To move water from one region to the other requires time, thus the 

 process is time-dependent. If the pressure change is rapid, there may be 

 insufficient time for very much transport to take place resulting in only 

 a small change of the water level. If there is sufficient time, an equi- 

 librium state can be reached where the pressure forces are in balance with 

 the gravitational forces. In the deeper regions of the sea, equilibrium 

 state occurs more rapidly than in the shallow regions near the coast. In 

 the nearshore regions, transport is impeded due to bed friction; more time 

 is required to transfer the water. For actual hurricanes, the pressure 

 disturbance moves with the speed of the storm, and can vary with the time 

 elapsed. Hurricanes which move at very high speeds may not allow suf- 

 ficient time for the water surface to reflect maximum and minimum levels 

 that are associated with the actual pressure difference. 



Consequently, the speed of the hurricane may play a role in the 

 actual amount of water being set up. Harris (1963) indicates that the 

 difference in water level between any two points in the. storm field is 

 proportional to the difference in atmospheric pressure provided that the 

 storm speed (Vp) is small compared to the shallow- water wave speed, /gD. 

 When the storm speed is comparable to the shallow- water wave speed, the 

 water level will be amplified by resonance (Harris ,1957) . 



The difficulty in establishing a relation which gives the actual 

 amount of water set up during a hurricane is because the value has never 

 jeen observed for past hurricanes. The total rise at shore, even with 

 the best measurement devices, is only an estimate. Separating pressure 

 setup from other components of the total rise is impossible. Thus, 

 methods which give only a reasonable approximation of this effect must 

 be used. One such approximation that has been used frequently in prac- 

 tice was developed by Myers (1954) . He based his analysis on the mean 

 radial pressure distribution of 69 historical hurricanes. The pressure 

 p was defined as 



-R/r 

 P = Po + (Pn - Po) ^ (31) 



based on the mean values of the experienced hurricanes. Here the pressure 

 p is the pressure at a radial distance, r, from the storm center, p^ is 

 the central pressure, R is the radius of maximum winds, and Pjj is the 

 peripheral pressure (theoretically at r = °°) . Subtracting p^^ from both 

 sides of the equation the relation can be written as 



Pn - Po = CPj, - Po) (1 - e'^^"") (32) 



When pressure is in inches of mercury, the setup of water in feet due to 

 the pressure difference based on the above equation is given by 



S^p = 1.14 (pj^ - Pq) (1-e-R/^) (33) 



25 



