David Moore, 29, of Victoria Grove, Southsea, was servicing the Upper Lode lock gate near Tewkesbury, Gloucestershire, four years ago, when the "primitive" dam failed and he became trapped under 3.5m of water.

Operations engineer Roger Byrne, who has an honours degree in civil engineering funded by British Waterways, has told an inquest he had not considered the risk of pressure differential.

The inquest jury heard the temporary dam beside which Mr Moore was working, was sealed only by a "hessian sausage" of sacking.

Mr Byrne said: "The method had been used successfully for 40 years. One member of the direct labour team had been there for 35 years, the others for 20 years. Taking advice from them, it had been used before."

Gloucestershire coroner Alan Crickmore asked if he was concerned by the suction effect of high pressure of water flowing to a low pressure area.

He replied: "I possibly knew that there may be some suction. I didn't realise how dangerous it was."Southsea

The coroner said: "When you stand in a bath full of water going down the plug hole, you can easily stand as a child and have your foot sucked into the hole. That's the effect of it."

The coroner then asked: "What calculations did you did you do to ascertain the likely stability and strength of the hessian seal?" Mr Bryne, who worked for British Waterways since 1982, replied: "I didn't do any calculations...to my knowledge there were none."

The Crown Prosecution Service decided no criminal charges were justified, following the death of Mr Moore. The hearing at the Manor by the Lake Hotel, Cheltenham, is expected to end next Friday.