The influence of the opening width of check dams on bedload continuity of mountain rivers

The design of open check dams at mountain rivers for flood protection has an essential impact on the sediment budget of downstream river reaches. A key issue is the bedload transfer hindering and retention which can be influenced by the geometry of the openings of check dams. Contemporary design concepts provoke the retention of too much or insufficient bedload with regard to the downstream transport capacity. This dysfunction can cause continuous maintenance works or endanger downstream infrastructures, respectively. Herein an experimental analysis of the influence of check dam openings on the bedload transport capacity is made as a function of the constriction ratio of the opening width and the mean width of the undisturbed channel. Constriction ratios of more than 0.9 have no significant influence on the bedload transport capacity, while constriction ratios of less than about 1/3 block every grain larger than 1/40 of the channel width. much sediment and regular maintenance works in terms of reservoir dredging are required to prevent progressive backfilling. In this case, the longitudinal river connectivity is still interrupted in terms of eco-morphologic aspects like the migration of aquatic organisms and the substrate availability in the form of sediment transported by the river. Accordingly, an adequate design of open check dams is crucial for reducing the human impact on the sediment budget of the whole river which manifests impressively at marine estuaries like described by Syvitski et al. (2012). Despite the need for sediment continuity, check dam openings also have an upper size limit which is imposed by safety aspects. It is necessary to retain the part of the sediment supply which exceeds the downstream bedload transport capacity at critical points, like the passage of settlements during floods. Therefore, check dam openings should be sufficiently wide for ensuring the longitudinal connectivity in terms of sediments but still keep their role of protecting downstream infrastructures in case of floods. Open check dams can be clogged by two different mechanisms: (i) mechanically, i.e. when obstacles which are larger than the opening are transported by the river or (ii) hydraulically, i.e. when the discharge capacity of the opening is exceeded (Piton & Recking 2015). This study focusses on the bedload retention due to hydraulic obstruction, where bedload is intended to pass undisturbed most of the time. The bedload transfer capacity