c. 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 



(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 intensity, 

 path or track, overwater duration, atmospheric pressure variation, speed of 

 translation, storm size, and associated rainfall. Basin characteristics that 

 influence water level changes are basin size and shape and bottom con- 

 figuration 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- 

 54 and 3-55. Figure 3-54 shows the difference between observed water levels 

 and predicted astronomical tide levels during Hurricane Carla (1961) at 

 several Texas and Louisiana coastal tide stations. Figure 3-55 shows high 

 water marks obtained from a storm survey made after Hurricane Carla. Harris 

 (1963) gives similar data from other hurricanes. 



d. Initial Water Level . Water surfaces on the open coast or in enclosed 

 or semienclosed basins are not always at their normal level prior to the 

 arrival of a storm. This departure of the water surface from its normal 

 position in the absence of astronomical tides, referred to as an ini.t'Lal water 

 level, is a contributing factor to the water level reached during the passage 

 of a storm system. This level may be 0.6 meter (2 feet) above normal for some 

 locations along the U.S. gulf coast. Some writers refer to this difference in 

 water level as a forerunner in advance of the storm due to initial circulation 

 and water transport by waves, particularly when the water level is above 

 normal. Harris (1963), on the other hand, indicates that this general rise 

 may be due to short-period anomalies in the mean sea level not related to 

 hurricanes. Whatever the cause, the initial water level should be considered 

 when evaluating the components of open-coast storm surge. The existence of an 

 initial water level preceding the approach of Hurricane Carla is shown in 

 Figure 3-54 and in a study of the synoptic weather charts for this storm 

 (Harris, 1963). At 0700 hours (eastern standard time), 9 September 1961, the 

 winds at Galveston, Texas, were about 16 kilometers per hour (10 miles per 

 hour) but the open coast tide station (Pleasure Pier) shows the difference 

 between the observed water level and astronomical tide to be above 0.6 meter 

 (2 feet). 



3-111 



