Non-cohesive sediment cleaning patterns and processes upstream of a Type A piano key weir for steady and unsteady flow conditions

The piano key weir has been used in channel and spillway rehabilitation projects due to their compact footprint, hydraulic efficiency and cost-effectiveness. Despite these advantages, sediment deposition upstream of the weir can impact hydraulic performance, raising concerns about the long-term functionality and performance during flood events. Therefore, the present study investigates the self-cleaning behavior of non-cohesive sediments deposited immediately upstream of a Type A piano key weir in a run-of-river setting. Laboratory flume experiments were conducted under clearwater conditions, examining both steady and, for the first time, unsteady inflow conditions. Analyses of the sediment-flow interactions and flow field characteristics were augmented with Reynolds-Averaged Navier-Strokes (RANS) based single fluid computational fluid dynamics (CFD) simulations. Findings revealed that frontal scour was primarily driven by localized vortices formed by the piano key weir geometry while general upstream riverbed degradation was governed by inflow conditions, including unit discharge and the level of sediment deposition relative to the weir crest. Frontal scour and riverbed degradation reached the base elevation of the weir for densimetric Froude numbers exceeding 2.15. Notable sediment transport was initiated at densimetric Froude numbers as low as 1.45. The self-cleaning efficiency was closely linked to local velocities and bed shear stresses. For the steady-state cases, the rate of sediment removal reduced with time until equilibrium conditions were established. Tests showed that the unsteady inflow conditions were equally effective at sediment removal, as 80-85% of total transported sediment occurred during initial 30% of time of laboratory testing for any given unit discharge.