4.35 EFFECT OF EXTREME EVENTS 



Infrequent events of great magnitude, such as hurricanes, cause sig- 

 nificant modification of the littoral zone, particularly to the profile 

 of a beach. An extreme event could be defined as an event, great in 

 terms of total energy expended or work done, that is not expected to occur 

 at a particular location, on the average, more than once every 50 to 100 

 years. Hurricane Camille in 1969 and the East Coast Storm of March 1962 

 can be considered extreme events. Because large storms are infrequent, 

 and because it does not necessarily follow that the magnitude of a storm 

 determines the amount of geomorphic change, the relative importance of 

 extreme events is difficult to establish. 



Wolman and Miller (1960) suggested that the equilibrium profile of 

 a beach is more related to moderately strong winds that generate moderate 

 storm waves, rather than to winds that accompany infrequent catastrophic 

 events. Saville (1950) showed that for laboratory tests with constant 

 wave energy and angle of attack there is a particular critical wave steep- 

 ness at which littoral transport is a maximum. Under field conditions, 

 there is probably a similar critical value that produces transport out of 

 proportion to its frequency of occurrence. The winds associated with this 

 critical wave steepness may be winds generated by smaller storms, rather 

 than the winds associated with extreme events. 



A review of studies of beach changes caused by major storms indi- 

 cates that no general conclusion that can be made concerning the signifi- 

 cance of extreme events. Many variables affect the amount of damage a 

 beach will sustain in a given storm. 



Most storms move large amounts of sand from the beach offshore, but 

 after the storm, the lower waves that follow tend to restore this sand to 

 the face of the beach. Depending on the extent of restoration, the storm 

 may result in little permanent change. Depending on the path of the storm 

 and the angle of the waves, a significant amount of material can also be 

 moved alongshore. If the direction of longshore transport caused by the 

 storm is opposite to the net direction of transport, the sand will prob- 

 ably be returned in the months after the storm and permanent beach changes 

 effected by the storm will be small. If the direction of transport before, 

 during, and after the storm is the same, then large amounts of material 

 could be moved by the storm with little possibility of restoration. Suc- 

 cessive storms on the same beach may cause significant transport in oppo- 

 site directions, (e.g. Everts, 1973.) 



There are some unique events that are only accomplished by catastro- 

 phic storms. The combination of storm surge and high waves allows water 

 to reach some areas not ordinarily attacked by waves. These extreme con- 

 ditions may result in the overtopping of dunes and in the formation of 

 washover fans and inlets. (Morgan, et al., 1958; Nichols and Marston, 

 1939.) Some inlets are periodically reopened by storms and then sealed 

 by littoral drift tremsported by normal wave action. 



4-37 



