water Article Numerical Investigation of Hydraulics in a Vertical Slot Fishway with Upgraded Configurations Mohammad Ahmadi 1 , Amir Ghaderi 2, * , Hossein MohammadNezhad 3 , Alban Kuriqi 4, * and Silvia Di Francesco 5   Citation: Ahmadi, M.; Ghaderi, A.; MohammadNezhad, H.; Kuriqi, A.; Di Francesco, S. Numerical Investigation of Hydraulics in a Vertical Slot Fishway with Upgraded Configurations. Water 2021, 13, 2711. https://doi.org/10.3390/w13192711 Academic Editor: Agostino Lauria Received: 26 August 2021 Accepted: 27 September 2021 Published: 30 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, Faculty of Engineering, Islamic Azad University, Shabestar Branch, Shabestar 51368, Iran; sthfar@gmail.com 2 Department of Civil Engineering, Faculty of Engineering, University of Zanjan, Zanjan 537138791, Iran 3 Department of Civil Engineering, Faculty of Engineering, Urmia University, Urmia 5756151818, Iran; mohammadnezhad.hossein@gmail.com 4 CERIS, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal 5 Department of Engineering, Niccolò Cusano University, 00166 Rome, Italy; silvia.difrancesco@unicusano.it * Correspondence: amir_ghaderi@znu.ac.ir (A.G.); alban.kuriqi@tecnico.ulisboa.pt (A.K.); Tel.: +98-9384503512 (A.G.) Abstract: The implementation of vertical slot fishway (VSF) has been demonstrated to be an effective mitigation measure to alleviate extensive river fragmentation by artificial hydraulic structures such as weirs and dams. However, non-suitable flow velocity and turbulent kinetic energy significantly affect fish swimming behavior and, as a result, hinder such facilities’ performance. Therefore, this study’s main objective is to propose a new configuration of VSF that can allow the passage of different fish species under frequent variations of flow discharge. To achieve that objective several novel configurations of VSF were numerically investigated using the FLOW-3D ® model. Namely, five variants of angles between baffles, four different pool widths, and another upgraded version of VSF by introducing cylindrical elements positioned after the opening behind the baffles were tested. Results show that smaller angles between baffles increase the Vmax and decrease the maximum turbulent kinetic energy (TKE max ); the opposite result was obtained when increasing angles between baffles. Namely, the Vmax was increased up to 17.9% for α =0 ◦ and decreased up to 20.37% for α = 37 ◦ ; in contrast, TKE max decreased up to −20% for α =0 ◦ and increased up to 26.5% for α = 37 ◦ . Narrowing the pool width increased the Vmax linearly; nevertheless, it did not significantly affect the TKE max as the maximum difference was only +3.5%. Using cylinders with a large diameter decreased the Vmax and increased TKE max ; in contrast, using cylinders with smaller diameters further reduced the Vmax velocity inside the pool while increasing the TKE max . However, in the case of cylinders, the dimension of the recirculation depended on the configuration and arrangement of the cylinder within the pool. Overall, the maximum velocity was reached at near 77% of the water depth in all cases. Finally, solution-oriented findings resulted from this study would help water engineers to design cost-effective VSF fishways to support the sustainable development of hydraulic structures while preserving aquatic biodiversity. Keywords: ecohydraulics; fishway; fish migration; hydraulic model; hydropower; FLOW-3D 1. Introduction For decades, dams have been built for multiple purposes, such as irrigation, water supply, navigation and fish farming, flood control, recreation, and energy generation [1,2]. In recent years, hydropower, including both small and large examples, has attained an increasingly significant role within available renewable energy sources, currently represent- ing the most significant contributor of its kind to power generation, especially in Europe, and is also gaining tremendous attention in developing countries, as well [3,4]. Due to its technical-economic specificities, hydropower is becoming a key player in helping to face global energy challenges, fighting climate change, and achieving sustainable development Water 2021, 13, 2711. https://doi.org/10.3390/w13192711 https://www.mdpi.com/journal/water