3D-hydronumerical models are growing in popularity throughout various disciplines to achieve a better understanding of complex flow in hydraulic structures. Due to increasing computational resources over the last decades, these models enable us today to economically simulate a broad range of situations, where laboratory or analytical approaches are limited. But, the issue with 3D-hydronumerical simulations are the numerous variables to be handled by the user. Slight modifications of numerical parameters may lead to differing results. Furthermore, certain con-structional designs of fishways were identified in laboratory studies for their unpredictable flow regimes when modifying minor geometrical parameters. These volatile designs might affect the suitability of fishways in-situ, even though they comply common constructional guidelines. Hence, this article presents the evaluation of in total 10 slightly modified vertical-slot-fishway simulations, to identify (un)stable fishway designs and varies them through two different turbu-lence models (URANS and DES). Its evaluation indicates, that some fishway design built world-wide are affected in 3D-hydronumerical models by both, the constructional modifications and the simulation approach. The recognition of unstable designs and their consequences for fish passage criteria has to either be avoided in future designs or used on purpose to diversify flow regimes in technical built fishways.

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