630 SAND-MOVEMENT ON THE NEW SOUTH WALES COAST, 



When the wind blows obliquely to the shore, it either creates a 

 current or accelerates or retards the existing current, in any case 

 tending to move the beach-material in the same direction as the 

 wind blows. But independently of the current, there is a move- 

 ment due to the obliquity of the breaking wave to the shore. 

 An alongshore wind on a steep coast causes obliquely running 

 waves to break before they have time to swing round and face 

 the shore; but on a sandy beach the waves due to a moderate 

 wind swing round almost parallel to the shore before breaking. 

 With a strong wind, or gale, the velocity is too great to allow of 

 this, and in consequence we have waves breaking more or less 

 obliquely to the shore, the obliquity varying as the velocity of 

 the wind, and with the steepness of the beach. Even with a 

 moderate breeze there may be two sets of waves running, one 

 coming in from the offing, due to distant storms, and the other 

 caused by the alongshore wind. In deep water these two sets of 

 waves pass through each other without any appreciable inter- 

 ference of the transmission by each of its proper impulse in the 

 direct line of its motion. When these two sets of waves are 

 running the combined billow formed by the coincidence of the 

 two crests, breaks before the time proper to the alongshore wave, 

 and therefore before it can swing round to face the shore.* A 

 number of short breakers are thus formed in the direction of the 

 mechanical resultant of the two forces, and the result in moving 

 the sand and shingle on the shore line is considerable. 



It will be manifest that the undertow or offshore current will 

 be greatest at the southern end of the beach as the result of a 

 northerly wind, and at the northern end in consequence of a 

 southerly wind; and, as the result of the undertow is to cut down 

 the beach, the effect of a prevailing wind blowing obliquely to 

 the shore will be to deepen the water at the far end of a beach 

 and to allow the sand to accumulate at the near end. 



An onshore wind blowing at right angles to the shore-line does 

 not move the sand or shingle alongshore, but it has the effect of 



*Cornish, V., "On Sea-beaches and Sand-banks," Geog. Journ., Vol.xl., 

 1898. 



