Flow profiles and hydraulic behavior of rectangular piano key weirs: effect of sloping crest and orientation across different scales

• Flow profiles are analyzed for PKWs with inclined crests in both flow-aligned and opposite-flow configurations. • Inclined crests enable adaptive flow regulation, engaging only the lower part of the crest during low flows and the entire crest during flood events. • Scale effects are examined for PKWs with inclined crests. • A larger scale ( W/B = 2/3) enhances flow stability, while a smaller scale ( W/B = 1/3) increases nappe curvature and turbulence. • Opposite-flow inclined crests amplify turbulence by approximately 25 %, improving energy dissipation and aiding in flood mitigation. Piano Key Weirs (PKWs), renowned for their enhanced discharge capacity within a compact footprint, represent a significant advancement in hydraulic engineering. While extensive research has focused on conventional PKW designs, a critical knowledge gap persists regarding flow behavior over PKWs with inclined crests. This study presents the first comprehensive experimental investigation into the hydraulic performance of rectangular PKWs featuring crests inclined in both the flow and opposite-flow directions. We specifically explore the impact of two distinct geometric scales, defined by the channel width-to-total weir length ratio ( B/W ) of 1/3 and 2/3, on flow characteristics. Experiments were systematically conducted under varying discharge conditions in a 10 × 0.75 × 10 m flume at Tarbiat Modares University. Our findings reveal that inclined crests offer a novel mechanism for adaptive flow regulation based on upstream water levels. At low upstream flows, only a partial crest length is engaged, progressively utilizing the entire crest as flow increases, particularly under flood conditions. Furthermore, the inclination direction and geometric scale significantly influence critical flow patterns, including nappe trajectory, flow aeration, and energy dissipation efficiency. The larger scale ( B/W = 2/3) demonstrated enhanced flow stability and uniformity, while the smaller scale ( B/W = 1/3) exhibited higher nappe curvature and localized turbulence. These insights provide invaluable data for optimizing PKW designs for adaptive flow management and energy dissipation, offering crucial guidance for practical applications and shedding new light on scaling effects in complex weir hydraulics.