There is an inherent risk of marine accidents occurring where goods are transported by sea, just as there are risks associated with other forms of transport, although these risks are far less per tonne mile than occur with other forms. Such accidents may occur if a ship is unsuccessful in its attempt to avoid another vessel or obstruction. Harbour authorities make an important contribution to reducing the risk of such events by undertaking their responsibilities as conservancy authorities over various measures to provide for navigation safety. Furthermore, where response plans have been drawn up, an appropriate, co-ordinated approach to any incident will ensure that any potential damage to the environment is limited, particularly where hull ruptures and loss of cargo or fuel spillage occur. The potential impacts of such oil spills and discharges are discussed in the Waste Management Section.

When a vessel runs aground it is inevitable that this event will disturb the seabed. The length of time a vessel stays aground may influence the extent of damage caused, however, waiting for the tide to re-float the vessel may be less harmful than vigorous action by tugs. Grounding of a vessel may cause resuspension of sediment resulting in turbidity and mobilisation of any contaminants in the sediment. The disturbance to the benthic community will be short lived and dependent upon the type of benthic animals in situ. Hard bottom communities are generally less resistant to increases in turbidity than those adapted to a silty estuarine environment. Some loss may occur as a result of burial. In general, the impact of a grounding incident and the length of time required for habitat recovery is likely to be greatest for sensitive, slow growing species and communities in the intertidal and shallow subtidal that are unable to move away, such as maerl or seagrass beds. In hard bottom areas, physical damage to rocky communities, such as those of reef habitats, may be an issue, although the greater risk of hull damage will normally mean that navigators will allow greater safety margins to minimise risk.

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