PART II: GENERAL OPERATION OF DETACHED BREAKWATERS 



6. As previously mentioned, detached breakwaters protect a zone of the 

 beach from direct wave action and transform the incoming waves. The area 

 immediately behind the breakwater is sheltered because wave energy is dissi- 

 pated on or reflected off of the structure. Wave energy is also reduced as 

 waves diffract around the breakwater ends, resulting in a lateral spread of 

 wave energy and a reduction in energy reaching the shore at any given point. 

 The net effect is to reduce the capacity of waves to entrain and transport 

 sediment in the breakwater shadow and to drive sand into the sheltered area 

 immediately behind the breakwater where it is deposited, thus causing a bulge 

 or salient to develop along the shore (Figure 2) . Littoral drift entering the 

 breakwater zone of influence may be permanently trapped or removed at a later 

 date when transport conditions change. If wave conditions are relatively con- 

 stant, a state of dynamic equilibrium may be attained. 



AVERAGE LONGSHORE COMPONENT 

 OF WAVE ENERGY (PLUS DIRECTION) 



PREDOMINANT 

 WAVE DIRECTION 

 AND MAGNITUDE 



\ 



AVERAGE LONGSHORE COMPONENT 

 OF WAVE ENERGY (MINUS DIRECTION) 



/ 



WAVE DIFFRACTION 

 PATTERNS 



UPDRIFT AND 



DOWNDRIFT 



SHORELINE 



IMPACTS 



BREAKWATER HEIGHT 

 AND POROSITY' 



3 



ACCRETED 

 SHORELINE 



LITTORAL 



TRANSPORT RATE 



Figure 2. Segmented detached breakwater design considerations 



7. The functioning of a detached breakwater is best understood by 

 comparison with a more traditional sand-accreting shoreline structure, the 

 groin. Both groins and detached breakwaters are methods of beach erosion 

 control which can involve the use of a single structure or a group of 

 structures designed as a system. Groins are generally built perpendicular to 

 shore, while detached breakwaters are generally built parallel to the shore. 

 Groins do not appreciably reduce the wave energy striking the shore and they 



