offshore, the chance of tombolo formation increases, becoming almost certain 

 in usual circumstances when £ < 2X . There is the possibility of a double 

 tombolo formation with trapped water between them when the structure length 

 is further increased. 



Offshore breakwaters designed for an open coast are generally sited in 

 water depths between 1 to 8 meters (3 to 25 feet). If the project length is 

 so great that economic considerations preclude moving the structure far enough 

 offshore to satisfy the i < X criterion, alternate methods for increasing 

 the energy flux into the protected region must be employed. 



-..,-. ,. New Shoreline 



Original Shoreline 



oreiine / 



Approximate Location of 

 Cuspate Spit Apex 



i 



Normally Incident Waves 

 Figure 5-31. Location of cuspate spit apex. 



b. Wave Overtopping . The offshore breakwater can be designed so that a 

 part of the incident wave energy can be transmitted by overtopping which helps 

 to prevent the connection of the cuspate spit to the structure. An advantage 

 to using this method is that the shoreline of the cuspate spit tends to flat- 

 ten and spread laterally along the shore in a more uniform manner. However, 

 the transmitted waves have a shorter wave period than the incident wave and 

 are highly irregular. Tide level, wave height and period, structure slope and 

 roughness all have nonlinear effects on the amount and form of energy trans- 

 mission by overtopping. This makes the design procedure difficult unless 

 these parameters are nearly constant. Chapter 7, Section 11,3 discusses 

 procedures for altering the structure cross section so that sufficient energy 

 is transmitted by overtopping. If an existing structure is not performing as 

 required, it is conceivable that the crest elevation could be raised or 

 lowered, but this is often costly and impractical. 



c. Breakwater Permeability . Another means of preventing a tombolo forma- 

 tion is to make the structure permeable, so a part of the incident energy is 

 passed through the breakwater. This energy is transmitted at the period of 

 the incident wave period and is generally more predictable and regular than 

 overtopping transmission. With transmission through the permeable structure, 

 the advancement of the shoreline is generally more uniform than with segmented 

 structures. However, the transmission is highly dependent on wave period. 

 If an existing structure is not performing as intended, it is impractical to 

 increase the permeability as a solution to the problem. Figure 5-32 shows 



5-69 



