Longshore drift is movement of sand and pebbles along a sea coast. The material is carried along the beach by the waves hitting the coast obliquely (swash) but is swept back at right-angles to the beach by the backwash. If the carrying power of the waves decreases the material may be deposited to form a spit.
To combat the effect of longshore drift groynes are constructed. Usually wooden wall-like structures, they run perpendicular to the coast at regular intervals and so stop the erosion of the coast. Another, increasingly used method to combat the effect is to replace the material in a process known as 'beach nourishment.