This relation provides a method of estimating the pressure setup on the 

 traverse line. However, Equation (33) does not reflect the effect of 

 the speed of the storm nor does it reflect the restrictions to flow. 

 Thus, it is assumed that the sea is in a state of equilibrium when con- 

 sidering only the effects of atmospheric-pressure variation. For rapid 

 storms, the estimate may be much too high, and when resonance occurs, the 

 estimate may be too low. The best estimate possible would appear to occur 

 when the storm speed is slow compared to the shallow-water wave speed. 



2. Initial Water Level and Astronomical Tide 



Because water level is often above the normal level prior to the 

 arrival of a hurricane, this condition must be taken into account when 

 making open-coast surge estimates for hypothetical hurricanes. It has 

 been observed for past hurricanes in the Gulf of Mexico that in the 

 absence of predicted astronomical tides, the water level has been as 

 much as 2 feet and more above the normal level at shore just before the 

 approach of the hurricane. Initial water levels observed for hurricanes 

 experienced along the East Coast of the United States are substantially 

 less than those in the Gulf and generally decrease with higher latitude. 

 At present there are no precise methods for prescribing the initial water 

 level that should be used for a particular location in conjunction with 

 a hypothetical hurricane. It has been the practice to base this initial 

 level on those previously observed in the region. This appears to be the 

 most satisfactory approach at this time. On this basis, when an open- 

 coast surge estimate is to be made for some specific location, all per- 

 tinent storm and water-level data should be analyzed to arrive at a 

 reasonable value. The final value selected should also reflect the 

 probability of occurrence of the hurricane event being studied. Thus, 

 for an extremely rare event such as the PMH, a more conservative (i.e., 

 a higher initial water level) value should be adopted provided that the 

 value selected is reasonable for the area. 



Normally, in application to hypothetical hurricanes, the astronomical 

 tidal rise is taken in phase with maximum level of the storm surge. For 

 an SPH, the spring high tide is usually taken; for the PMH, that tide 

 exceeded by only 10 percent of the spring tides is taken. The specific 

 tide amplitude chosen depends on the particular geographical location 

 being studied. 



Including the effects of the initial water level and astronomical 

 tide in the numerical integration procedure gives a more accurate esti- 

 mate of the water-level rise at the shore. This is due to more correctly 

 specifying the total water depth for all steps in time and space, and the 

 errors introduced by directly adding these effects to the wind surge are 

 generally very small. Thus, in most cases it is usually satisfactory 

 to include these effects after making the storm wind surge calculations. 

 Another method frequently employed is to include the assumed astronomical 

 tide level and the initial water level as constants in numerical inte- 

 gration procedure and correcting the final water level hydrograph for 

 the time-dependent tide. Taking the tide as a constant for the entire 



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