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The authors developed a novel design equation based on numerical model results by taking into account the effects of water velocity and residual tunnel cross-sectional area above the water surface. The discussers acknowledge the presented approach, but also express their concerns regarding the following points: (i) the validation of the numerical model is superficial as it is based on bulk parameters only and ignores known model limitations; (ii) effects of the air vent characteristics are not considered in the numerical model and consequently neither in the presented design approach; (iii) air entrainment is not considered in the authors' approach but may be important for Froude numbers above 6; and (iv) the general applicability of the design equation due to the small range of considered Froude numbers and the negligence of air vent characteristics which have been shown to significantly impact air demand in outlet tunnels. This discussion aims at clarifying some facts not clearly presented by the authors and identifying limitations of the authors' design equation. Overall, we highlight that the approach describes the unrestricted air discharge and should be limited to the parameter range and assumptions considered for its derivation, i.e. for large air vent parameters (A* > 0.1) and flows with Froude numbers below 10. [ABSTRACT FROM AUTHOR]

Numerical hydrodynamic models enable the simulation of hydraulic conditions under various scenarios and are thus suitable tools for hydropeaking related assessments. However, the choice of the necessary model complexity and the consequences of modelling choices are not trivial and only few guidelines exist. In this study, we systematically evaluate numerical one‐dimensional (1D) and two‐dimensional (2D) hydrodynamic models with varying spatial resolution regarding their suitability as input for hydropeaking‐sensitive, ecologically relevant hydraulic parameters (ERHPs), and their computational efficiency. The considered ERHPs include the vertical dewatering velocity, the wetted area variation between base and peak flow and the bed shear stress as a proxy for macroinvertebrate drift. Furthermore, we quantified the habitat suitability of brown trout for different life stages. The evaluation is conducted for three channel planforms with morphological characteristics representative for regulated Alpine Rivers, ranging from alternating bars to a braiding river morphology. For the prediction of habitat suitability and bed shear stress, a 1D model appears to be always insufficient, and a highly resolved 2D model is suggested. Reducing the spatial resolution of 2D models leads to computational efficiency similar to 1D, while providing more accurate results. Thus, our results suggest, that while a highly resolved 1D model is sufficient for accurate predictions of the dewatering velocity and wetted area in the less complex alternating bar morphology, a 2D model is recommended for more complex wandering or braiding morphologies. This study can serve as guideline for researchers and practitioners in the selection and setup of hydrodynamic models for hydropeaking. [ABSTRACT FROM AUTHOR]