In the era of high anthropogenic pressures on the quality of waters, a reliable water management strategy is essential. Mathematical models play an essential role in providing the information needed to take efficient measures to improve the water quality status of rivers. The current water quality simulators can be categorized into two broad categories. The first category consists of detailed water quality simulators that represent the physical reality in a detailed way. Water quality models developed using this types of simulators are characterized by long calculation times, and often require intensive input data. As a consequence of the excessive calculation time, they are not suitable for water quality management applications that require multiple simulations. In the second category are water quality simulators that represent the physical reality in a simplified way. Water quality models developed using this types of simulators have very short calculation times. They are therefore suitable for water quality applications that require multiple model runs and long-term simulations. This category can further be divided into empirical simulators and conceptual simulators. The empirical simulators rely on input-output relations, and hence the equations do not have a physical meaning. Consequently, they are not suitable for scenario investigations. On the other hand, conceptual water quality simulators use simplified equations that lump some of the physical/chemical processes but that have a physically/chemical meaning. Many conceptual water quality simulators have been used for water quality modelling as alternatives or complements to detailed water quality simulators. However, they have a limited or no applicability for: 1) complex systems that have two-way interactions – such as river-flood plain systems, tidal rivers, and looped sewer systems; 2) the simulation of pollutants that travel attached to fine sediments – such as phosphate and trace metals. These limitations challenge the versatility of the current conceptual water quality simulators. Besides these limitations, the numerical solvers of most conceptual water quality simulators may provide unreliable results for some applications, especially when the computation time step is large.With an aim of enhancing the applicability of conceptual river water quality simulators, a Conceptual Integrated Tool for Water quality Assessment (CIToWA) has been developed in the framework of this PhD research, hereby addressing the limitations described earlier. The tool is developed to: 1) present a robust solutions scheme 2) enable simulation of size-selective sediment-bound pollutant transport, and 3) enable simulation of complex stream networks using a conceptual approach. It is aims to maintain the computational speed of conceptual models but increases the versatility.
In CIToWA, the river system is divided in reaches, conceptual elements that divide the channel longitudinally in to different parts. The discharges and the velocities in these reaches must be provided by external simulation tools. The reaches are further considered to act as Continuously Stirred Tanks Reactors (CSTR), whereby complete mixing is assumed within each reach. The water quality tool integrates several components, developed for specific purposes. The first component is the simulator of dissolved oxygen, biochemical oxygen demand, the nitrogen compounds, the phosphorus compounds, and the algae.
A quasi-analytical solver is developed to simulate robust solutions of water quality problems, based on the CSTR principles and on ordinary first order reaction equations. The performance of the quasi-analytical solver is evaluated by simulating different experiments and testing the robustness of its solution under extreme conditions. The results are also compared with the performance of SWAT (Soil and Water Assessment Tool) – a commonly used water quality simulator - and with high and low order numerical solvers that are commonly used in other conceptual water quality simulators. The comparison shows that the quasi-analytical solver simulates robust solutions -even for large computation time steps- in all the experiments and performs better than all the solvers used in the comparison. This increases the reliability of the solutions as well as the speed of computation of CIToWA.
A method is also developed to simulate complex stream networks and systems characterized by bi-directional interactions. The method uses the direction of the flow to enable pollutant transport in a reverse direction (from downstream to upstream), if needed.
A second component of CIToWA deals with the sediment and sediment-bound pollutants. The sediment transport module considers the sediment in suspension and the sediment on the river bed, taking settlement and resuspension into account. Hereby, the Particle Size Distributions of the sediment (PSD) are taken into account by considering probability density functions. The temporal and spatial evolution of the mean and standard deviation of the PSD –caused by settlement, resuspension, and emissions– are determined by using statistical functions of mixture distributions. The concepts of this novel sediment transport simulator are validated by simulating extensive sediment mobility data of laboratory and field experiments from literature, as well as by a case study on the River Zenne (see below). CIToWA simulates the sediment-bound pollutant transport using a Langmuir adsorption model. The adsorption simulator integrates the quasi-analytical solver with the fine fraction of the sediment – the mass of the sediment to which pollutants adsorb. The latter module was also tested on a case study on the River Zenne.
An application of CIToWA in the complex river – canal system of the River Zenne (Belgium) illustrates its applicability for real cases. The system is characterized by bifurcations, such as overflows from the river to the canal and some of the reaches experience two-directional flows. The flow in the canal is characterized by high and frequent discontinuities as the flow is controlled by the sluice activities.
A comparison of the simulation results of CIToWA with those of a previous detailed water quality model of the River Zenne shows that CIToWA performs as well or better than the detailed model. The better performance of CIToWA is attributable to the fact that it runs fast (23 seconds to run the model with an hourly time step over the period 2007-2010) and hence automatic parameter optimization can be carried out – one of the key objectives of the research. The latter was indeed not the case for the detailed model, as the latter required 10 days of computation time to run the model for the pre-cited period.
Specifically with regard to the sediment-bound pollutant transport module, the fate of the inorganic phosphate in the River Zenne was considered. The comparison of the simulation results whereby adsorption to the sediments is considered with the simulation results whereby the adsorption-desorption processes are not taken in to account, shows that the adsorption model outperforms the model without adsorption in the canal – where the sediment is dominated by clay. A comparison of the adsorption model of CIToWA with the detailed water quality model shows that the new tool performs better than the detailed model. Given the fact that the new tool provides robust simulations in the complex stream network characterized by multiple branching and two-directional flows indicates that CIToWA can be applied to tidal rivers, river-flood plain systems and storm sewers systems characterized by two-directional flows. The plausible performance of CIToWA for the simulation of size-selective sediment transport and the association of inorganic phosphate to the fine sediments shows that the new tool can be used to simulate the transport of sediment and pollutants that adsorb to them, such as trace metals. The application on the River Zenne shows that CIToWA has an enhanced applicability, as compared to current conceptual water quality models.
