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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.
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.
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.
Sketch of a cross-section and top view of a perched beach.
The added value of this building solution to Building with Nature (BwN) type projects is that it enables creating natural coastal barriers while enhancing other ecosystem services such as recreation. This building solution is mainly applicable in the initiation and planning and design phase.
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.
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
The coastal profile between the breakwater and the coastline is dynamic, but averaged over its variability, it is likely to tend towards an equilibrium shape. This equilibrium profile can be described on the basis of Dean’s theory (1977, 1991), which was later adapted for equilibrium profiles on perched beaches by Gonzalez et al. (1999). The equilibrium beach profile is the underwater profile that is in equilibrium with the ambient wave and current conditions and the characteristics of the sediment.
The equation of the equilibrium beach profile is a power law in the form (e.g. Dean, 1977):
h = A * xm
h = water depth
A = parameter depending on the sediment characteristics = 0.067w0.44
w = fall velocity
x = distance from the water line
m = slope parameter
Characteristic features of equilibrium beach profiles are:
- Tendency to be concave upwards (m<1)
- Smaller grain sizes result in milder slopes and coarser grains in steeper slopes
- Beach face is approximately planar
- Steep waves result in milder slopes and a tendency to bar formation
The balance between the (wave-induced) forces and the sediment characteristics (important for bottom friction) determines the shape of the beach profile. The slope parameter m can be determined according to the assumption concerning the wave energy dissipation (Dean, 1977):
- Uniform average longshore shear stress: m=0.4
- Uniform average wave energy dissipation by turbulence per unit plan area: m=0.4
- Uniform average wave energy dissipation by turbulence per unit water volume: m=0.67.
This is based on the assumption of spilling breakers: H = 0.8 * h.
Field data were used to verify the analytical solution and to tune the parameters A and m. For this purpose, Dean (1977) compared 502 beaches at the east coast of the United States. This revealed that at this coast the third mechanism is dominant, resulting in a value of 0.67 for the parameter m. He also found that the parameter A depends on the fall velocity of the sediment and derived an empirical relation for the parameter A (see above).
Sill or submerged breakwater
The design of the submerged structure determines the amount of wave energy that passes the structure and arrives at the beach. Gonzalez et al. (1999) argued that submerged breakwaters or sills generally are too narrow to have significant wave energy dissipation by breaking. Also friction-induced attenuation over the structure is negligible, so wave reflection is then the main process determining the amount of wave energy arriving at the beach.
The water depth directly at the landward side of the sill (hi) is a function of the ratio between incoming and reflected wave energy:
he = the water depth at the seaward side of the structure,
R = the reflection coefficient, defined as Hr/He, the reflected wave height divided by the incoming wave height.
The value for R can be derived iteratively and is dependent on the breakwater dimensions (B,d), location of the breakwater (he) and the wave length (L). Subsequently, the water depth at the landward side of the sill can be used to position the equilibrium profile and determine the shoreline advance or retreat, given the grain size of the sediment used for the nourishment. Further derivation can be found in Gonzalez et al. (1999).
Reflection coefficient as a function of the breakwater width B and wave length L, and the water
depth over the breakwater d and the water depth at the toe of the breakwater he.
Fringing reefs, which are coral reefs along and close to the coast, can protect the coast by dissipating wave energy, rather than reflecting it. Munoz-Perez et al (1999) found that for a natural reef-protected beach to exist, the reef width must exceed three wave lengths. Then wave breaking over the reef occurs, which limits the wave energy reaching the beach. Also friction damping over a wide coral reef can reduce the hydrodynamic energy arriving at the beach. Note that the mildly sloping natural/artificial fringing reefs work in a different way (via wave dissipation) than relatively steep and reflective submerged breakwaters.
Dimensions and position of the submerged breakwater are important design parameters, because they influence the reflection coefficient and the position of the coastal profile. The grain size of the fill material can be varied to change the profile shape behind the structure. Hence, key design variables are:
- Height and width of the sill
- Water depth / offshore distance at which the sill is placed
- Characteristics of the fill material
- Material used to construct the sill
Accretion in the lee of a submerged breakwater as a result of oblique incident waves and significant longshore sediment tranport.
