landmass such as southern Florida and moves offshore. High offshore winds 

 in this case can cause the water level to drop several feet. 



Setdown in semienclosed basins (bays and estuaries) also may be sub- 

 stantial, but the fall in water level is influenced by the coupling to the 

 sea. There are some detrimental effects as a result of setdown, such as 

 making water-pumping facilities inoperable due to exposure of the intake, 

 increasing the pumping heads of such facilities, and causing navigational 

 hazards because of decreased depths. 



However, rises in water levels (setup rather than setdown) are of 

 most concern. Abnormal rises in water level in nearshore regions will not 

 only flood low-lying terrain, but provide a base on which high waves can 

 attack the upper part of a beach and penetrate farther inland. Flooding 

 of this type combined with the action of surface waves can cause severe 

 damage to low- lying land and backshore improvements. 



Wind-induced surge, accompanied by wave action, accounts for most of 

 the damage to coastal engineering works and beach areas. Displacement of 

 stone armor units of jetties, groins and breakwaters, scouring around 

 structures, accretion and erosion of beach materials, cutting of new in- 

 lets through barrier beaches, and shoaling of navigational channels can 

 often be attributed to storm surge and surface waves. Moreover, surge can 

 increase hazards to navigation, impede vessel traffic, and hamper harbor 

 operations. A knowledge of the increase and decrease in water levels 

 that can be expected during the life of a coastal structure or project is 

 necessary to design structures that will remain functional. 



3.862 Storms . A storm is an atmospheric disturbance characterized by 

 high winds which may or may not be accompanied by precipitation. Two 

 distinctions are made in classifying storms: a storm originating in the 

 tropics is called a tropical storm; a storm resulting from a cold and 

 warm front is called an extratropioat storm. Both of these storms can 

 produce abnormal rises in water level in shallow water near the edge of 

 water bodies. The highest water levels produced along the entire gulf 

 coast and from Cape Cod to the south tip of Florida on the east coast 

 generally result from tropical storms. High water levels are rarely 

 caused by tropical storms on the lower coast of California. Extreme 

 water levels in some enclosed bodies, such as Lake Okeechobee, Florida 

 can also be caused by a tropical storm. Highest water levels at other 

 coastal locations and most enclosed bodies of water result from extra- 

 tropical storms. 



A severe tropical storm is called a hurricane when the maximum 

 sustained wind speeds reach 75 miles per hour (65 knots). Hurricane 

 winds may reach sustained speeds of more than 150 miles per hour (130 

 knots). Hiirricanes, unlike less severe tropical storms, generally are 

 well organized and have a circular wind pattern with winds revolving 

 around a center or eye (not necessarily the geometric center). The eye 

 is an area of low atmospheric pressure and light winds. Atmospheric 

 pressure and wind speed increase rapidly with distance outward from the 

 eye to a zone of maximum wind speed which may be anywhere from 4 to 60 



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