Author(s): Lisa Besser; Mario Oertel

Linked Author(s): Mario Oertel

Keywords: Non-linear Weir; Piano Key Weir; Tailwater influence; Flow behavior; Energy dissipation

Abstract: Piano Key Weirs (PKW) are hydraulic control structures that have a significantly increased discharge capacity compared to linear weirs due to a longer effective spillway crest. The efficiency of these structures as well as the influence of different geometric configurations on the discharge capacity have already been investigated and analyzed in several studies. Nonlinear weirs are implemented in two major applications: (1) top of dam installation and (2) in-channel installation. For in- channel applications, different tailwater levels occur depending on varying discharge events influencing the hydraulics and the flow behavior downstream the weir. In this paper the influence of tailwater on the downstream flow regime is investigated within an experimental model. Flow velocities downstream of an exemplary Piano Key Weir are used to parametrize the resulting flow condition and energy dissipation is analyzed for different discharges in combination with different tailwater levels. Measurements are conducted for an A-type trapezoidal Piano Key Weir model and results are compared to a linear weir under similar inflow conditions. Results show that rising tailwater level weakens the complex three-dimensional downstream flow regime of the Piano Key Weir and reduces air entrainment and length of the aerated region. Velocity profiles transform from inclined velocity distribution in lower tailwater, typical for open channel flows, to nearly vertical profiles. Residual energy was found to be greater for Piano Key Weir but with energy dissipation taking place closer to the weir toe, while energy of the linear weir is mainly dissipated by hydraulic jump further downstream in the channel.

DOI: https://doi.org/10.3929/ethz-b-000675921

Year: 2024