Lee side erosion/accretion in case of breakwaters of finite length
Ranasinghe and Turner (2006) give an elaboration on the different successes of finite-length submerged breakwater applications. Field observations, model simulations and laboratory test are compared. They conclude that laboratory tests and numerical simulation results imply that shoreline accretion will occur in the lee of the submerged structures located on coastlines with significant ambient longshore sediment transport. In the lee of submerged structures located on coastlines with predominantly shore-normally incident waves, set-up over the structure will cause a water level elevation which induces circulation currents around the heads of the breakwater. The flow pattern closer to the beach as drawn by Ranasinghe and Turner, however, is dubious. As the structure will reflect part of the wave energy, set-up behind the breakwater will be less than that in the fully exposed adjacent zones. Hence there will be a wave-driven current into the sheltered area. Field observations seem to support this, although conclusions can never be definitive, because exclusively normal wave incidence never occurs.
Erosion in the lee of a submerged breakwater due to shore-normally incident waves.
In case the alongshore continuity of the perched beach is a problem, for example because it cannot be constructed between two natural fixed points like rocky outcrops for example, a permeable or submerged groin can be placed at either side of the submerged breakwater to mitigate lee side erosion. This is done at Pellestrina beach, Italy (Lamberti et al. 2005, Raudviki and Dette 2002, Muraca 1982).
The main objective of the construction of an underwater sill is to reduce erosion of the beach and thus reduce the volume of fill material needed for the beach nourishment. In addition, a perched beach can stimulate ecosystem development, depending on the design choices made. Fixation of all elements in the physical system by ecosystem engineers has to be prevented, because exchange of sediments between the submerged and dry parts of the beach and dunes must be possible to maintain the beach. When biodiversity is enhanced, the following ecosystem services might be beneficial:
- Stabilizing sediment in the lee of the structure by sea grass meadows;
- Damping of hydrodynamic energy by sea grass meadows, coral reefs, mussel/oyster reefs and mangroves (mangroves only when the structure is emerged);
Improve water quality by sea grass meadows and mussel/oyster reefs;
Add recreational value to the area: coral reefs are attractive for snorkeling, diving etc.
Mangrove trees on a breakwater at Pulau Hantu
Note that all of the ecosystems have specific habitat requirements. This can also have consequences for the perched beach design. For example, when the growth of coral reefs on the submerged structure is desired, the growth of corals is limited to certain water depths in combination with the turbidity (penetration of light). In addition, coral establishment requires hard substrate. This implies that the submerged structure has to be constructed of material suitable for corals, for example riprap. More information on habitat requirements can be found at the building solutions for coral reefs, sea grass, mangroves and shellfish.
Here, we present a couple of case studies from the USA, Italy and Australia where perched beaches have been applied for sustainable coastal management.
Constructed oyster reef
At Palm Beach a submerged breakwater with a length of 1.2 km, a width of 4.6 m and a height of 1.8 m was placed in 3 m water depth, some 70 m offshore. The submerged breakwater resulted in a reduction of wave heights with 10-15%. Longshore currents increased probably due to wave-pumping, i.e. ponding of water trapped behind the submerged breakwater. This led to an increased erosion behind the structure up to 2.3 times the erosion without breakwater. After 3 years, the structure was removed and replaced by a nourishment (Dean et al, 1997).
Schematisation of the flow pattern at Palm Beach, Florida, USA.
South of Rome at the Italian coast, an eroding beach was nourished in combination with the construction of a submerged breakwater. Erosion occurred due to decreased sediment supply from a river nearby. The tidal range is limited in the Mediterranean Sea (less than 0.5 m) and waves can exceed a significant wave height of 5 m with a period of 10 seconds, arriving at the coastline under an angle of 15 degrees to the shore normal.
The perched beach developments were hindcasted with a model based on the Unibest software package, which predicted the coastal evolution quite well, except near the water line, where the erosion was underestimated. The sill is located at 150 m from the shoreline at a depth of 4-5 m. The crest of the sill has a width of 15 m and is elevated till 1.5 m - MSL. The total length of the sill is 3 km. No cross-shore groins were constructed. After 3 years, the behaviour of the perched beach was satisfactory. Minor sand losses had occurred behind and downdrift of the breakwater. The rubblemound breakwater was fully covered with mussels, which is a bonus for marine fauna and leisure fishing (Lamberti et al. 2005, Ferrante et al. 1992). According to Gonzalez et al (1999), the difference in success for the Florida and Lido di Ostia projects can be explained by the coarser material used for the nourishment in Italy.
Perched beach at Lido di Ostia, Italy.
In 1995, a system of partially submerged groins and a long submerged breakwater was constructed at Pellestrina beach. Pellestrina beach is located at the seaward side of one of the barrier islands of the Venice Lagoon in Italy. The structures form a system of 18 partially closed cells. The spring tidal range amounts 1m and the 1/10 year wave height is 3.7m. In addition, seasonal winds (Bora) can result in significant wind set-up. The groins are about 150-210 m long with the crest at about +2.2 to 2.7m above the MWL and for their first 100 m they are on the beach. These groins are joined to the submerged breakwater through a submerged part with the crest at about 1.5m below the MWL. The system is is able to solve erosion problems (Lamberti et al. 2005).
