Torre Gomez, Guadalupe (2022) Experimental and Numerical Investigation to couple a Piano Key Weir with a Fish Pass. [Tesi di dottorato]

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Climate Change has led to a higher risk of flooding events and droughts directly affecting the environment and the society. These changes have brought to develop Piano Key Weirs (PKWs), which are characterized by a higher discharge efficiency. The location of PKWs in rivers allows increasing the water level for numerous purposes, such as hydropower exploitation or irrigation, while controlling the upstream water level during flooding events, which ultimately helps to avoid the overtopping of the banks. Nevertheless, the installation of hydraulic structures, such as PKWs, on rivers results in altered habitats, modified flow conditions, loss of river continuity and connectivity and especially severe damaging impacts on the aquatic environment. One of the effective solutions to overcome some of these problems is installing fish passes. The aim of this Ph.D. research is studying the viability of combining a PKW with a fishway, proposing a new structure which joins the beneficial effects of the PKW discharge efficiency with the possibility of creating a passage for fishes, aiming at overcoming the barrier created by the PKW structure on the river. Aiming to better understand the discharge efficiency of the two most efficient PKW types, type A and type B , an experimental and numerical assessment was performed to evaluate the discharge efficiency of three PKW geometries: a symmetric type A, PKWA, with Wi/Wo=1.5, the same model rotated 180°, resulting in a type A Wi/Wo=0.67, PKWA_reverse, and a type B model, PKWB, with the main geometric features than PKWA (L, P, Wi, Wo, Bb, Bh). The PKWA and the PKWA_reverse were experimentally and numerically tested while the PKWB was only studied numerically. The comparison between numerical and experimental results proved the effectiveness of using numerical testing. The obtained results showed that the PKWA discharge coefficient is up to ∼34% higher than the PKWA_reverse, resulting in an increase of the upstream head on the PKWA_reverse up to ∼32%, remarking the relevance of the Wi/Wo ratio in the discharge efficiency of PKWs. Conversely, the tested PKWB resulted more efficient for lower head (H/P ≲ 0.35) than the PKWA, however, when increasing the upstream head, the PKWA model proved to be more efficient. This change in efficiency is herein explained because of the hydraulic behaviour of the inlet and outlet key of both geometries. Results proved that the discharge coefficient is strongly dependant on the specific geometry, hence, it is not possible to generalize about the efficiency of the different PKW types. Furthermore, a Denil fish pass was selected to be implemented in the outlet key of the tested PKWB, which was modified to foster the fish pass. A numerical assessment of the combined structure was performed aiming to verify the advisable flow conditions for fish passage and to evaluate the loss in efficiency of the PKWB-FP in comparison with the PKWB. After following the geometry design recommendations from the FAO (Food and Agriculture Organization of the United Nations, 2002) for the fish pass, numerical results showed that the velocity field obtained in the middle of the fish pass and in the baffles exceeded the maximum advisable velocity of 2 m s-1. In terms of discharge efficiency, the PKWB-FP discharge coefficient is up to ∼42.4% smaller than the PKWB, resulting in an increase of the upstream head up to ∼51.8%. This assessment proved the possibility of effectively combine a fish pass with a PKW, resulting in a new structure that works as a fish pass at regular conditions and as a PKW at flood conditions. The two main advantage of installing this structure in rivers are: (1) to restore continuity for fish movements and (2) either to increase the upstream water level in rivers, while maintaining inundation risks comparable to the previous ones or to decrease upstream inundation risk if the weir crest level is maintained constant.

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