With respect to sand supply, a given length of coastline may have a surplus, be in balance, or have a deficit in its sand supply budget. A sand supply would be in balance for a particular area if the amount of sand leaving the area was being replaced by an equal amount of sand arriving from adjoining areas. A large reduction in the sand supply, or a long term reduction in supply to an area, creates a deficit in the sand budget that must be balanced, usually by increased erosion. Sand supply influences erosion by supplying the material that builds and maintains beaches, offshore bars, and the general beach profile. These features dissipate wave energy and the absence of these features, where they would normally be present, usually results in increased erosion.
Human activity can significantly affect sand supply.
Coastal navigation and shore protection structures can adversely affect sand supply and the transport of sand along the coastline. Groins and harbor structures in particular may interfere with the transport of sand along the coastline. Seawalls or revetments may cut off the littoral transport system from its sand source. Individually, the effects of one or several structures may, or may not, be noticeable, but the cumulative effect of hundreds of such structures can be significant. Dredging operations may hinder or benefit littoral transport and the sand supply.