Vanja Travaš

Gubitci vode u vodoopskrbnim sustavima uzrokuju značajne ekonomske i ekološke štete. Pored povećanja troškova distribucije pitke vode, oni nepovoljno utječu i na vodne resurse, što je osobito izraženo u područjima s ograničenim zalihama vode. Unatoč uloženim naporima, u Republici Hrvatskoj gubici i dalje čine značajan udio ukupne potrošnje te prosječno dosežu i do 50 % zahvaćenih količina vode. Procjena gubitaka temelji se na specifičnim pokazateljima stanja vodnokomunalne infrastrukture, među kojima se posebno ističe ILI pokazatelj (engl. Infrastructure Leakage Index), predložen od Međunarodne udruge za vodu (engl. International Water Association), definiran kao omjer stvarnih i neizbježnih gubitaka vode. U radu je prikazana teorijska osnova procjene ILI pokazatelja s naglaskom na ključne parametre koji na njega utječu (stvarni i neizbježni gubici, duljina cjevovoda, prosječni tlak u sustavu i broj korisnika). Također je prikazan praktičan primjer procjene ILI pokazatelja temeljen na računalnom modelu određene DMA zone (engl. District Metered Area). Primjenom programa EPANET modelirani su tlakovi unutar zadane DMA zone, a na temelju dobivenih podataka procijenjeni su stvarni i neizbježni gubici vode. Analizom su identificirana područja unutar DMA zone, koja zahtijevaju implementaciju mjera za smanjenje gubitaka vode i poboljšanje operativne učinkovitosti sustava. Zaključeno je da se smanjenje ILI pokazatelja na ciljanu vrijednost može postići isključivo iterativnom primjenom numeričkih analiza i terenskih mjerenja.

Leakages in water supply networks can be minimized through pressure reduction or by repairing the resulting damage. The effective implementation of the latter approach necessitates the localization of leaks, for which various methods exist. This paper presents a specific software and hardware configuration for constructing a digital twin of pressurized piping systems that can be utilized for this purpose. The proposed method can identify both the flow and position of leaks and is based on solving an equivalent optimization problem, where the objective function depends on the difference between the measured pressure values at specific points and those predicted at the same points under the same boundary conditions by a computational model. Given the multimodality of the formulated problem, particle swarm optimization is implemented for this task. To enable real-time localization, the measured pressure values are transmitted to a microcontroller, which processes the data and sends it to a Python script. This script employs a while loop to continuously monitor the criteria for leak detection, and when these criteria are met, it initiates a procedure for leak localization. The theoretical aspect of the procedure, as well as its practical implementation, is presented below as an example.

The paper presents a relatively simple calculation procedure for modeling the drainage dynamics of rain gardens, i.e. adequately designed infiltration surfaces that are used in urbanized areas for the temporary retention of rainwater. To that end, it is necessary to quantify the infiltration of water into the porous soil, which in general can be very demanding because the hydraulic process in question is described by Richards' partial differential equation, the numerical integration of which is a very challenging task (due to the nonlinearity between the hydraulic conductivity and the degree of saturation determined by its solution). At the same time, relying on several working assumptions stated in the paper, the mathematical description of the subject process can be reduced to the solution of one non-linear algebraic equation, which then creates a relatively simple calculation procedure that can be applied in everyday professional work (if the introduced assumptions can be justified). The calculation procedure is reduced to the integration of Darcy's law, which leads to the Green-Ampt model. The paper presents the argumentation for the aforementioned, calculation procedure intended for practical work, and also offers a numerical example.

The paper presents a calculation method that can be used to model the dynamics of emptying and filling retention basins intended for the temporary retention of water, i.e. to achieve the necessary transformation of the flood wave hydrograph. The calculation method is based only on the principle of mass conservation and is therefore very simple and attractive for practical application (assuming that dynamic phenomena can be reasonably neglected). To verify the effectiveness of retention for the relevant hydrological extreme conditions, the temporal change in water level must be determined by solving the mass conservation equation, which correlates the resulting change in water volume in the retention and the difference in mass flows at the inlet and outlet of the retention in the same time period. The presented procedure takes into account the inflow of water from the surface flow and the inflow of water resulting from precipitation above the retention basin. Therefore, for water flows at the outlet, the following are taken into account: the water flow under pressure through the outlet, the water flow over the spillway crest and the infiltration flow at the bottom of the retention basin. The calculation method was reduced to a simple numerical solution of an ordinary and non-linear differential equation of the first order.

