As the water becomes more shallow as it reaches the coast, the waves slow down and become closer together and steeper. 

There are many effects to shoaling, most importantly: energy dissipation and sediment transport. 

The gradual shallowing of the seabed allows the energy in waves to slowly dissipate.  Therefore, waves are not as aggresive when they reach the shoreline.   If a coastal structure were placed at the shoreline that created an abrupt stop for the waves, energy dissipation would be more extreme. 

Since shoaling involves the effects that waves have with the bottom of the seabed, sediment transport is a big issue.  The wave's energy dissipation occurs because of friction with the shallowing bottom.  This will cause sediments to be suspended in the water, and released once the energy dissipates. 

Shoaling is a natural process, and human interference with coastal areas has created unnatural slopes and bluffs, and encouraged sediment transport with alterations to the coastline.

As seen in the animation, refraction can play a large part in coastal erosion and sediment transport.  Like shoaling, refraction is a natural process.  As an extrusion from the coastline is gradually eroded, the sediments are brought onto areas on either side of the extrusion, creating beaches.  This erosion will continue until the coastline is uniform. 

Diffraction

Diffraction is one of the most important areas to explore when designing a coastal structure because it involves the change in wave propogation due to the presence of obstacles.  When designing a coastal structure, an engineer must look at how wind and waves may approach the barrier.  Then, it is necessary to examine the effects that the barrier has on wave speed and energy in the target area.