5 . Initial Generalized Surge Frequency 



In the absence of more hurricanes of record, a generalized method is 

 presented for deriving, at least to the first approximation, the open- 

 coast surge frequencies. This simply means that all 19 hurricanes of 

 record will be used initially to derive a surge-frequency relationship 

 representative of the entire Texas Coast. To enable determination of the 

 surge frequency for any specified position on the coast, a ratio of surge 

 heights is employed. The ratio taken is the actual or observed siirge 

 height which occurred at some particular location to the surge height 

 computed for the same location by the Bathystrophic Storm Tide technique. 

 The surge height obtained analytically is chosen because the hurricane 

 parameters are prescribed prior to carrying out the computations. With 

 reference to Section II, paragraph k and Figure 2, the Beta Hurricane 

 having parameters of LR and MVp is selected (any other hypothetical param- 

 eters would be appropriate) for expressing the ratio. The ratio of the 

 actual surge height to the preassigned hypothetical surge height will be 

 designated hereafter as H/Hg. From the peak surge heights given in Table 

 1 and Hg values given on Figure 2, the exceedence frequency* per 100 events 

 for the surge ratio is indicated by the circles shown on Figure 3. Curve 1 

 on the figure is a smooth line passing through the points. From Equations 

 (l) and (2) or from Figure 1, frequency curves of surge heights at 50-mile 

 intervals to the left and right of the peak surge were constructed and are 

 shown as curves 2 through 8. The surge ratio probabilities, given by the 

 eight curves on Figure 3, are mutually exclusive events. They may, there- 

 fore, be added to obtain the total surge-ratio probability representative 

 of the entire coast, or stated mathematically, the probability frequency 

 per 100 years for every H/Hg is given by 



100 E (Ef^) 



(T) 



Y (EAX) 



Where E is the number of events (19 for this particular case), fj- the 

 frequency ratios read from Figure 3, Y is the number of years of record 

 used (66), and AX is the increments of distance along the coast employed. 

 The total distance along the Texas Coast is ^tOO miles, therefore EAX = 8 

 for the present problem. Figure h shows the results of these computations, 

 Since Ho is known for the entire coast, the surge height H and associated 

 frequencies can be evaluated for any position along the shoreline. 



6. Surge Frequencies at Galveston, Texas 



A question could be raised on the validity of the initial generalized 

 frequency curve (Figure h) derived, due to the relatively few events used; 

 therefore, verification of the relationship is required. Of all the sta- 

 tions along the Texas Coast, Galveston has the most complete record of 

 hurricane surge elevations, and based on these observations of record, 

 this station was selected for carrying out the verification. 



*Exceedence frequency - the percentage of values that exceed a specified 

 magnitude . 



II 



