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Institut für Wasser- und Umweltsystemmodellierung - IWS



"One-dimensional Multi-strip Transport Model for Long Distance Sediment Transport in Navigation Channels, Case Study the River Elbe"

Many European rivers are being regulated using groyne structures, which have a great influence on the flow and suspended sediments transport. Fine sediments associated with contaminants are deposited in the groyne fields and could be resuspended and remobilised by high erosive flood events. The exchange process of suspended sediment between a main channel and adjacent groyne fields is a key process of sediment transport in such waterways. To describe long-term and large scale transport processes, a one-dimensional model is suitable. For assessing the resuspension, transport and redeposition of the very fine sediments deposited in the groyne fields, a one-dimensional multi-strip model has been developed, which allows to account for the exchange between the side strips and the main channel. The model is based on a finite difference formulation focussing on flow and suspended sediment transport in three river compartments. The one-dimensional 3-strip model comprises the flow and transport equations coupled by exchange parameters between the strips. Due to the complex flow field inside the groyne field, the exchange coefficient is determined by a 2-d flow model and entered to the 1-d model. The exchange terms are determined according to the river discharge. When the water level is lower than the groyne crest the exchange process between the strips is dominant. When the groynes are completely submerged advective f! low occurs, which can be approximately captured by the 1-d model. This study aims at modelling the interaction between the groyne fields and the main channel of 38 km long stretch of the river Elbe during extreme flood event occurred in August 2002. The River Mulde, left tributary, was severely polluted by wastewater from the mining and chemical industries discharged to the river. Therefore, the modelled area starts around 3 km upstream of the mouth of the Mulde. The goal was to determine the effect of the flood on the deposited sediments in groyne fields along the River Elbe, due to urgent issues about the impact of resuspended contaminants on the aquatic environment. Generally, during the first 14 days of simulated period, deposition was the dominant process, where about 771 m3 of sediments were deposited. Within the next 12 days erosion occurred with the maximum eroded volume of about 50000 m3. Erosion of deposited suspended sediments started and ended when the discharge was approximately 1400 m3/s. The results showed that before the extreme flood, deposition in the groyne fields occurred. During the flood, the erosion of the main channel was dominant. However, erosion of some deposited material in the groyne fields occurred as well, with the maximum erosion depth of about 2.2 cm in the right groyne field at km 285.9. Furthermore, 17% of the total eroded sediments eroded from the groyne fields. Erosion in the groyne fields started 1 day after the erosion in the main channel occurred. The multi-strip model and its use could be extended to all rivers if the division of cross-sectional flow areas, in terms of different flow velocities and depths, is relevant. The modelled results showed that erosion and deposition occur in the same cross-section at the same time. Deposited material in some groyne fields during low discharge was eroded during flood event. Nevertheless, further use could be for flood events when the flow characteristics of flooded areas are different from the ones in a main channel. Also, the deposition of eroded material from the river bed to the flood plains is important when the eroded sediments are contaminated and flood plains are used for agricultural purposes.