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Coastal Processes - OverviewThe Sefton coast, which extends over 36 kilometres (21 miles), comprises soft and granular estuary deposits of sand, silt, clay and peat. There are no outcrops of rock on the shoreline. Hence, the forces of nature readily mould it, so the shoreline is constantly changing in response to the fluctuating influence of wind and water and as a result of human activity. Its overall shape derives from two major river estuaries, the Mersey and the Ribble. Two other much smaller rivers, the Alt and Crossens, each have important local zones of secondary influence.In the distant past both human and wild life moved with, and adapted to, changes in the position of the coast. Between the last Ice Age, and medieval times the sea occasionally broke through the coastal dune barrier and flooded low-lying inland areas. Large tracts of sand were periodically stripped by gales from the dune coast and sand penetrated inland across fields and settlements. In more recent times the dunes have been partially stabilised by maintaining their natural vegetation, trees have been planted for shelter and artificial sea defences have been built to protect developed shorelines. The inland lakes and mosslands have been drained and claimed for agricultural production. Before the beginning of the eighteenth century the form of the coast derived mainly from conditions offshore. Maps and charts demonstrate the effect of recent human influence. Dredging, river training, and coastline hardening have imposed a pattern of accretion and erosion on the shoreline, where previous conditions were much more variable. At Crosby and Southport the shoreline has been partly fixed in place by coastal defence work. However, the natural forces remain at work and sand drift at Crosby is tending to bury parts of the sea wall, whilst sand dunes are developing in front of the sea wall north of Weld Road. From Hightown to Birkdale the coastline is in a more natural condition, with generally wide sandy beaches still backed by an extensive system of sand dunes. Formby Point, midway between the Mersey and Ribble estuaries is the meeting zone of the two major estuarine regimes. Tidal streams converge offshore, with the result that a large inter-tidal sand spit (Taylor's Bank) has developed, aided by the construction in the first half of the 20th century of the River Mersey training walls. The predominant source of the Sefton coast beach and dune sand is the bed of the Irish Sea. During the last Ice Age advancing ice sheets pushed glacial deposits into the Irish Sea basin and over 10,000 years these have been broken down, sorted and transported by tidal and wind-driven currents. The prevailing westerly weather and tidal streams both tend to move seabed deposits towards the coast and into the river estuaries, which are both zones of net sediment accumulation. The River Mersey contributes very little sediment to the Sefton coast; it is mostly intercepted in the Ship Canal. The River Ribble carries fine silt downstream, which settles out on the northern beaches whenever, or wherever, sheltered conditions prevail. A further source of estuarine accretion (infilling by sediment) is the sand eroded from Formby which is moved by tidal and wave-driven currents northwards into the Ribble Estuary and southwards into the Mersey Estuary. Although the Altcar sand dunes and the dunes to the north of the Ainsdale National Nature Reserve are accreting, the sand dune system around Formby Point has experienced continuous erosion throughout the 20th century. The balance of evidence suggests that the present phase of erosion was primarily triggered at the end of the nineteenth century by a significant increase in the frequency of storm force westerly winds and destructive waves. The erosion was compounded by the combined effects of dredging, spoil dumping and training wall construction which significantly altered the bathymetry in Liverpool Bay, leading to increased wave focusing onto Formby Point. This focusing is greatest to the north of Wicks Lane and it particularly affects the National Trust frontage. The erosion rate is greatest at the boundary between the Formby National Trust and the Ainsdale National Nature Reserve, with an average loss of approximately 3.5 metres per year over the past 20 years. Individual erosion events often result in a short-term loss of several metres. These events are generally transitory and conditions then settle back into the long-term trends identified. The Sefton coastline also has to be considered in the light of the effect of possible climatic change:- global warming, sea level rise and increase in storminess. Numerous predictions have been made as to the magnitude of these effects, with widely differing answers, however, current predictions have tended to come together in a narrower range. Recent climate-modelling work suggests a possible rise in average sea level of 0.3 metres over the next 60 years. On top of this one has to consider the increase in maximum wave height and meteorological surge effects on any storm event. Although the environmental influences and the directions of sediment movement are well understood, there is uncertainty over just how much sediment remains in the Irish Sea "reservoir". More research is needed to gain a better understanding of how the Sefton Coast might respond to future climatic change. Text taken from Coastal Defence Issues and Strategy - 1999 Update, 2000, Sefton Council.
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