Longshore Drift

Longshore drift is the process of moving material along a beach. It occurs when waves strike the coast at an angle with the swash carrying shingle and sand up the beach obliquely. The backwash will then carry some material directly down the steepest gradient of the beach. As this process is repeated, material will be moved laterally. The direction of this movement will depend on the direction of approach of the waves. On the east coast of England, most of the movement is southwards except in north Norfolk where it is westwards. At a river mouth or at a curve in the coastline, the material may be carried straight on and deposited to eventually form a spit - a low lying ridge of sand and shingle, like an extension of the beach out into deeper water. As this spit extends, it may even divert the course of the river.

This diagram show the movement of material along the coast by longshore drift

Examples of this type of formation on the east coast are Spurn Head which has diverted the mouth of the Humber, Blakeney Point in Norfolk at the mouth of the Glaven and Orford Ness which has diverted the mouth of the Alde about 15km. The end of the spit may curve where the effect of wave refraction prevents further extension in an almost straight line. The spit will continue to grow until the point of equilibrium is reached between deposition caused by longshore drift and erosion which removes material at the far end.

This aerial photograph shows Blakeney quay - a long shingle spit.

The image shows the growth of Spurn Head. From this, you can see that the formation of a spit is not just one steady growth but can be interrupted as part is eroded and development restarts. This cycle seems to take place about every 250 years.

At the mouths of some estuaries there may be two spits appearing to grow towards each other from opposite banks. This has actually happened at Poole Harbour in Dorset but in other examples it is simply one spit divided by the river but extending from the windward side under the surface of the water.

This diagram shows the stages in the growth and decline of Spurn Head at the mouth of the Humber.