The effects of passing seagoing vessels on the inland ships in the outer harbour and locks of Hansweert

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The lock of Hansweert, located in the province of Zeeland in the Netherlands, serves as a crucial inland shipping node between the Western Scheldt estuary and the Rhine Delta. The outer harbour of the lock complex provides the connection to the Western Scheldt and accommodates waiting facilities for inland ships. The Western Scheldt is a vital gateway for maritime traffic, linking the Port of Antwerp to the North Sea. As the navigation channel of the Western Scheldt is located close to the outer harbour of Hansweert, multiple ship incidents at this location are attributed to the water motions generated by passing seagoing vessels on the Western Scheldt. This research investigates how the water motions induced by passing vessels in the Western Scheldt contribute to unsafe situations in the outer harbour and locks of Hansweert, and what preventative measures can be identified to effectively minimize incidents and mitigate risks.

Investigating the incident records reveals that the key contributors are the primary water motions generated by the passing vessels in the Western Scheldt. The phenomenon, experienced as a sudden lowering of the water level and suction forces, can lead to the breakage of mooring lines and uncontrolled movements of inland ships, resulting in a range of safety hazards and operational disruptions. Several documented incidents, field studies and interviews highlight the urgency for effective measures to mitigate the potentially harmful effects of passing vessels on the ships in the Hansweert outer harbour and locks.

A seven-week measurement campaign, involving 1281 passages of so-called oversized vessels, reveals distinct patterns of water level fluctuations during a vessel’s passage. A vessel is considered oversized if its length exceeds 210 metres or if its draught is larger than 10 metres. These patterns are described as a translatory drawdown wave travelling into the harbour, reflecting against the lock complex and oscillating back and forth in the outer harbour until dampened. The key parameter characterizing this wave is the lowering of the water level, referred to as the drawdown height. The average measured drawdown height approximates 6 centimetres, with maximum observations up to 40 centimetres. The main factors influencing the drawdown height are the vessel’s passing distance to the outer harbour, its speed relative to the currents and its dimensions, shown by a correlation analysis between the parameters describing the passing vessel and the generated drawdown height. Extreme drawdown events were exclusively observed during a combination of a relatively high speed through the water of the seagoing vessel and small passing distances relative to the harbour’s entrance.

The impact of the drawdown effects on the inland ships is determined by the forces generated by the pressure difference along the ships, caused by the inclination of the water level. A critical drawdown height of 12 centimetres is set, based on existing force criteria and the linear relation between the drawdown height and water level slope. To improve on the existing drawdown height prediction methods, a site-specific drawdown height prediction equation has been derived. Validation of this equation using the observations made during the measurement campaign yields a Pearson correlation coefficient of 0.81 and an Mean Absolute Error score of 2.2 centimetres.

Preventative measures are identified, aiming to minimize incidents and mitigate the risks related to the water motions induced by passing vessels. The predicted drawdown, generated by the passing vessel, is kept below the critical level by recommending a maximum speed related to the passing distance and dimensions of the vessel. Practically, this measure could be applied as a calculation tool or as an overlay on the pilot’s electronic sea chart. Coupling this information with awareness campaigns for pilots will contribute to minimizing the adverse effects on the ships in the outer harbour. The resilience against drawdown-induced risks could be strengthened by restricting the maximum combined width of ships moored alongside. Furthermore, by limiting the excessive slack in the lines of the moored ships, through signage and floating bollards, the movements of the ships will be restricted, reducing the risk of line breakage. Notifications of anticipated critical drawdowns would allow traffic controllers or lock operators to caution the inland ships and delay the lock chamber door openings, whilst alerting the passing vessel. Incorporating the mitigation measures recommended in this research could positively impact the safety of navigation in the Hansweert outer harbour and locks.