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Shoreline Management Coastal Features - Sand Dunes Probably the most famous features found on the Sefton Coast is the Sand Dunes System. Sefton Is home to the longest continuous belt of sand dunes in Britain stretching from the River Alt at Hightown to Weld Road at Birkdale. Definition Sand Dunes are mounds or ridges of drifted sand. Sand dunes can form both at the coast and in inland areas where there is sufficient sand and wind. However, the shape (morphology) of coastal dunes and inland dunes differ considerably even though the process of formation is very similar. Formation They are unique as coastal features as they are formed by air movement rather that water movement. There are several basic requirements for the formation of sand dunes, these are: A supply of dry sand over a wide beach An area of low lying land behind the beach Predominantly onshore winds Dunes normally form at the top of a beach, when the onshore winds blow sand from the lower shore to the top of the shore. The sand builds up at the top of the shore, especially around small objects such as strandline material. These small objects slow the speed of the wind so that it is no longer able to blow the sand. These mounds of sand are colonised by salt tolerant plants, which further reduce the wind speed and increase sand deposition. With increased dropping of sand the height of these 'embryo dunes' increases. These embryo dunes are less frequently covered by the tide and waves and subsequently more plant species are able to grow here. The embryo dunes continue growing, unless they are destroyed by waves or storms. When the dunes are no longer covered by the highest tides, they are generally known as 'mobile dunes'. These mobile dunes are less salty than the embryo dunes as rain washes out some of the salt. Sand continues to be blown from the beach into and over these dunes and the mobile dunes appear to move backwards. Mobile dunes are characterised by areas of sand and a few species of dune building plants. As they get further from the sea the amount of salt in the dunes becomes less and a larger variety of plants can grow. When the dunes are almost totally covered with vegetation they are classed as 'fixed dunes'. Erosion Erosion of the dunes can and does occur naturally due to changes in the coastal processes or following disease or damage to vegetation. There are two main types of erosion: Surface erosion by wave action (deflation) Marine erosion of the toe and seaward face of the dunes There are clear changes in beach profile as they become flatter as a result of destructive wave action. The upper beach level falls allowing the high tide mark to reach the toe of the dunes. Waves impact directly onto the dune toe, causing the front face of the dune to collapse onto the beach, and sand is then carried down the beach. These eroding dunes are characterised by steep high frontal dunes. However, erosion can also occur and be accelerated as a result of human activities including excessive trampling, driving vehicles over dunes, removal of sand, mechanical beach cleansing, changes to training banks/coastal defence works and dredging. Survey and Monitoring Coastal dunes form a very complex system due to the interactions of the physical forcing factors (e.g. wind) and vegetation. As a result there a wide range of different methods of surveying and monitoring. A number of vegetation and wildlife surveys are conducted. The dunes are monitored for changes in their morphology, position, extent, composition. Lidar surveys are the quickest method to assess the topography of a dune system. aerial photography used to identify areas of bare sand and canges in dune position dune front survey monitors position of frontal dunes used to assess rates of accretion and erosion. Coastal defences survey looks at the condition of the dunes and highlghts areas of concern. Home Page - Top www.seftoncoast.org.uk

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