To examine the effectiveness of various technical solutions for minimizing the adverse effects of saltwater intrusion in Lake Vrana, Croatia, a reliable mathematical model for describing the exchange of fresh- and saltwater between the lake and its surroundings is needed. For this purpose, a system of two ordinary and nonlinear differential equations is used. The variable coefficients represent hydraulic conductivity functions that are used to quantify groundwater flow and should be appropriately estimated by relying on data obtained by in situ measurements. In the abstract space of all possible hydraulic conductivity functions, the method of particle swarm optimization was used to search for functions which will minimize the difference between the predicted (modeled) and realized (measured) water surface elevation in the lake through the time span of 6 years (which includes relevant hydrological extremes – droughts and floods). The associated procedure requires the parameterization of conductivity functions which will define the number of dimensions of the search space. Although the considered mass exchange processes are significantly nonlinear, and the parametrization of hydraulic conductivity functions can define a search space with a relatively large number of dimensions (60 dimensions were used to estimate the hydraulic conductivity functions of Vrana lake), the presented example confirms the effectiveness of the proposed approach.

Microplastics (MPs) are found to be omnipresent in marine environments and their production and disposal are constantly increasing. In order to control and mitigate their negative effects on the environment, it is important to understand the processes involved in their transport and sinking. Most of the conducted experimental studies on MP in water have focused on exploring the settling behavior of MP particles under static flow conditions. This paper presents the results of an experimental study of MP particles in a horizontal stream of water. A series of laboratory experiments were carried out to measure the drag coefficient of quasi-radially-symmetric MP particles with different shapes (spheres, cubes, and crosses) and sizes (1.5 to 3.0 mm). In the study, the relationship between the drag coefficient and flow conditions represented by the local Reynolds number is investigated. The experimental procedure was validated, and a total of 400 measurements were performed. Values of the measured drag coefficient ranged from 0.8 to 2.5 for the values of Re from 40 to 300. The results show that the shape of MP particles in the water stream considerably influences the particle drag coefficient, with a rather complex relationship between the drag coefficient and flow conditions.

In order to achieve optimal flow velocity in stationary flow condition in pressurized pipe systems, it is necessary to determine an optimal value of pipe diameter. It should be noted that even for a simple pipe network with pipes connected in only few numbers of loops, the determination of the optimal pipe diameter is an unintuitive problem and from the mathematical point of view a nonlinear problem that requires some iterative process with a relatively complicated update procedure between iterations. Usually, the optimization algorithms are based on stochastic methods. A deterministic approach is proposed that satisfies the prescribed constrains in terms of the specified flow velocity in the pressurized system of pipes. The presented iterative algorithm for optimizing the pipe diameter is implemented in computer code PIPENET3D (written in FORTRAN90) and used for hydraulic analysis of steady flow in an arbitrary pressured pipe system.

Krvavica, N.; Travaš, V., and Ožanić, N., 2017. Salt-wedge response to variable river flow and sea-level rise in the microtidal Rječina River Estuary, Croatia. A finite-volume model for two-layer shallow-water flow is presented and applied to study the dynamic response of a salt wedge in a microtidal estuary to changes in river flow rate and sea-level rise (SLR). First, the shape of the arrested salt wedge was computed for different hydrographic conditions. Next, the response of the salt wedge to highly variable river flow was investigated. Finally, this model was applied to predict the impacts of the SLR on salinity intrusions in the Rječina River Estuary. To assess the model performance and to examine the salinity structure in the estuary, a field-sampling campaign was conducted during 2014 and 2015. Field observations revealed negligible longitudinal density variations in both freshwater and saltwater layers and highly stratified conditions for all considered river flow rates and sea levels. F...

A finite volume model for two-layer shallow water flow in microtidal salt-wedge estuaries is presented in this work. The governing equations are a coupled system of shallow water equations with source terms accounting for irregular channel geometry and shear stress at the bed and interface between the layers. To solve this system we applied the Q-scheme of Roe with suitable treatment of source terms, coupling terms, and wet-dry fronts. The proposed numerical model is explicit in time, shock-capturing and it satisfies the extended conservation property for water at rest. The model was validated by comparing the steady-state solutions against a known arrested salt-wedge model and by comparing both steady-state and time-dependant solutions against field observations in Rječina Estuary in Croatia. When the interfacial friction factor λ_i was chosen correctly, the agreement between numerical results and field observations was satisfactory.