WAVE WORK 



561 



Fig. 4. Undermixixg of Sandbar causing Collapse of House. 



along the bottom as undertow. In the second place^, on-shore winds, by 

 driving the surface of the sea landward, insure a vigorous undertow sea- 

 ward; this undertow carries the beach material out to deep water, 

 thereby aiding beach destruction. Vigorous on-shore winds drive in 

 large waves, whereas off-shore winds, no matter how violent, can not 

 form large waves in the immediate vicinity of the shore. During the 

 recent storms on-shore winds raised the high-tide level on the New 

 Jersey coast from one to several feet above its normal elevation, much 

 beach sand was sucked out to sea by the resulting undertow, and large 

 waves were driven upon the shore with terrific violence. 



Other things being equal, the greatest damage will occur where the 

 land exposed to wave erosion is lowest. Waves may expend their energy 

 in two ways : in eroding the land or sea-bottom, or in transporting 

 debris. If the land is high, the waves break at the base of a high 

 cliff which sheds much debris into the water as its base is undermined. 

 This debris must be removed by the waves if effective erosion is to con- 

 tinue, as otherwise the cliff would soon be protected by the accumulated 

 waste. Eemoval of the debris requires much of the waves' energy, and 

 leaves them less competent to wear back the cliff. If the land is low, 

 the low cliff sheds but a small amount of waste upon the shore, the 

 waves quickly dispose of it and energetically continue their landward 

 advance. It is true that the effect of cliff height may be more than 

 offset by other factors, among which the 'form of the adjacent sea- 

 bottom is important. Of still greater importance for such a region as 

 the one in question is the effect of artificial sea defences, such as break- 

 waters, bulkheads and similar devices. At Seabright and adjacent 



