3.864 Initial Water Level . Water surfaces on the open coast or in en- 

 closed 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 

 initial water level, is a contributing factor to the water level reached 

 during the passage of a storm system. This level may be 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 (1963b) on the other hand, indi- 

 cates 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 preceeding the 

 approach of Hurricane Carla is shown in Figure 3-41 and in a study of the 

 synoptic weather charts for this storm. (Harris, 1963b.) At 0700 hours 

 (Eastern Standard Time) 9 September 1961, the winds at Galveston, Texas 

 were about 10 mph, but the open coast tide station (Pleasure Pier) shows 

 the difference between the observed water level and astronomical tide to 

 be above 2 feet. Rises of this nature on the open coast can also affect 

 levels in bays and estuaries. 



There are other causes for departures of the water levels from normal 

 in semienclosed and enclosed basins, such as the effects of evaporation 

 and rainfall. Generally, rainfall plays a more dominant role since these 

 basins are affected by direct rainfall and can be greatly affected by 

 rainfall runoff from rivers. The initial rise caused by rainfall is due 

 to rains preceding the storm; rains during the passage of a storm have a 

 time-dependent effect on the change in water level. 



3.865 Storm Surge Prediction . The design of coastal engineering works is 

 usually based on a life expectancy for the project and on the degree of 

 protection the project is expected to provide. This design requires that 

 the design storm have a specified frequency of occurrence for the partic- 

 ular area. An estimate of the frequency of occurrence of a particular 

 storm surge is required. One method of making this estimate is to use 

 frequency curves developed from statistical analyses of historical water 

 level data. Table 3-8, based on National Ocean Survey tide gage records, 

 indicates observed extreme storm surge water levels including wave setup 

 The water levels are those actually recorded at the various tide stations, 

 and do not necessarily reflect the extreme water levels that may have 

 occurred near the gages. Values in this table may differ from gage-station 

 values because of corrections for seasonal and secular anaomalies. The 

 frequency of occurrence for the highest and lowest water levels may be 

 estimated by noting the length of time over which observations were made. 

 The average yearly highest water level is an average of the highest water 

 level from each year during the period of observation. Extreme water 

 levels are rarely recorded by water level gages, partly because the gages 

 tend to become inoperative with extremely high waves, and partly because 

 the peak storm surge often occurs between tide gage stations. Post-storm 

 surveys showed water levels, as the result of Hurricane Camille, August 



3-85 



