Oyster reef shore protection
Contents
Oyster reef ecosystem services
Oyster reefs are an example of biogenic reefs. Biogenic reefs are structures built with ecosystem engineer species that sustain themselves with self-generative properties. If properly designed, these reefs can fulfil a shore protection function by attenuating incident waves.
Oyster beds that were common in coastal waters in the past have largely disappeared[1]. Overharvest is the mean reason for the disappearance of oyster reefs, but other factors have also contributed, such as diseases, non-native species invasions, alterations of shorelines, changes in freshwater inflows and increased loadings of sediments, nutrients and toxins. With the disappearance of oyster reefs, many ecosystem services have been lost, including water purification, enhancement of fish stocks, carbon sequestration, enrichment of biodiversity and ecosystem stability. Oyster reefs protect sedimentary coasts from erosion by attenuating waves and trapping sediment.
Oyster reef restoration
The recent restoration of oyster reefs has been largely motivated by the coastal protection function they can perform, reducing the need for hard artificial structures. Oysters naturally aggregate and attach themselves to older shells, rocks, or submerged surfaces, creating a rocklike reef structure. The development of oyster reefs can be stimulated by creating an appropriate substrate, for example limestone or concrete or loose shells within a rigid frame[2] (Fig. 1). Oyster reef growth and development also require appropriate water salinity, dissolved oxygen and well-timed larval supplementation[3]. Oyster reefs not only reduce coastal erosion by waves and currents, but also offer a habitat for many species (e.g. algae, sponges, crustacea, finfish) and serve as a highly valued food source. Although reefs can grow vertically thus keeping pace with sea level rise, oyster reefs cannot replace revetments for providing protection against flooding during storm surges as they do not grow around or above high tide[4][5]. A critical threshold for intertidal oyster reef establishment is 50% inundation duration. An evaluation of oyster reef restoration projects on the Atlantic and Gulf coasts of the United States revealed that a suitable oyster habitat requires inundation more than one-half of the time, which reduces the effectiveness of wave attenuation[6]. Many of the oyster reef living shoreline approaches therefore failed to optimize the ecological and engineering goals. Success of eco-engineered oyster reefs is dependent on numerous factors, in particular the reef design (dimensions, material) and the choice of a site providing suitable physical (substrate, exposure), environmental and biological factors (ensuring recruitment)[7].
Appendix Wave attenuation formula
Wave attenuation can be expressed by means of the wave transmission coefficient [math]K_t = H_t / H_i[/math], where [math]H_i[/math] is the spectral wave height of the incoming wave (approximately equal to the significant wave height, see Statistical description of wave parameters) and [math]H_t[/math] the spectral height of the transmitted wave. A laboratory experiment was carried out by Xiang et al. (2024[9]) with an oyster reef built with shells with average dimensions of 7.96 cm in length, 5.55 cm in width, and 1.60 cm in thickness. The porosity of the oyster shells, determined by measuring the volume of the water that filled the gaps among the shells, was found to be 0.67. The slope of the reef structure was [math]\tan\alpha = 0.2[/math]. The wave attenuation by this submerged oyster reef could be represented by the formula
[math]K_t \equiv \Large\frac{H_t}{H_i}\normalsize = c_1 \Big( 1 - 0.9 \exp(-\large\frac{h_c}{H_i}\normalsize) \Big) + c_2 \Big( \Large\frac{B}{H_i} \Big)^{c_3} \normalsize [1 - \exp(c_4 \xi)] , \qquad (1) [/math]
with coefficients [math]c_1=0.67, \; c_2=0.51, \; c_3=-0.65, \; c_4=-0.41[/math]. The meaning of the symbols in Eq. (1) is:
- [math]h_c =[/math] the distance between the reef crest level and the still water level (positive for a submerged reef, negative for an emerged reef)
- [math]B =[/math] the width of the reef crest
- [math]\xi = \Large\frac{\tan \alpha}{\sqrt{H_i / L}} = [/math] the surf similarity parameter.
- [math]L = g T_p^2 / (2 \pi) =[/math] the wavelength of the incident wave
- [math]T_p =[/math] the peak spectral wave period
- [math]g =[/math] the gravitational acceleration
The general applicability of Eq. (1) is limited, because oyster reefs in the field can take different shapes, with different surface roughness and porosity. Fig. A1 compares the prediction of Eq. (1) with the measured wave transmission over an oyster reef in the Eastern Scheldt[8]. The measured wave attenuation is larger than the prediction when the water level is just above the reef crest (small [math]h_c/H_i[/math]) and smaller for higher water levels.
As the oysters grow, the reef surface height, the surface area and the surface roughness increase, which enhances wave attenuation. Oyster reefs may therefore dynamically adjust to sea level rise.
The experiments by Xiang et al. suggested that the wave attenuation can be substantially increased by incorporating macro-roughness elements (concrete structures) in the oyster reef.
Related articles
- Wave transmission by low-crested breakwaters
- Nature-based shore protection
- Artificial reefs
- Restoration of estuarine and coastal ecosystems
References
- ↑ Beck, M.W., Brumbaugh, R.D., Airoldi, L., Carranza, A., Coen, L.D., Crawford, C., Defeo, O., Edar, G.J., Hancock, B., Kay, M.C., Lenihan, H.S., Luckenbach, M.W., Toropova, C.L., Zhang, G. and Guo, X. 2011. Oyster reefs at risk and recommendations for conservation, restoration, and management. Bioscience 61: 107–116
- ↑ Goelz, T., Vogt, B. and Hartley, T. 2020. Alternative substrates used for oyster reef restoration: a review. Journal of Shellfish Research 39: 1–12
- ↑ Theuerkauf, S.J. and Lipcius, R.N. 2016. Quantitative validation of a habitat suitability index for oyster restoration. Front. Mar. Sci. 3, 64
- ↑ Scyphers, S.B., Powers, S.P., Heck, K.L. and Byro, D. 2011. Oyster Reefs as Natural Breakwaters Mitigate Shoreline Loss and Facilitate Fisheries. PLoS ONE 6 (8), e22396
- ↑ Borsje, B.W., van Wesenbeeck, B.K., Dekker, F., Paalvast, P., Bouma, T.J., van Katwijk, M.M. and de Vries, M.B. 2011. How ecological engineering can serve in coastal protection. Ecol. Eng. 37: 113–122
- ↑ Morris, R.L., La Peyre, M.K., Webb, B.M., Marshall, D.A., Bilkovic, D.M., Cebrian, J., McClenachan, G., Kibler, K.M., Walters, L.J., Bushek, D., Sparks, E.L., Temple, N.A., Moody, J., Angstadt, K., Goff, J., Boswell, M., Sacks, P. and Swearer, S.E. 2021. Large-scale variation in wave attenuation of oyster reef living shorelines and the influence of inundation duration. Ecol. Appl. 31, e02382
- ↑ Chowdhury, M.S.N., La Peyre M., Coen, L.D., Morris, R.L., Luckenbach, M.W., Ysebaert, T., Walles, B., Smaal, A.C. 2021. Ecological engineering with oysters enhances coastal resilience efforts. Ecological Engineering 169, 106320
- ↑ 8.0 8.1 Sigel, L., 2021. Effect of an Artificial Oyster Reef on Wave Attenuation. MSc Thesis. TU Delft University of Technology
- ↑ Xiang, T., Bryski, E. and Farhadzadeh, A. 2024. An experimental study on wave transmission by engineered plain and enhanced oyster reefs. Ocean Engineering 291, 116433
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