Definition of Subaerial coastal sand dunes:
Ridges or moulds of loose, wind-blown sand (fine to medium) forming on the backshore and forming the coastal features at certain locations. Dunes are more or less vegetated. Dunes are active coastal form elements acting as a flexible sand reservoir.
This is the common definition for Subaerial coastal sand dunes, other definitions can be discussed in the article
Definition of Subaqueous dunes, frequently called "sandwaves" in shelf seas:
Bed forms induced by the interaction of the seabed with (mainly tidal) currents.
This is the common definition for Subaqueous dunes, frequently called "sandwaves" in shelf seas, other definitions can be discussed in the article
Notes subaerial coastal sand dunes
Because of the importance of the wind to the mobilisation of sediment, sand dunes develop from sand grains in the size range 0.2-2.0mm. At eroding coasts dunes are moving backwards in parallel with the erosion process. Dunes act as a kind of flexible natural protection against erosion and flooding, see Dune erosion. If the vegetation is damaged by too much traffic or grazing etc. the integrity of the dunes may be endangered, see Shore protection vegetation.
Notes subaqueous dues
Sandy seabeds are often covered with dune fields in regions where the maximum tidal current velocity is in the range 0.5-1.5 m/s. Dune spacing (more than 10 m and less than 1000 m) depends mainly on water depth. Dunes can reach a height of 10-50% of the water depth; high dunes present a risk for navigation.
Wikipedia article on sand dunes .