for the PMH. Moreover, k is considered a function of the latitude for' 

 the PMH while k for the SPH was taken as a constant. 



4. Design Storms other than SPH and PMH 



It may be advantageous in some cases to select a design storm other 

 than the SPH or PMH based on the risks or economics of a particular 

 coastal structure. In such circumstances, the information presented in 

 HUR 7-97 can be used to obtain a different probability of occurrence of 

 the CPI, and obtain hurricanes of various intensities. HUR 7-97 is 

 recommended instead of Graham and Nunn's earlier investigation (1959) 

 because of the updating of the basic data. It should be noted that the 

 CPI for a PMH was not assigned to any particular probability of occur- 

 rence, but was taken as only an extreme event because of the relatively 

 small amount of data presently available. Generally, if a design storm 

 is selected other than SPH and PMH, it would be a storm less severe than 

 the SPH. For instance, a coastal structure such as a stone jetty which 

 offers protection only for normal conditions, could for some cases be 

 designed for a storm less severe than the SPH. This is because it may 

 be more economical to provide occasional maintenance than to build a much 

 more costly and stable structure initially. A design storm for such a 

 condition would be selected according to the CPI frequency considered 

 satisfactory for the particular project. The radius of maximum winds 

 R and the forward speed Vp would be taken as those values considered 

 appropriate for that particular geographical location. 



Section V. ESTIMATING THE PRESSURE EFFECT, INITIAL WATER LEVEL 

 AND ASTRONOMICAL TIDE 



1. Pressure Setup 



The effect of atmospheric-pressure setup, S^^, which enters in the 

 total depth calculation in space and time is evaluated separate from the 

 relations previously described. Although several methods have evolved 

 for estimating the pressure effect, none appear to be completely satis- 

 factory for all storm-surge problems. The method presented here appears 

 to be satisfactory for some cases, but for other cases the approximation 

 may not be very reliable. Prior to giving the pressure-setup relation, 

 a discussion will be made of the total problem to clarify the limitations 

 of the relation presented. 



The atmospheric pressure, p, acting on the water surface during a 

 hurricane is a function of position in the storm field and time, or 

 symbolically p = f(x,y,t). In general, the atmospheric pressure is 

 minimum at the hurricane eye, where p = Pq, and increases radially out- 

 ward to the edge of the storm, or storm periphery where p = Pj^. Because 

 of the pressure variation in the open sea, water will rise in regions of 

 low pressure and fall in regions of high pressure. The higher pressure 

 region depresses the sea level by transporting the water to the lower 

 pressure region where it causes the sea level to rise. 



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