Section IV. DESIGN STORMS 



1. General 



From the coastal engineering viewpoint, it is necessary to define 

 the design hurricane prior to discussing the actual application of the 

 numerical scheme previously described. Design analysis of coastal 

 projects by engineers must provide the degree of protection necessary 

 for the purpose of the project. A high degree of protection is required 

 when a coastal structure provides protection of lives. A high degree 

 of protection is also required when extensive property damage can be 

 caused by a hurricane. When the risks are small, it may be better to 

 accept a lower degree of protection to obtain lower costs for the project. 

 Thus, the selection of a design hurricane for a coastal protection proj- 

 ect is dependent on the economics of providing the required degree of 

 protection. 



To give the most probable degree of protection required for a 

 particular study area, it has been the practice to select a hurricane 

 with a given set of characteristics for the particular geographical 

 location. Since the characteristics are specified, the storm is called 

 a Hypothetical Hurricane or Hypo -Hurricane, USA-CERC (1966). Furthermore, 

 for such a storm the characteristics are taken as invariant, and the 

 storm is presumed to follow a prescribed path. Two design storms, which 

 are dependent upon geographical location, have been established for 

 practical application in the analysis of the design of coastal structures. 

 These are referred to as the Standard Project Hurricane (SPH) and Proba- 

 ble Maximum Hurricane (PMH) . The establishment of these design storms 

 was carried out jointly by the U. S. Weather Bureau and the U. S. Army 

 Corps of Engineers. 



2. Standard Project Hurricane 



Graham and Nunn (1959) developed the criteria for defining the SPH 

 through a detailed analysis of past documented hurricanes. The objective 

 for developing such a design storm was to provide a standard or criterion 

 which would serve as a basis in the design of coastal structures subject 

 to hurricane surges. In this analysis, statistical quantities were 

 compiled for the experienced hurricane characteristics for various de- 

 lineated zones along the Atlantic and Gulf Coasts of the United States. 

 The characteristics considered were: central pressure index (CPI) , radius 

 of maximum winds (R) , forward speed (Vp) , maximum gradient wind speed 

 (Vgx) , and maximum wind speed (Vx) . The SPH is based on a CPI having 

 an occurrence probability of once in 100 years. 



Furthermore, the radius of maximum winds and forward speed are to 

 be taken in such a manner that the highest surge is produced at the 

 coast being investigated. The CPI, as well as R and Vp, are representa- 

 tive of positions for the various zones. More specifically, Graham and 

 Nunn defined the SPH as "A hypo hurricane that is intended to represent 



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