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.
Fig. 2. Sketch of a cross-section and
top view of a perched beach.
Perched beach designs have been tested both in laboratory experiments and in reality, with varying success. Depending on the local conditions, increased or decreased erosion may occur. This underlines the importance of a thorough knowledge of the environment in which the perched beach is going to be constructed.
With this manual, anyone having a basic level of knowledge and/or working experience in sandy coastal systems can make a first-order assessment. To fully assess the suitability for perched beaches one should have additional expertise on nourishments, local coastal processes and thorough knowledge of the environment.
The added value of this Building Block to Building with Nature (BwN) type projects is that it enables creating natural coastal barriers while enhancing other ecosystem services such as recreation.
Fig. 3. 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
Reduce the sand volume needed by nourishments
Reduce erosion by altering hydrodynamics
Can enhance biodiversity by providing habitat for seagrass, corals, shellfish and mangroves. These could then damp hydrodynamic energy. The sea grass meadows can then stabilise the sediment in the lee of the structure. Sea grass meadows together with shellfish reefs could improve water quality. Coral reefs add recreational value to the area because they are attractive for snorkeling and diving.
If the perched beach cannot be constructed along the whole beach strip, lee-side erosion occurs.