Author(s): Burkhard Rosier; Jean-Louis Boillat; Anton J. Schleiss
Linked Author(s): Anton J. Schleiss
Keywords: Side overflow; Side weir; Bed forms; Bed morphology; Alluvial channel roughness
Abstract: In flood protection engineering, side weirs or overflow dams are used to divert water in a controlled way into flood plains as soon as the discharge capacity of the channel is reached. Due to the lateral loss of water, significant morphological bed changes take place. On the one hand, the reduced sediment transport capacity results in local sediment deposition and thus an increase of lateral overflow intensity. On the other hand, bed form characteristics such as length, height, steepness, stoss and lee slope angle and consequently alluvial roughness within the vicinity of the lateral overflow are considerably affected by the side overflow. This latter aspect represents the focus of the present study. The results from an experimental flume study show that the shape of observed dunes is highly three-dimensional and that three distinct regions along the channel axis can be identified. The first one reaches from the channel entrance to the upstream weir corner, the second one comprises the reach of the weir, and the third one represents the reach downstream of the weir. In addition to this streamwise distinction, also a lateralwise distinction appears to be necessary. Form induced roughness can only roughly be predicted by the help of approaches from literature. A promising alternative for a more reliable prediction is the probability density distribution of bed form geometry.