(Figure 13). A large seasonal change in wave direction increases the proba- 

 bility of this occurring. Shore-connected double tombolos are generally an 

 undesirable condition due to the large pool of stagnant water and associated 

 beach use problems. 



43. If the shoreline to be protected is very long, a segmented detached 

 breakwater utilized to generate a series of small tombolos may be appropriate. 

 A single long breakwater would need to be placed farther offshore in deep 

 water to reduce the quantity of trapped sediment and prevent double tombolo 

 formation. Such a plan is usually uneconomical. Generally, the condition 

 necessary for developing a segmented breakwater project is one where the shore 

 to be protected is approximately five or more times longer than the chosen 

 distance offshore for breakwater construction (Table 1) . Each segment should 

 be between one and two times as long as the distance offshore if tombolos are 

 desired, and the gaps should be sized according to the desired shoreline posi- 

 tion opposite each gap. Unless the gap-to-incident wavelength ratio is very 

 small, there will be little reduction in wave height at the shoreline directly 

 opposite each gap. Without a substantial sediment supply, the shoreline will 

 probably not accrete and may even erode in these areas, including into the 

 placed fill (Figure 18) . If this formation is not acceptable and a more uni- 

 form shoreline advance is desired, a perched beach plan might be appropriate. 



44. To ensure a tombolo, the solid portion of the breakwater should be 

 constructed to prevent wave transmission over or through it, thus minimizing 

 wave activity in its lee. The structure should therefore have low permeabil- 

 ity, with a crest height and slope sufficient to minimize overtopping by storm 

 waves . 



Prevention of tombolos 



45. For many situations it is desirable to design a breakwater project 

 to prevent tombolo development and encourage only salients. With the desired 

 shoreline advance and sinuosity determined, the breakwater should be located 

 such that the design shoreline would be less than halfway between the original 

 shoreline and the breakwater. Tombolos are prevented by allowing sufficient 

 wave energy to enter the protected region through one or more of the following 

 techniques . 



46. Structure length versus distance offshore. From Table 1 and an 

 overview of international breakwater projects, it appears that a tombolo can 



40 



