being eaten away by erosion with alarming regularity; at Covehithe 

 the land is retreating at the rate of about seventeen feet a year. 



Although many coasts seem to have a "permanent" beach, some- 

 times the beach is maintained only at the expense of cliff erosion. 

 Where a coast curves smoothly "longshore" movement continually 

 washes material away. Flamborough Head off England's east coast, 

 for example, prevents the replenishment of Holderness beaches. 

 Although waves are constantly moving beach material south in 

 this area, new beach material, won from the cliffs by erosion, replaces 

 the old. But we should remember that these "permanent" beaches 

 continually change — sometimes they build up when they reclaim 

 material from the sea; other times they are lowered as their sands 

 are washed out to sea. The steep storm waves are the chief force 

 that robs the beaches of their sand. When these waves reach their 

 break point, they are called destructive waves, and whenever they 

 are reinforced by an onshore wind their powers of destruction are 

 greatly increased. The excess water pushed shoreward by the wind 

 must flow back to the sea, and does so as an undertow that scours 

 the bottom. This is the reason why after stormy periods the level 

 of a beach is lower than normal. 



While carrying on their destructive work, these storm waves 

 can also be constructive by throwing coarse gravel, called shingle, 

 high up the beach, higher than normal waves ever reach. It is the 

 storm wave, then, that helps make the familiar shingle ridges that 

 line many beaches high above the normal high- water mark. Chesil 

 beach in Dorset, which has large shingle deposits, reaches up to 

 forty-three feet above high-tide level near Portland. 



Destructive waves do not move material very far offshore, and 

 it is usually returned to the beach in time. Permanent loss of mate- 

 rial results from its being carried along the beach and eventually 

 deposited outside the beach area. This longshore movement can be 

 devastating to areas suffering from erosion. For this reason coastal 

 engineers build groynes on beaches from which the material is 

 liable to be swept away and carried along the coast. If you look at 

 a groyned beach, you will usually find that the sand is piled much 

 higher along one side of the groynes — from the direction of long- 

 shore movement. Inadvertently, in our attempts to preserve one 

 section of beach or coast, we may destroy another. While groynes 

 can protect a windward beach, they can be dangerous to the leeward 

 beaches by starving them of sand, and so making conditions right 

 for erosion. In the United States, along the west coast near Los 

 Angeles, where some beaches have been lost for this reason, or 

 where a breakwater has acted as a large groyne, large quantities of 

 sand have been artificially dumped on the leeward beach in an 

 attempt to maintain its height. 



The coastal engineer wants to know exactly how much material 

 is being carried along the shore by longshore movement. One of 

 the techniques developed recently is to follow the movement of 

 radioactive material with Geiger or scintillation counters. Along 

 the Suffolk coast at Orford Ness, where this technique was used, 

 scientists found that the movement of material between the offshore 

 zone and the beach was very limited. Even strong currents seem 

 unable to move shingle against waves approaching from a different 

 direction. On the other hand, waves reinforced by northerly winds 

 were able to move great quantities of shingle in a southerly direc- 



Groynes, like these along the beach at 

 Brighton, England, prevent beach material 

 from being washed away by longshore move- 

 ment of the water. Evidence of the longshore 

 movement can be seen by the way sand 

 and pebbles have been piled up along one 

 side of the groynes. 



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