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Perched Beaches

Perched beaches can be applied to reduce the sand volume needed for regular coastal maintenance by nourishments, but also to alter the hydrodynamics and reduce erosion. A perched beach is at the seaward side supported by an underwater sill or breakwater, see Figure 2 and 3. Landward from this sill, nourishments may be applied and a dynamic equilibrium profile may develop. If the latter happens, erosion will be reduced. In addition, the sill will reduce seaward sediment losses and thus the need for regular nourishment of the coast. As the hydrodynamic energy on the beach is reduced due to the submerged structure, a steeper beach profile may form and the shoreline may shift seawards.

A problem with a perched beach can occur if it cannot be constructed along the whole beach strip and leeside erosion occurs. This phenomenon is caused by a net circulation around the sill/breakwater head, as further elaborated in the part 'How to use'. Littoral transport might be reduced by the use of (permeable) cross-shore groins. A construction with a submerged breakwater in combination with cross-shore groins is built at Pellestrina beach, see Figure 1 and the ‘Practical Application’ page.

The perched beach concept provides several opportunities to enhance biodiversity. In the lee of the structure, reduced hydrodynamic energy may yield suitable conditions for sea grass meadows. The structure itself may provide hard substrate for coral establishment, oyster reefs etc. Mangroves may cover the breakwater if it is (partly) emerged.   Read more

 

How to Use

With a perched beach design, the aim is to create and maintain a dynamic equilibrium profile landward of a submerged breakwater and thereby reduce coastal sediment losses. The design of a perched beach should be based on several considerations, like dimensions and location of the sill or breakwater, the sediment used for the nourishment, the dynamic equilibrium state and the habitat requirements for certain species. In this chapter an overview of the basic theory on equilibrium profiles and perched beaches and some guidance for perched beach design is given.

The different elements of the design (profile, sill) are interrelated, because the design process is cyclic. The first choice regards the location of the shoreline: should it shift seawards, stay in the same position or is a certain amount of retreat allowed? Depending on this choice, the design of the profile and the sill can be further considered.

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Practical applications

On the right a couple of cases is presented providing some application experiences that can be used as inspiration for future designs

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 References

 

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