The commonly held view that a wreck gradually sinks into the seabed due to its own weight is not always true, at least not in tidal waters like the North Sea. The changing tides with their tidal streams, especially the ebb stream, cause sediment to be washed out around the wreck, which leads to the formation of scour holes.
Therefore, the water depth above a wreck, which is crucial to the safety of navigation, is continually changing. On an echograph, it is often possible to distinguish scour holes caused by the flood stream from those caused by the ebb stream because the latter are deeper. Such scour holes, whose size and depth depend on the current strength and seabed properties, may cause gradual changes in the wreck’s position.
A wreck may slide down into the scour hole, with either bow or stern lifted higher.
In some cases, a wreck with a scour hole under its middle section may break apart and slide into the depression, with both bow and stern lifted higher.
Or the scouring action causes bow and stern to sink into the scour hole.
A wreck which initially lies on its side may be put in an upright position by the scouring action, causing the superstructure to be elevated, which potentially poses a hazard to navigation.
A wreck may also break apart due to corrosion, and parts of it may become dislodged. Drift ice or ice ridging may change the shape and position of a wreck. There are quite a number of factors that may physically change a wreck, affecting the shallowest depth of water above it. Therefore, it is absolutely necessary to continually monitor and re-survey wrecks located in congested waters. Possible changes undergone by underwater obstructions are illustrated by the examples in the following.