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.

The largest previously undiscovered feature found so far is an 18-mile channel carved by a river when Britain was still connected to the Continent. It vanished beneath the waves when the seas rose at the end of the last Ice Age, about 12,000 years ago, and is now 40m (130ft) underwater. The river channel, which had cut its way into the bedrock, is between 80m and 150m wide and would have been more than 10m deep, although it is now filled with sediment.

Scientists are using high resolution, multibeam sonar to take thousands of simultaneous readings, allowing them to find objects on the seabed as small as 15cm (6in) across and gauge depths with far more precision than was previously possible.

Rob Spillard, of the Maritime and Coastguard Agency, said: “We have had a team of around a hundred people working on the project over the past year. It’s the first time anything like this has been done on such a scale, with so many people working together. The sonar allows us to collect a huge amount of data which is checked closely before being made into a map. The findings will be added to nautical charts to help ships navigate the waters around Dorset but it has also thrown up a huge amount of other information, including giving us very detailed pictures of shipwrecks.”

The sonar, besides gauging depth, is able to determine the type of surface making up the seabed. Richard Edmonds, science manager for the Jurassic Coast World Heritage Site, said: “The pictures the study has produced are hugely exciting. I was absolutely blown away when I first saw them. The detail is mind-boggling and you can see the intricate patterns of the structure of the seabed.

“We’re very excited about the project — when the river bed was uncovered, the land would have been used by woolly mammoths, reindeer and wolves, as well as early humans.”

The first phase of the two-year study, known as the Dorset Integrated Seabed Study (Doris), has been completed and the second year of video and photography is due to begin. Analysts will use the maps to identify patterns in the seabed before filming underwater animal and plant life to create an elaborate picture of previously hidden habitats.

The project is being led by the Dorset Wildlife Trust. Simon Cripps, the director of the trust, said: “We have no real idea what goes on under the sea. This study will give us an understanding of what is actually physically down there — it’s very exciting. This is the first time anyone has studied it on three critical levels: how deep it is, the material of the seabed and the animals and plants living there.

“It’s like putting a 3-D jigsaw together in three layers. The results will be quite spectacular.”

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