nautical miles from the center. From the zone of maximum wind to the 

 periphery of the hurricane, the pressure continues to increase; however, 

 the wind speed decreases. The atmospheric pressure within the eye is the 

 best single index for estimating the surge potential of a hurricane. This 

 pressure is referred to as the centvdl pressure index (CPI) . Generally 

 for hurricanes of fixed size, the lower the CPI, the higher the wind speeds, 

 Hurricanes may also be characterized by other important elements, such as 

 the radius of maximum winds (R) which is an index of the size of the storm, 

 and the speed of forward motion of the storm system (Vj.) . A discussion of 

 the formation, development and general characteristics of hurricanes is 

 given by Dunn and Miller (1964) . 



Extratropical storms that occur along the northern part of the east 

 coast of the United States accompanied by strong winds blowing from the 

 northeast quadrant are called northeasters. Nearly all destructive north- 

 easters have occurred in the period from November to April; the hurricane 

 season is from about June to November. A typical northeaster consists of 

 a single center of low pressure and the winds revolve about this center, 

 but wind patterns are less symmetrical than those associated with hurri- 

 canes . 



3.863 Factors of Storm Surge Generation . The extent to which water 

 levels will depart from normal during a storm depends on several factors. 

 The factors are related to the: 



(a) characteristics and behavior of the storm; 



(b) hydrography of the basin; 



(c) initial state of the system; and 



(d) other effects that can be considered external to the system. 



Several distinct factors that may be responsible for changing water levels 

 during the passage of a storm may be identified as: 



(a) astronomical tides 



(b) direct winds 



(c) atmospheric pressure differences 



(d) earth's rotation 



(e) rainfall 



(f) surface waves and associated wave setup 



(g) storm motion effects . 



The elevation of setup or setdown in a basin depends on storm inten- 

 sity, path or track, overwater duration, atmospheric pressure variation, 

 speed of translation, storm size, and associated rainfall. Basin charac- 

 teristics that influence water-level changes are basin size and shape, 

 and bottom configuration and roughness. The size of the storm relative 

 to the size of the basin is also important. The magnitude of storm 

 surges is shown in Figures 3-41 and 3-42. Figure 3-41 shows the differ- 

 ence between observed water levels and predicted astronomical tide levels 

 during Hurricane Carla (1961) at several Texas and Louisiana coast tide 

 stations. Figure 3-42 shows high water marks obtained from a storm survey 

 made after Hurricane Carla. Harris (1963b) gives similar data from other 

 hurricanes . 



3-84 



