An ancient river channel, shipwrecks and giant underwater gravel dunes are among previously unknown features discovered during the most detailed survey to date of the Channel seabed.

The survey, covering 500 square miles off the Dorset coast, is being carried out in advance of the 2012 Olympics. Sailing events will take place off Weymouth and Portland, and organisers are anxious to avoid any unpleasant surprises, such as uncharted rocks, that have holed small boats in the past.

The £300,000 project has already led to the redrawing of marine charts in use for nearly 75 years. It will also enable marine conservationists to record the variety of habitats in the area.