<p>Dam construction reduces flow velocity, causing sediment accumulation in reservoirs and shortening dam lifespan. Among various sediment management approaches, free-flow flushing is considered an effective strategy for improving sediment removal efficiency. Despite its potential, the influence of outlet shape on flushing efficiency (FE) remains insufficiently studied. This study investigates the effects of outlet shape, inflow discharge (Q_in), and sediment level (h_s) on FE through 48 large-scale flume experiments, involving four outlet shapes, three sediment levels, and four discharges. Results indicate that increasing the Froude number (Fr = 1.22–2.13) enhances FE by 5–11% in the optimal model. Higher sediment levels were directly associated with greater flushing volumes, with the most efficient model showing a 14% increase in FE at <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\text{h}}_{\text{S}}/{\text{h}}_{\text{bo}}=1.28\)</EquationSource> </InlineEquation>. Among the outlet geometries, the flat rectangular shape achieves the highest sediment removal volume. In contrast, the upright rectangular shape shows the lowest efficiency. Finally, the flat rectangular outlet shape characterized by a higher Froude number (Fr = 2.13), and maximum sediment level (<InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({\text{h}}_{\text{S}}/{\text{h}}_{\text{bo}}=1.28\)</EquationSource> </InlineEquation>) was the most efficient model. Statistical analysis resulted in a dimensionless equation to estimate sediment flushing volumes for the tested conditions. These findings emphasize the importance of outlet design in improving sediment management, supporting sustainable reservoir operation and reducing maintenance costs.</p>