Lengthening the structure or reducing its distance offshore beyond the 

 condition given in Equations 13 and 14 will increase the extent of the tombolo 

 and assure tombolo development. This, however, may eventually form a 

 double tombolo planform with trapped water between, which may lead to 

 undesirable water stagnation problems. To further assure tombolo 

 development, the breakwater should be constructed to prevent or minimize 

 wave transmission through the structure. Crest elevation and slope should be 

 designed to minimize wave overtopping. Likewise, to prevent tombolo 

 formation and allow only salients to develop, wave energy in the lee should be 

 increased by increasing wave transmission and overtopping of the structure. 

 Increasing gap width will also increase wave energy behind the structure. 



Pope and Dean (1986). Based on prototype data, Pope and Dean (1986) 

 defined a shoreline classification scheme that included five types of beach 

 response: permanent tombolos, periodic tombolos, well-developed salients, 

 subdued salients, and no sinuosity. A relationship was developed that gives 

 the beach response classification scheme as a function of the ratios of segment 

 length to gap length LJL and effective distance offshore to the average water 

 depth at the structure X/a s . Figure 28 shows the relationships between all 

 prototype projects relative to these two dimensionless parameters. The 

 projects plotted in Figure 28 show a grouping that may define fields of 

 predictable beach planform response for low to moderate wave climates. It 

 should be noted that these results are only preliminary and further verification 

 is required. 



Ahrens and Cox (1990). Ahrens and Cox (1990) used the beach response 

 index classification scheme of Pope and Dean (1986) to develop a predictive 

 relationship for beach response based on a ratio of the breakwater segment 

 length to breakwater segment distance from the original shoreline. The 

 relationship defining a beach response index I s is: 



; _ (l-72-0.4l£fJ 



X 



For the five types of beach response defined in Pope and Dean (1986), the 

 following values of I s were specified: 



I s = 1 (Permanent tombolo formation) 



I s = 2 (Periodic tombolos) 



I s = 3 (Well-developed salients) 



I s = 4 (Subdued salient) 



I s = 5 (No sinuosity) 



Chapter 2 Functional Design Guidance 



47 



