Abstract
Duckbill spillways are widely used in irrigation networks due to their extended effective crest length, which allows them to limit upstream water level fluctuations under varying flow rates. However, sediment accumulation upstream of these structures is a critical operational challenge that can significantly affect their hydraulic performance. This study investigated how different sediment-deposition patterns affect duckbill-weir performance under various channel slopes in an adjustable rectangular flume. It also examined how two crest geometries (sharp and rounded) influence the discharge coefficient with and without sediment. Findings show that upstream sediment causes a substantial 24–28% reduction in the discharge coefficient, leading to higher upstream water levels. Although steeper channel slopes generally increase the coefficient, the negative impact of sediment becomes more severe at higher slopes. Crest geometry comparison revealed that the rounded crest offers a clear advantage under sediment-free conditions (about a 6.5% increase in the discharge coefficient), but this benefit sharply declines when sediment is present (only about a 2.5% increase). Overall, the results highlight that sediment accumulation is a major factor limiting the performance of duckbill weirs. Therefore, accounting for sediment effects during both design and operation is essential for maintaining effective water-level control in irrigation distribution systems.
Keywords: Channel slope, Crest geometry, Discharge coefficient, Duckbill spillway, Sediment deposition
Methodology
A laboratory flume with adjustable longitudinal slope was employed to simulate flow over duckbill spillways under two operational scenarios: initial operation without sediment and advanced operation with full sediment accumulation. Two crest geometries were tested, including a sharp-edged crest and a rounded crest commonly recommended for improving flow detachment. The sediment layer upstream of the spillway was shaped to represent a real-world deposition pattern observed after long-term network operation. Flow depth and hydraulic response were recorded for each condition, and the discharge behavior was evaluated based on changes in the discharge coefficient.
Results and Discussion
The findings revealed that sediment accumulation plays a significant role in altering the hydraulic entrance conditions of duckbill spillways. In sediment-free conditions, flow approaches the crest smoothly, preserving energy and leading to efficient discharge. However, in the presence of sediment, especially when it extends to the proximity of the spillway nose, the flow experiences partial blockage and redirection. This condition increases upstream water head and reduces flow velocity just before passing over the crest, resulting in a noticeable decrease in discharge efficiency.
Moreover, while increasing the channel slope generally improved flow capacity by supplying additional flow energy, this benefit was partially offset by the disruptive effect of sediment. In steeper slopes, although the flow had higher energy, the interaction between flow and deposited sediment generated stronger turbulence and localized energy losses, indicating that the positive effect of slope becomes limited if sediment is not managed properly.
The comparison between crest geometries demonstrated that the rounded crest exhibited better performance under clear-water conditions by facilitating smoother flow transition and reducing flow separation. However, when sediment was present, the effective overflow edge shifted from the physical crest to the sediment surface, masking the hydraulic benefits of the rounded crest. This finding suggests that crest geometry optimization alone is insufficient in networks with persistent sedimentation and must be complemented with sediment management strategies.
Conclusion
This research highlights that upstream sediment deposition is a key parameter influencing the hydraulic performance of duckbill spillways and cannot be neglected during design and operation. The study emphasizes that channel slope and crest geometry interact with sediment conditions, and their combined effect determines the actual field performance of the structure. Therefore, for long-term efficiency, spillway design and maintenance strategies must incorporate predictive assessment of sediment behavior and targeted flushing or dredging plans. The outcomes of this study provide practical insights for irrigation authorities, offering guidance for both initial design decisions and adaptive management during operation.