ORIGINAL SHORELINE 



>(< I l»_ ■- " " 



UNDISTURBED WAVE 

 REACHES SHORELINE 



DIFFRACTED WAVES 

 INTERSECT HERE 



INCIDENT WAVES 

 NORMAL TO SHORE 



Figure 17. Condition ensuring tombolo development 

 assuming linear wave theory 



the distance offshore. A sufficient supply of sediment is necessary in order 

 to form the tombolo. Refraction, shoaling, breaking, and the previously dis- 

 cussed higher order effects on celerity are not included, but this relation- 

 ship is probably conservative. When comparing the ratio of length to distance 

 offshore versus shoreline response, it appears that tombolos can occur when 

 the ratio approaches 1.0 (Table 1). Bishop (1982) also reached this conclu- 

 sion. The recommended criterion to assure tombolo development (assuming suf- 

 ficient sand supply) may be a compromise. For example, 



a > i.5x 



l > 1.5x 

 s — 



for a single detached breakwater 



for a segmented detached breakwater 



42. Lengthening the structure or reducing its distance offshore beyond 

 the above condition will increase the size of the tombolo and may eventually 

 induce the formation of double tombolos, with trapped water in between. This 

 trend is suggested by the double beach ridge tendency at Colonial Beach 



38 



