SIR — Re '£2.4 billion lagoon dream' (Free Press, April 18: The first thing to understand is that the cliffs and beaches we know so well are the gift of the tides and longshore currents of the Bristol Channel. The erosion of some cliffs provides the debris which recharges the beaches and pebbly foreshores which, in turn, break the force of storms and foreshores which would otherwise batter the softer, more erodible cliffs. It is a dynamic and constantly changing system sustained by Nature. It is always best to work with Nature rather than to try to impose our man-generated schemes, however they may have been modelled or borrowed from another area where they worked. In history, it was the approach of William Smith, the so-called 'Father of English Geology', when he solved the problem of coastal erosion on the Holkham Estate in Norfolk for the Earl of Leicester. His solution was to cart loads of flint gravel several miles to the west of the gaping breach to the coastal banks at Holkham, leaving it to the North Sea to shift the flints eastwards to neatly fill the hole better than teams of estate workers with shovels. This moral came to mind when I tried to take in the possible consequences of the proposed lagoon. To create the retaining wall to the Bristol Channel would test the skills of civil engineers in creating the lagoon, as would a regime of opening and closing it for craft up to the size of the Balmoral or proposed car ferries. The tramway to Doniford has much appeal but, facing the seas we have experienced in recent months, it would only safely function in favourable - i.e calm - weather conditions. Returning to the dynamic model which we record at present, not only is it responsible for the profit and loss of our cliffs and beaches but it is vital for sea life and marine ecology. An imposed control of tides could well produce a stagnant lagoon, prone to algae and weed growth which, once arrived, could be difficult to eradicate. The Press has recently listed invasive species of plants and animals which have come from the bilge washings of ships coming into our ports and competed with our native sea life disastrously. We need the constantly refreshed tidal regime of the Channel. A final concern: The conditions of Swansea Bay are dramatically different from our coast. The science is not transferable. We must hope that our local authorities look at this most closely. Dr Eric Robinson, Whitehall, Watchet.