid rheology is mandatory for the choice of an appropriate simplified model. DOI: 10.1061/(ASCE)HE.1943-5584.0000881.

Simplified wave models applicability to shallow mud flows modeled as power-law fluids

Simplified wave models — such as kinematic, diffusion and quasi-steady — are widely employed as a convenient replacement of the full dynamic one in the analysis of unsteady open-channel flows, and especially for flood routing. While their use may guarantee a significant reduction of the computational effort, it is mandatory to define the conditions in which they may be confidently applied. The present paper investigates the applicability conditions of the kinematic, diffusion and quasisteady dynamic shallow wave models for mud flows of power-law fluids. The power-law model describes in an adequate and convenient way fluids that at low shear rates fluids do not posses yield stress, such as clay or kaolin suspensions, which are frequently encountered in Chinese rivers. In the framework of a linear analysis, the propagation characteristics of a periodic perturbation of an initial steady uniform flow predicted by the simplified models are compared with those of the full dynamic one. Based on this comparison, applicability criteria for the different wave approximations for mud flood of power-law fluids are derived. The presented results provide guidelines for selecting the appropriate approximation for a given flow problem, and therefore they may represent a useful tool for engineering predictions.

Boundary conditions effect on linearized mud-flow shallow model

The occurrence of roll-waves in mud-flows is investigated based on the formulation of the marginal stability threshold of a linearized onedimensional viscoplastic (shear-thinning) flow model. Since for this kind of non-Newtonian rheological models this threshold may occur in a hypocritical flow, the downstream boundary condition may have a nonnegligible effect on the spatial growth/decay of the perturbation. The paper presents the solution of the 1D linearized flow of a Herschel and Bulkley fluid in a channel of finite length, in the neighbourhood of a hypocritical base uniform flow. Both linearly stable and unstable conditions are considered. The analytical solution is found applying the Laplace transform method and obtaining the first-order analytical expressions of the upstream and downstream channel response functions in the time domain. The effects of both the yield stress and the rheological law exponent are discussed, recovering as particular cases both power-law and Bingham fluids. The theoretical achievements may be used to extend semi-empirical criteria commonly employed for predicting roll waves occurrence in clear water even to mud-flows.

On the applicability of minimum channel length criterion for roll-waves in mud-flows

Abstract The paper addresses the prediction of roll-waves occurrence in mud-flows. The spatial growth of a point-wise disturbance is analytically described, based on the linearized flow model of a Herschel and Bulkley fluid, in the neighborhood of an initial uniform base condition. The theoretical achievements allow to generalize to mud-flows the minimum channel criterion commonly used for the prediction of roll-waves in clear-water. The applicability of the criterion is discussed through the comparison with literature laboratory data concerning unstable flows without rollwaves.

Evaluation of Different Formulations to Optimally Locate Sensors in Sewer Systems

AbstractEfficient management of a sewer system includes the control of the conveyed wastewater quality to adequately operate treatment plants and protect the receiving water bodies. Moreover, these...

Illicit intrusion characterization in sewer systems

Abstract A very important aspect in sewer systems management is represented by the detection of illicit intrusions, which recently supported the development of online sensors for wastewater quality monitoring. The present paper proposes a new methodology for characterizing an illicit intrusion in a sanitary or combined sewer system, using on-line pollutant concentration measurements. The source identification is formulated as an optimization problem, solved combining the hydraulic and quality simulation tool storm water management model (SWMM) with the GALib code. The methodology, which includes a pre-screening procedure useful for complex and large systems, is applied to a literature scheme and a real test-case, showing promising results. An uncertainty analysis is also performed for checking its robustness with respect to inflow uncertainty and in the presence of measurement errors.