TU Berlin

Chair of Water Resources Management and Modeling of HydrosystemsAbstract

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Abstract

ost organic and inorganic pollutants such as heavy metals are adsorbed to fine suspended particles, which are transported through the river system and deposited in near bank groyne fields, harbors, headwater sections and reservoirs. The sedimentation of sediment-bound contaminants in river systems plays an important role in water resources management. Therefore, it is a challenging task to model and predict the rate or contaminated suspended sediment in spatial and temporal distribution in river systems. The processes that are involved in the topic mentioned above are well explained physically, but not numerically, in three dimensions. Therefore, there has been research that tried to address the contaminant transport in river systems, but with assumptions and simplifications which cannot be applied to each case study. For this reason, a contaminant transport module has been developed based on the numerical code TELEMAC system. The new developments were done for the module SUBIEF-2D, part of TELEMAC system that deals with only suspended sediment transport. The new module, called CTM-SUBIEF-2D, which is a short form for Contaminant Transport Modulc-SUBIEF-2D, describes the transport of dissolved and particulate substances, in two dimensions, in the water body and the transient storage zone (river bed) with emphasis on the interaction between particulate and dissolved phases by first order sorption kinetics. Sedimentation and erosion are also taken into account. Diffusion between dissolved phases in the water body and the transient storage lone takes place. The interaction between contaminants in the water body and contaminants in the transient storage zone occurs through a mixing layer, which is assumed to have a constant thickness if short-term simulations are applied. The sorption interaction is only considered between one sediment fraction and one type of contaminant however, the module deals with different fractions and contaminants. Degradation processes are also considered for the dissolved and particulate phases in both the water body and the transient storage zone. In this study, pure suspended sediment concentration, dissolved and particulate concentrations both in the water body and the transient storage zone are taken into account as variables. 
The developed module consists of five coupled transport equations. Each equation represents one of the variables mentioned above. Methods of Operator Splitting, Stream Upwind/Petrov- Galerkin "SUPG" and Positive Streamwise Invariance "PSI" are used as solution algorithms. A linearization of the system of equations is necessary because of the non linear coupling terms; therefore, the Picard linearization method is used. The developed module is designed for short-term simulations using time steps up to 60-70 seconds.

Since not sufficient data are available for sensitivity analysis, calibrating and validating the parameters of the developed module, it is tested for a typical rectangular channel. In this test, the parameters of the newly developed module are investigated sensitivity assessment. 
Then, the developed module is applied to study the flow characteristics, suspended sediment behavior and contaminant transport in a medium size flood retention reservoir, called Hüttenbühl, which is located in southern Germany. Besides the complex geometry and operation rules, the unsteadiness of the flow field during the flood retention periods controls the transport and sedimentation in the flood retention reservoir. In this study, numerical simulations under unsteady state conditions are performed for two sediment size fractions and two different outlet operation rules in terms of outflow discharge and water level to predict the behavior of contaminants as a function of space and time. The simulation time is 70 hours. The aim of these simulations here is to find out the optimum operation rules that fulfill sedimentation mitigation and reduction of contaminant concentration. The numerical results of the flood retention reservoir show the transport behavior of contaminant, suspended sediment and the spatial and temporal distribution of contaminated deposits for different reservoir operation rules in terms of the outlet discharge. It is shown that the amount of sediments and contaminants deposited in the reservoir can be reduced by the appropriate operation of the bottom outlet without losing the flood retention efficiency. 
The developed module is also applied to an impoundment in the River Rhine, called Iffezheim which is located at kilometer 334 of the Upper Rhine, to investigate the impact of the flood event of May 1999 on that region. The Iffezheim impoundment consists of II weir, a hydropower plant and a navigation lock. The flow and transport are assumed to be quasi-steady state since the time scale of variation of the water elevation level during the flood event is very small compared to the flow time scale. The simulation time is done for a year of the flood event May 1999. The numerical simulations are performed for three fraction sizes of sediments and two stages of the inflow discharge, i.e. normal discharge and flood discharge, under different operation rules of the weir and the hydropower plant. One of these fractions of sediments is assumed to be only contaminated. The numerical results show the flow field, suspended sediment concentration, sedimentation and erosion rates, and the und spatial distribution of the deposited contaminated particles for a complete year of the flood event 1999.
The developed module CTM-SUBIEF-2D simulates the complex physical interaction processes and shows reasonable results for the applied real cases, i.e. the flood retention reservoir and the impoundment lffezheim in river Rhine. The numerical results from the developed module prove a qualitative agreement with the results from the field measurements and the physical model. Therefore, the designed module can be used for comprehensive contaminated suspended sediment transport in river systems. 
For future work, additional improvements will have to be made regarding a larger simulation time step und mixing layer thickness if long-term simulations are needed. These improvements will also consider flocculation and consolidation of sediments on the river bed. Chemical and biological reactions might be important for some sedimentation problems, and therefore, the developed module can be coupled with other numerical models that deal with these reactions.

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