Pellestrina beach, Venice Lagoon, Italy.
At the Gold coast in Australia an artificial reef is constructed with large filled sandbags (up to 350 ton). The aim of the artificial reef is to provide beach protection and to enhance surf conditions. For this reason, the structure is V-shaped. The design is created to have two twin reefs with a stream for peddling in between. The structure extends from 100 m to 600 m offshore and over 350 m alongshore. The bed slope is approximately 1:50. The crest was initially located 1 m below MLW. A large nourishment was applied at the beach during the construction phase. Initially substantial erosion occurred (up to 2 m), which resulted in significant lowering of the structure’s crest. The crest was heightened to the initial level afterwards. At the beach a salient formed in the lee of the structure, resulting in an accretion of approximately 10-20 m in two years (Ranasinghe and Turner, 2006). For more information, visit the knowledge page on artificial reefs.
Artificial reef at the Gold Coast, Australia.
Shellfish such as oysters and mussels are reef-forming ecosystem engineers. Their reefs can protect shorelines and stabilize eroding (intertidal) areas. Apart from locally fixing sediment, they are able to influence tidal flow and wave action at larger scales, causing changes in depositional patterns. Oyster reefs are applied in many places around the world as a coastal protection method (e.g. living shorelines), reducing hydraulic forces and enhancing sediment entrapment. In this project we constructed artificial reefs in the Oosterschelde (SW Netherlands) to reduce the erosion of tidal flats. To be successful, the artificially placed substrate needs to develop into a living, persistent oyster reef and at the same time protect the tidal flat against erosion. Read more about the project here.
Artificial shellfish reef in the Eastern Scheldt
Overview of perched beach supporting structures
Ranasinghe and Turner (2006) made an overview of submerged breakwaters around the world with their main dimensions and characteristics and their success, see the table below. Also in Lamberti et al. (2005) an overview of European projects with low crested structures is given, including main dimensions and hydrodynamic characteristics.
Overview of submerged breakwaters around the world with their main dimensions and characteristics and their success.
- Dean, R.G. (1977). Equilibrium Beach Profiles: U.S. Atlantic and Gulf Coasts, Ocean Engineering Technical Report no. 12 .
- Dean, R.G. (1991). Equilibrium Beach Profiles: Characteristics and Applications, Journal of Coastal Research 7(1): 53-84 .
- Dean, R.G., Chen, R., Browder, A.E. (1997). Full scale monitoring study of a submerged breakwater, Palm Beach, Florida, USA, Coastal Engineering, 29: 291-315
- Di Risio, M., Lisi, I., Beltrami, G.M., DE Girolamo, P. (2010). Physical modeling of the cross-shore short-term evolution of protected and unprotected beach nourishments, Ocean Engineering, 37: 777-789
- Ferrante, A., Franco, L., Boer, S. (1992). Modelling and monitoring of a perched beach at Lido di Ostia (Rome), Proc. 23th Int. Conf. on Coastal Eng. ASCE, 3305-3318
- Gonzalez, M., Medina, R., Losada, M.A. (1999). Equilibrium profile model for perched beaches, Coastal Engineering, 36: 343-357
- Groenewoud, M.D., van de Graaff, J., Claessen, E.W.M., van der Biezen, S.C. (1996). Effect of submerged breakwater on profile development, Proc. 25th Int. Conf. on Coastal Eng. ASCE, 2428-2441
- Lamberti, A., Mancinelli, A. (1996). Italian experience on submerged barriers as beach defence structures, Proc. 25th Int. Conf. on Coastal Eng. ASCE, 2352-2365
- Lamberti, A., Archetti, R., Kramer, M., Paphitis, D., Mosso, C. and Di Risio, M. (2005). Eurpean experience of low crested structures for coastal management. Coastal Engineering 52: 841-866
- Munoz-Perez, J.J., Tejedor, L., Medina, R. (1999). Equilibrium Beach Profile Model for Reef-Protected Beach, Journal of Coastal Research 15(4): 950-957
- Muraca, A. (1982). Shore Protection at Venice: A case study, Proc. 18th Int. Conf. on Coastal Eng. ASCE, 1078-1093
- Ranasinghe. R., Turner, I.L. (2006). Shoreline response to submerged structures: A review, Coastal Engineering, 53: 65-79
- Raudkivi, A.J., Dette, H.H. (2002). Reduction of sand demand for shore protection, Coastal Engineering, 45: 239-259
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