|D50||Median particle/ grain size in sediments|
|Deepwater||Water sufficiently deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wave length is considered deepwater.|
|Defra||Department for Environment Food and Rural Affairs|
|DEM||Digital Elevation Model|
|Depth-limited||Situation in which wave generation (or wave height) is limited by water depth.|
|Design Wave Height||The wave height adopted for the purposes of designing coastal structures such as breakwaters and seawalls. It is chosen to ensure that the structures are not at undue risk of wave damage.|
|Detection limit||The smallest concentration or amount of a substance that can be reported as present with a specified degree of certainty by a definite, complete analytical procedure.|
|Diastrophic||Movement of the earth’s crust to produce surface irregularities|
|Diffraction||The "spreading" of waves into the lee of obstacles such as breakwaters by the transfer of wave energy along wave crests. Diffracted waves are lower in height than incident waves.|
|Directional Waverider Buoy||A floating device used to measure ocean wave height, period and direction.|
|Dissolved oxygen||The amount of oxygen in solution.|
|Diurnal||Having a period of a 'tidal day' i.e. about 24.8 hours.|
|Diurnal inequality||The difference in height of the two high waters or of the two low waters of each tidal day; also, the difference in speed between the two flood tidal currents or the two ebb currents of each tidal day.|
|Downdrift||In the direction of net longshore transport of beach material.|
|Dunes||Accumulation of windblown sand on the backshore, usually in the form of small hills or ridges.
A type of subaenal bedform indicating significant sediment transport over a sandy beach.