4.35 EFFECT OF EXTREME EVENTS
Infrequent events of great magnitude, such as hurricanes, cause significant 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
détermines 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 steepness 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 itidicates that no general conclusion that can be made conceming the significance 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 resuit 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 probably be retumed in the months after the storm and pemanent 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 opp
site directions, (e.g, Everts, 1973.)
There are some unique events that are only âccomplished by £***
Phic storms. The combination of storm surge and high waves
0 reach some areas not ordinarily attacked by waves.
formation of
itions may resuit in the overtopping of dunes an in
Marston
^ashover fans and inlets. (Morgan, et al., 1958; ^cl° and then seal’ed
J939.) Some inlets are periodically reopened by s orm
y littoral drift transported by normal wave action.
4-37
Infrequent events of great magnitude, such as hurricanes, cause significant 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
détermines 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 steepness 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 itidicates that no general conclusion that can be made conceming the significance 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 resuit 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 probably be retumed in the months after the storm and pemanent 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 opp
site directions, (e.g, Everts, 1973.)
There are some unique events that are only âccomplished by £***
Phic storms. The combination of storm surge and high waves
0 reach some areas not ordinarily attacked by waves.
formation of
itions may resuit in the overtopping of dunes an in
Marston
^ashover fans and inlets. (Morgan, et al., 1958; ^cl° and then seal’ed
J939.) Some inlets are periodically reopened by s orm
y littoral drift transported by normal wave action.
4-37