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Perched Beaches

 

On steep, eroding beaches, the use of a perched beach may be useful to reduce sand losses or to reduce the sand volume required for nourishment. A perched beach is at the seaward side supported by an underwater sill or breakwater. Landward from this sill, where a nourishment may be applied, a dynamic equilibrium profile will develop. Perched beaches can be applied to reduce seaward sediment loss, thus reducing the sand volume needed for regular coastal maintenance by nourishments.  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.

 

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How to Use

 

The aim of a perched beach 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. 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. Below, you can find an overview of the basic theory on equilibrium profiles and perched beaches and some guidance for perched beach design.

 

 

Sketch of perched beach concept with: water depth over the breakwater, d, water depth at seaward and

shoreward side of the toe structure, he and hi, respectively, advanced beach width, Δy, and beach berm height, B0

 

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Practical applications

 

Here, we present a couple of case studies from the USA, Italy and Australia where perched beaches have been applied for sustainable coastal management. 

 

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 References

 

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