Concept for stabilizing the bed of the river Elbe: implementation in the "Klöden" pilot stretch using a 3D CFD model
Erosion of river beds is a natural process that is compounded by human intervention and causes many concerns, especially in terms of floodplain ecology and shipping usage. In a 160 km long section of the river Elbe the WSD East is confronted with erosion rates of up to 1 cm p.a. between Mühlberg and the mouth of the river Saale. Over a long period of time, this leaves deep tracks in the river bed while also increasingly separating the river off from its foreland. In May 2009, the WSD East presented a concept for bed stabilisation in cooperation with the BfG and the BAW, as well as the states of Saxony and Saxony-Anhalt. The resulting measures are to be implemented swiftly in pilot projects. Among others, the package includes adapting the river training structure in the waterway to the current water level, as well as bed load management. In addition, reconnecting the foreland to the main course of the river will also provide relief for the river bed that is already in a state at bankful stages. The sections of the river selected for the pilot projects included "Klöden" (between Pretzsch-Mauken and the mouth of the river Schwarze Elster, approx. 10 km) as well as "Wittenberg/L." and "Coswig".
In the Klöden pilot section, the catalogue of measures will be examined in various combinations and variations. To this end, the BAW applies among others a 3D CFD model (computational fluid dynamics) with a horizontal grid resolution of 2 m x 2 m in the river bed and 5 m x 5 m on the foreland, together with a vertical resolution of 0.5 m. The grid has a modular structure for corresponding flexibility in use while saving computing time. The flow is calculated using the UnTRIM numerical method. With the ultrafine nets, it is possible to establish model variations quickly for development of an optimum package of measures. The model serves for analysis of surface current by assuming a solid bed. A thorough analysis of the current situation provides the basis for designing different variants. Analysing the hydraulic changes in the status compared to the current status makes it possible to reduce the erosion. Other methods are used in addition to the 3D HN model, e.g. 1D numerical sediment transport models, for estimating the long-term morphological consequences of a variant also for longer sections of the river.
The two illustrations show a comparison of the current status (picture 1) with the results of a study that also includes reactivation of the Bösewig (bayou) in addition to river engineering measures (picture 2). The calculated water level has been colour-coded with the calculated flow velocities (the paler the colour, the faster the flow velocity). It can be clearly seen how the flow velocities decrease over the entire depicted length. Lower flow velocities also indicate reduced river bed erosion.