dune succession

plagiosere

dune slack

beach

A model dune system (Maine Geological Survey)

 

Aberlady Bay

 

 

 

 

 

 

Processes of sand movement across a dry beach

 

 

 

 

 

Model of the structure of a sand dune

 

 

 

 

 

 

 

 

 

Blow-out at Aberlady

 

 

 

 

 

 

 

 

Sand dune geomorphology

Coastal dunes develop where there is a supply of sand from the beach and where there is a frequent strong onshore wind. The scale and character of the coastal dune system depends on the combination of physical factors, such as the wind and wave regime, the sand supply from the beach and offshore bars, and biotic controls, such as plant succession and grazing pressure. Unconsolidated sand is highly unstable and the survival of dunes depends on the ability of plants to maintain ground cover and on a steady supply of sand. The falling tide exposes the beach to drying and wind action but most sand comes from driest, uppermost part of the beach, above the high water mark.

Dune sand is usually very well sorted sand of medium to fine size - it is  beach sand minus its large shell fragments. Dry sand is easily picked up at even moderate wind speeds and moves mainly by saltation across the beach and on to the dunes.

Mobile sand dunes have a well-developed slip face on the lee side of the dune ridge. As the dune migrates the sand avalanches, producing well-developed thin layers. These laminae are usually steeply sloping and several units arranged one above each other produce cross bedding. The dip of cross beds usually lies close to the angle of rest between 30 and 34. At steeper slope angles, sand avalanches down the slip face. Bedding surfaces separating individual units may be more or less horizontal. Surfaces left from periods of dune erosion are often curved.

Changing wind patterns or directions and the effects of large storms may strongly modify dune shapes. Wherever the vegetation cover is disturbed by wind, fire or human activity then there is potential for mobilisation of sand and the development of blow-outs.