Coastal engineering is the study of the processes ongoing at the shoreline and construction within the coastal zone. The field involves aspects of nearshore oceanography, marine geology, and civil engineering, often directed at combating erosion of coasts or providing navigational access.
Waves, generated primarily by the wind, propagate from the ocean to the shoreline across the continental shelves. These waves undergo many processes before they dissipate in the surf zone: refraction, diffraction, shoaling, and breaking. The energy and momentum associated with the waves arriving at the surf zone is used to create longshore and cross-shore currents that move the sand comprising beaches. This sand transport, if it carries more sand away from a site than towards it, results in beach erosion.
The ongoing rise in the sea level due to the glacial melting since the last ice age and now perhaps accelerated by the Greenhouse Effect creates a pervasive mechanism for shoreline retreat.
Tidal inlets, connecting bays or lagoons to the ocean, also contribute to the shoreline retreat by capturing beach sand into ebb and flood shoals.
The processes of coastal erosion are very complex, involving three-dimensional flow fields created by the breaking waves, unsteady turbulent sediment transport in both the water column and on the bottom, and an moving shoreline. Much research is being conducted worldwide to develop predictive models of this erosion process.
Numerous devices have been devised to stop the erosion process. These can be divided into two basic types: hard and soft structures.
Hard structures have been the traditional tool of the coastal engineer. These include groins (structures oriented perpendicular to the shoreline to slow the transport of sand along a shoreline), jetties (placed at inlets to keep sand from the navigational channel, breakwaters (to reduce wave action in harbors), and sea walls (to prevent the erosion of the upland).
Soft structures are those that are more natural. The primary example is beach nourishment, which is the placement of sand on an eroding beach. Nourishment is a short-term measure as it does not fix the cause of the erosion; however, it is the only method that involves adding sand to the coastal system.
Source: The Coastal Engineering Page