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| Sefton Coast Partnership | Shoreline Management | Nature and Wildlife | Coastal Heritage | Visiting the Coast | Students and Teachers | Coastlines | |
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Summer 1995Wet. Wet. Wet.Why dunes are really wetlandsArticle By John Houston, Coast Management OfficerThe winter of 1994/1995 was one of the wettest periods on record in the north west. Visitors to the sandhills were perhaps surprised to find lakes of water lying in the dunes and many of the paths flooded. Surprised, because our generalised view of sand dunes is that they are dry places, with only a few wet areas acting like oases in the desert. Dune systems are in fact huge reservoirs of fresh water and the water that we see is only the "tip of the iceberg". Over the centuries a large aquifer has developed under the Sefton Coast forming a wall of water which stops the sea water from seeping through the dune system and entering the low-lying farmland. Water enters the dune aquifer from rainfall and the flow of the is both seaward onto the beach and landward to the Alt and the Crossens drainage basin. In some places the flow is interrupted or channelled by layers of peat. The water level that you can see is called the watertable. The watertable levels fluctuate to a steady annual rhythm. The highest levels are in March after the winter period. With less rain in the spring and summer and with plants drawing up the water from the ground (a process called transpiration)water levels start to fall. Sun and wind also speed up the process through evaporation. By October the annual water levels are at their lowest, but as the plants start to die back and rainfall increases the watertable begins to rise. There can be up to 1 metre difference (3 foot) between summer and winter levels. Although farmers may wish for a constant and predictable amount of sun, rain and wind, such a situation would be disastrous for the natural cycles in the sand dunes. On top of the annual fluctuation in water levels are longer term fluctuations, perhaps over 7-10 years of 'wet' years and 'dry' years. Scientists are not yet sure whether this follows any pattern but there seems to be a number of 'dry' years followed by a number of 'wet' years. (The winter of 2000/2001 was another wet period) Water level data has been collected at the Ainsdale Sand Dunes National Nature Reserve since the late 1960s and some patterns are beginning to emerge. Water levels from a series of dips-wells on the site are measured every month by the wardens and this builds up a long term record of dune water levels. The work is carried out in conjunction with Liverpool University. There can therefore be a large difference between the lowest recent recorded levels and the highest levels. This is about 2 metres and is an important influence on dune forming process. To explain this, imagine a series of 'dry' years with a falling watertable. As each summer becomes drier the wind can scour even deeper into the dry sand until it reaches the water table. Wind cannot shift wet sand, so what happens is that as all the dry sand is blown out of a hollow so a flat-bottomed valley, called a slack, forms. In dry years these embryo slacks, as they are called, are no more than damp sand but imagine what they will look like a wet year! As the water levels increase so the slacks flood but it takes dry years to create good slacks and wet years to turn them back into pools that will remain wet until late summer. Managers on the coast advocate a dynamic approach to dune management. This means allowing wind formed blowouts to develop into new slacks. After a few dry years the very high water levels of 1995 have produced a series of new slacks along the coast and natterjacks toads have already been spotted testing the water! Dune systems
thrive on fluctuations, not least a changing watertable and many
animals and plants are adapted to cope with change.
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