Читать книгу Taming the Flood: Rivers, Wetlands and the Centuries-Old Battle Against Flooding - Jeremy Purseglove - Страница 34

So much for the rivers themselves. Enthusiastic attempts to drain wetlands throughout the 1970s and early 1980s have in many cases failed to produce the high-quality farmland which was the object of these expensive exercises. It has now become increasingly clear that one of the real harvests of wetland drainage can be the physical ruin of the land. The most familiar example of such degradation is peat wastage. Peat wastes as a result of shrinkage, oxidation, and bacterial action, all triggered by the drying effect of drainage upon peat. A ten-year trial by the Ministry of Agriculture, started in 1980 on the Norfolk peat marshes near Acle, is indicating a fall in ground level of an inch each year. The level of the peat fenland in Cambridgeshire has fallen by up to 15 feet in many places since Vermuyden’s initial scheme was completed in the 1650s. The Holme Post, recording a drop of nearly 13 feet since 1850, presents an unrealistically optimistic picture of the problem, since it now stands in a damp nature reserve.

More convincing estimates of wastage can be made by assessing the height at which roads and even some wartime pillboxes stand proudly above the adjacent level of the fens. As the peat wastes, drainage of the land deteriorates, and so the drains are deepened, thus triggering further wastage. The lowest deposits of peat are also the most acid and so least valuable to the farmer. The Fens now resemble a gigantic and very profitable grow-bag. Like a grow-bag, however, they cannot go on producing their rich harvests of vegetable crops such as celery and carrots for ever, since once the peat has all wasted away, poor acid subsoils, especially clays, are often all that remain beneath. It is estimated that by the first decade of next century, only 20 per cent of the peat soils now present on the 138,600 acres marked by the soil map of the Ely district will remain.^fn6 Once the peat has gone in such places, some land may be insufficiently fertile even for growing potatoes, and the very best land in the country will have been reduced to mineral soils of only average quality. Such downgraded land will still have to be pumped and embanked, since it will have dropped even futher below sea level,¹⁰ a situation worsened both by the fact that, due to the tilt of the land, eastern England is steadily falling in relation to the level of the North Sea, and by a possible rise in sea level due to the greenhouse effect.^fn7