2. The shoreline area in which the sand travels is altered. If the 

 shoreline is interrupted by a structure or altered by excavation 

 or dredging, it must be expected that the flow of sand will be 

 interrupted. The consequence is usually an accumulation of sand 

 on the updrift side of the structure or within the excavation 

 and at least temporary starvation of the beach on the downdrift 

 side due to elimination of the sand supply to it. 



3. The source of energy for the system is altered. There are 

 several ways in which this disruption can occur. Sometimes only 

 subtle changes in offshore bathymetry can produce marked 

 variations in the energy of the waves that impinge along a 

 shoreline [60]. Such changes can occur naturally or can be 



the result of excavation or deposition of dredge spoil. 



All too often protective structures fail to accomplish their intended 

 purpose, or they necessitate corrective action elsewhere on the coastline, 

 because of their disruption of the beach equilibrium. The construction 

 disturbances may also cause long-term adverse effects such as elimi- 

 nation of beach area and maintenance dredging of harbor inlets. To 

 prevent such troubles it is necessary that a thorough study and comprehen- 

 sive protection plan be developed before structures are authorized. 

 Detrimental structures already in place should be scheduled for removal 

 or replacement at the earliest opportunity. 



The solution to beachfront management is not to go exclusively with 

 either structural or nonstructural techniques but to achieve a balanced 

 plan emphasizing the nonstructural. Although it might be simplest to let 

 nature take its course, extensive areas of the coast are already occupied 

 and must somehow be maintained safely until setbacks and other protective 

 land-use plans can be implemented. Yet even these systems should be 

 allowed to remain as close to their natural dynamic states as possible. 

 Some structural interference may be necessary, however, to stabilize 

 inlets for navigation purposes. 



The principal shore protection structures include seawalls, 

 revetments, groins, jetties and breakwaters. Each serves a special 

 purpose, and each affects beaches in a specific manner. They may have 

 complex secondary effects. There are numerous example of structures 

 causing, or contributing, to destabilization of beaches. 



4.4.2 Seawalls 



A seawall is a solid barricade built to protect the ocean shore and 

 to prevent inland flooding. In principle, the seawall is designed to 

 absorb and reflect wave energy, as well as to hold beach sand or fill in 

 place and to raise the problem area above flooding elevations. 

 Unfortunately, seawalls are expensive and commonly accelerate the loss 

 of sand as the wall deflects the wave forces downward into the beach 



106 



