New numerical flow model for flood protection

Computation of water-level situtations and sediment transport are helping to identify measures needed for the protection of high-water protection dykes, as Dr Roberto Kohane of ERM Lahmeyer International reports.

The Bayernwerk Wasserkraft AG (Bavarian Hydropower Company - BWK) operates a power station at the Feldkirchen barrage on the upper reach of the River Inn. Distinct manifestations of silting are appearing here as well as at other barrages on the Inn, impairing the flood safety of the dykes.

Flooding in the past, in particular, the extreme high-water level in August 1985, showed that the required freeboard in the backwater area of the barrage is no longer being reached. For evaluating the necessity of measures to be carried out on the existing high-water protection dykes, Lahmeyer International has, as part of the study, prepared and applied a new numerical flow and sediment transport model, to examine the transport of fine suspended sediment in river barrage reaches.

With the help of the model, the temporal and spatial morphological development in the backwater area of the barrage was described and a computation of water-level situations for discharge conditions of a design flood was carried out. The model was calibrated and validated with available field data.

To complement the available data relating to discharge and water level at the barrage as well as the concentration of suspended sediment at the Oberaudorf/Reisach monitoring station, further field surveys were undertaken to determine the sediment properties and the most important parameters of the sediment transport model. These surveys took place during a one-day monitoring campaign with the participation of the Barrage Monitoring Department of the BWK Works Management in Lech, the Construction Department of the BWK Works Management and the Institute of Hydraulic Engineering at Stuttgart University.

The model for flow and transport of suspended sediment used in the study is based on a one-dimensional quasi-steady flow computation that takes into account the flow resistance caused by form of the moveable bed and surface roughness as well as the discharge in a channel of compound cross-section.

For the modelling of the transport of suspended sediment a convection dispersion model based on a quasi two-dimensional approach was used. The model calculates the depth-averaged concentration of suspended sediment, the erosion rate, and the deposition rate at various computational sections in the river's main stream and the floodplains. Based on the erosion and deposition rates, variations in the height of the of river bed level through deepening or elevation can be calculated for a given time increment.



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