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Open check dams are built to retain bed load in case of major floods and may have filtering, sieving as well as dosing effects on sediment transport. The combination of an upstream reservoir with an open check dam is designated as bed load trap. These structures are crucial elements for flood protection in mountainous regions as the sediment transport capacity of mountain streams is ample, highly unpredictable and therefore an imminent danger for river dwellers. The contemporary design criteria for open check dams are based on theoretical approaches, laboratory experiments or engineering experience. A multitude of different opening types and shapes were studied and built in the past, mostly based on design parameters which are related to grading curves of the river bed. In a series of laboratory experiments, the opening geometry and position is currently optimized in view of sediment transition for frequent floods in order to ensure sediment dynamics downstream. Herein, the results of a first series of experiments are presented, which were dedicated to the evaluation of the discharge capacity of check dam openings, under the influence of bed load transport on a rough river – bed under supercritical flow conditions. Existing experimental and theoretical formulae for purely hydraulic discharges are reviewed and it is found, that the actually applied value for the orifice coefficient μ are too small. These results may also apply for bridges, which serve in some cases intentionally or unintentionally as open check dams.