Section III. FREQUENCY OF HYPOTHETICAL HURRICANES 



1. General 



It may be seen from the final generalized surge frequency relation- 

 ship that the "Beta Hurricane" of large radius and mean forward speed, 

 has a return frequency of ahout once in 100 years. This, however, gives 

 no insight of frequencies of other "Beta Hurricanes" or in general all 

 hypothetical hurricanes (all parameters variable) with prescribed parame- 

 ters. Clearly it would be advantageous to assign probability frequencies 

 to all hypothetical hurricanes possessing parameters reasonably character- 

 istic of the area involved. It is not implied that all parameter combi- 

 nations are important in connection with design of hurricane protection 

 systems, but it would allow predicting the probable frequencies of the 

 ones that are of interest. In the following paragraphs a method is 

 presented for assigning frequencies to hypothetical hurricanes. 



2. Probability Frequencies of Hurricane Parameters 



Since the prescribed hurricane parameters (i.e., R, Vp, and CPl) are 

 the only basis for predicting the hypothetical hurricane frequencies, then 

 the actual probabilities of each parameter must be established. Based on 

 the historical records given in Table 1, return frequencies of Vp, R, and 

 CPI were constructed as shown on Figures 15, l6, 17, respectively. It 

 appears that a forward speed of 11 knots is more frequent than those less 

 than or exceeding this value, and accordingly the curve on Figure 15 was 

 derived. A plot of R versus CPI, as mentioned previously, indicates there 

 is a partial tendency for the radius of maximum winds to decrease with 

 increasing intensity of hurricanes, and the return frequency of R was 

 derived on this basis. 



3. Probable Storm Frequencies 



It would be more appropriate in deriving hypothetical hurricane 

 frequencies to study the influence of the parameters by the statistical 

 method of joint distribution if sufficient events were available for 

 such an analysis. However, the actual recorded events are limited in 

 number; therefore, an alternate solution must be obtained. If it is 

 assumed, for the present, that all variables are statistically independ- 

 ent, one can resort to an elementary mathematical principle of simultan- 

 eous events. This principle essentially states that if one event occurs 

 h ways, and another j ways, then the two events occurring simultaneously 

 will occur h X j ways. To account for the partial dependence of R to CPI 

 and other unknown factors, and based on the above principle, it will be 

 ass\imed that the hurricane probability frequency P, expressed in events 

 per 100 years, can be written in the form: 



P = K (f^) (fr) (fr) (7) 100 (8) 



^ R CPI Vf ^ 



where N is the number of events, Y is the years of record, and K is the 



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