39. MALLORY, J.K., "Abnormal Waves on the South East Coast of South 

 Africa," The International Hydrographic Review, Monaco, Vol. LI, 

 No. 2, July 1974, pp. 99-129. 



Keywords. Case Study; Continental Shelf; Current Refraction; Currents, 

 Nearshore; Currents, Ocean; Currents, Opposing; Fetch; Forces on Struc- 

 tures; Interactions, Short Wave-Long Wave; Meteorology; Observation; 

 Waves, Deepwater; Waves, Wind. 



Discussion. This paper gives a detailed account of the conditions in 

 which unusually large waves have caused damage to several vessels when 

 proceeding in a southwesterly direction of the southeast coast of South 

 Africa. Eleven cases of ships encountering such conditions, all on the 

 southwesterly flowing Agulhas Current, are recounted. Severe structural 

 damage was sustained by some of the vessels; in particular, the tanker 

 "World Glory" broke in half in June 1968. 



The abnormally high waves occur in winter when wind waves coming 

 from the southwest meet the Agulhas Current which is directly against 

 them. They are reported to last for only a few minutes, but heights in 

 excess of 18 meters have been reported. There are consistent reports of 

 a long, deep trough occurring in front of the most severe wave. This 

 configuration has led to cases where the ship's forepart has failed to 

 rise as the crest advances and causes damage by its impact on the deck. 



Detailed descriptions of the environment in which these waves are 

 generated are given. They occur off the narrow Continental Shelf; wave 

 conditions are often markedly more severe just outside the 200-meter 

 contour. The possibility that the numerous submarine canyons refract 

 the waves is also mentioned. 



The Agulhas Current is at its strongest just off the Continental 

 Shelf, attaining a depth of 300 meters and a maximum speed of 2 to 2.5 

 meters per second just seaward of the shelf edge. A counter-current is 

 observed inshore after the passage of a cold front. 



In all cases of "freak" waves, a southwesterly wind had prevailed 

 for a long time over a long fetch. Specific details are given for one 

 incident, and it is concluded that waves could have a strong wind 

 generating them for as much as 1932 kilometers. It is suggested that 

 other locally-generated waves are present at the same time, and that the 

 combination of these waves, amplified by the current, produces the 

 really dangerous waves. 



Coastal Engineering Significance. This is an admirably well-documented 

 study of abnormal wave behavior in a region where high waves meet a 

 strong current. It provides stimulating information, and it can be used 

 as both a basis and an inspiration for much theoretical work in this 

 highly important field for navigators. 



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