Passing Under a Bridge with Rising Water and Limited Air Draft
You are the skipper of a 24-metre inland vessel with a fixed air draft of 4.5 metres. You are approaching a bridge on the Dender River. The bridge has a clearance gauge showing 4.8 metres, but the river is in flood and the water level is rising at about 5 cm per hour. The current is strong, about 3 km/h against you. The bridge span is 20 metres wide, and you need to pass under the central arch. There is a small boat ahead of you moving slowly, and a larger vessel behind you. The weather is clear, but the river is carrying debris. You have a crew of three. You need to decide whether it is safe to pass under the bridge and what precautions to take.
1. Calculate the available clearance and assess the risk of passing under the bridge now.
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The gauge shows 4.8 metres clearance, and your air draft is 4.5 metres, giving a margin of 0.3 metres (30 cm). However, the water is rising at 5 cm per hour. If you wait, the clearance will decrease. You need to estimate the time it will take to pass under the bridge. At a speed of, say, 6 km/h through the water, with a 3 km/h current against you, your speed over ground is 3 km/h. To cover the distance to the bridge and pass under, say 200 metres, it will take about 4 minutes. In that time, the water will rise by about 0.33 cm, which is negligible. However, you must also consider the effect of squat and the possibility of the vessel pitching in the current. The margin of 30 cm is small, especially with debris that could block the arch or affect your course. Also, the gauge may not be accurate. Therefore, it is risky to pass. You should consider waiting for the water level to stabilise or recede, or find an alternative route. If you decide to proceed, you must do so at very low speed to minimise squat and have a crew member at the bow to monitor clearance.
2. What precautions should you take if you decide to proceed under the bridge?
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If you decide to proceed, first ensure that the small boat ahead has cleared the bridge and is not obstructing your path. Communicate with the vessel behind to inform them of your intentions and ask them to keep clear. Post a lookout at the bow with a VHF radio to monitor the clearance and report any debris. Reduce speed to the minimum required to maintain steerage, as squat increases with speed. Keep to the centre of the arch to avoid the bridge piers. Be prepared to stop or reverse if the clearance appears insufficient. Have the anchor ready to deploy in case of engine failure. Monitor the water level gauge continuously. If at any point the clearance seems less than 0.5 metres, abort the passage. Also, be aware of the effect of the current on your steering; you may need to use more rudder to maintain course. After passing, check the vessel for any contact with the bridge.
3. What are the legal and environmental considerations regarding the rising water and debris?
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You must comply with any local regulations regarding navigation during flood conditions. Some waterways may be closed to navigation when water levels are high. Check notices to mariners or contact the waterway authority. The debris in the river poses a risk of damage to your hull or propeller. You should keep a sharp lookout and avoid debris if possible. If you hit debris, check for damage and pollution. If you cause a wake that damages other vessels or the bank, you may be liable. Therefore, reduce speed to minimise wake. Also, consider the environmental impact of your wash on the riverbanks, which may be eroded. If you see pollution, report it. Ensure your vessel is not causing any pollution from engine exhaust or leaks. Finally, record the passage in your logbook, including the gauge reading and time, for future reference.