Nature-based shore protection

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Nature-based foreshores can attenuate waves, fix and trap sediment, mitigate erosion, accrete with rising sea level and recover from moderate damage. These properties contribute to reducing the dimensions and costs of primary shore and flood defenses while providing additional ecosystem services. Their limitation is that the protective ecosystem itself must survive or recover from the hydrodynamic forcing they are buffering. Monitoring and management are required to prevent irreversible degradation of the protective ecosystem by these and other natural and anthropogenic stresses.


Introduction

Protecting highly exposed low-lying coasts from flooding require massive defenses. A voluminous dune belt can fulfil such a protection function. Other nature-based features, such as natural reefs or vegetated foreshores, can also contribute to protection, but generally not as standalone solution. Such natural foreshores can form a buffer where waves and currents dissipate part of their energy. The resulting reduction in hydrodynamic loading can allow a leaner and less costly design of primary flood defenses, which are generally engineered structures.

The effectiveness of such protection depends on the ability of the natural foreshore to retain and stabilize sediment, to accrete with sea level rise and to recover naturally from damage by severe storms. Nature-based protection is therefore most applicable where conditions are compatible with the development and persistence of the protective habitat.

Nature-based shore protection is practiced under various names: Building with Nature, Ecological Engineering, Engineering With Nature, Living Shorelines, Soft Engineering, Working with Nature. In addition to shore protection, nature-based measures provide many other ecosystem services. The overriding premise is to work within the given, natural boundary conditions and not to create artificial landscapes as foreign bodies in the coastal environment[1].

This article discusses two natural forelands that contribute to shore protection: salt marshes and seagrasses. The shore protection function of other natural forelands is discussed in separate articles: Oyster reef shore protection, Mangroves and Coral reefs.

Beaches and dunes can also be considered living shorelines. The article Shore protection vegetation gives and introduction to the role of vegetation for the protection of dune coasts. Shore nourishment is a widely practiced nature-based coastal protection measure for beach and dune coasts worldwide. Several Coastal Wiki articles are devoted to this topic, see Shore nourishment and further references therein.

The shore protection function of natural shores is largely due to the wave attenuation by vegetation. Some empirical quantitative estimates are given in the article Wave damping by vegetation.


Seagrasses

Shore protection functions

Seagrass meadows contribute to shore protection by attenuating waves and currents, trapping sediment and stabilizing the seabed. Reduced water motion suppresses sediment resuspension and enhances deposition, while roots and rhizomes bind the sediment[2][3][4]. Seagrass meadows can therefore mitigate seabed and shoreline erosion[5].

Wave attenuation depends strongly on the fraction of the water column occupied by the canopy, as well as on meadow extent and density and plant characteristics such as blade stiffness. Wave damping is significant when the ratio of canopy height to water depth exceeds about 0.2[6]. Flume experiments with artificial vegetation simulating a Posidonia oceanica meadow showed wave-height reductions up to 67% for a high submergence ratio of about 0.7[6]. In a sheltered Florida bay, significant wave height decreased by about 30% over 39 m of submerged seagrass, although attenuation decreased with increasing wave height and turbulence[7]. See Wave damping by vegetation for further quantitative estimates.

Limitations

Seagrass meadows are mainly effective as protective foreshores on low- and moderate-energy coasts. Strong waves can break leaves, uproot plants through erosion around roots and rhizomes, or bury and suffocate plants through sediment movement. Wave attenuation also decreases when water depth increases relative to canopy height. This limits the protective effect during high storm surges, when it is most needed, although the longer waves generated during storms may still be attenuated by seagrass[8]. Sea-level rise can similarly reduce wave attenuation and sediment trapping if meadow growth cannot adjust to increasing water depth.

Persistence of the protective meadow also depends strongly on environmental quality. Seagrass requires sufficient light and good water quality and can be degraded by eutrophication, dredging, bottom trawling and other human pressures.

Restoration and creation

Causes of seagrass decline should be removed before restoration is attempted. Historical presence of seagrass is a useful indicator that habitat conditions may be suitable[9]. Natural recolonization can be assisted by planting where necessary. Creation of new protective meadows is feasible only where water depth, wave climate, substrate and water quality permit their establishment and persistence. See Seagrass meadows#Requirements for development and restoration and Principles of coastal habitat restoration.

Salt marshes

Shore protection functions

Salt marshes contribute to shore protection by attenuating waves, trapping sediment and stabilizing the marsh bed. Their shallow platform reduces wave energy, while vegetation provides additional friction and roots and rhizomes stabilize the sediment against erosion. Sediment trapping promotes vertical accretion, enabling marshes with sufficient sediment supply to grow with gradual sea-level rise.

Wave attenuation can be substantial. Vegetated salt marshes can under favourable conditions dissipate more than 90% of incident wave energy over tens of metres[10]. Field measurements on the North Norfolk coast showed average wave-energy dissipation of 82% over salt marsh compared with 29% over an unvegetated sand flat, with an average wave-height reduction of about 63% over 200 m[11]. Numerical modelling confirmed vegetation friction as an important contribution to this attenuation. Tall, rigid vegetation generally dissipates more wave energy than short, flexible vegetation, although the effect also depends on vegetation density and hydrodynamic conditions[12].

The reduction of wave loading can reduce the dimensions and costs of the primary flood defense. A modelling study for the Shanghai coast estimated that restoration of one kilometre of tidal flat ranging from high marsh to bare tidal flat could reduce the required sea-dike height by 0.67–0.84 m for a 1-in-200-year storm[13]. Natural foreshores can thus complement engineered flood defenses while simultaneously providing other ecosystem services.

Limitations

The protective capacity of a salt marsh depends on maintaining sufficient width, elevation and vegetation cover. Deep inundation during severe storm surges reduces the relative contribution of vegetation to wave attenuation, while very high waves can break plant stems[14]. Lateral wave erosion and migrating tidal channels can reduce marsh width, thereby diminishing wave attenuation; human disturbances can aggravate these processes. See Tidal channel meandering and marsh erosion and Dynamics, threats and management of salt marshes.

Marsh vegetation promotes vertical accretion by trapping sediment, but marshes can drown where accretion cannot keep pace with relative sea-level rise[15]. Landward migration can compensate for marsh loss where space is available, but dikes and seawalls can prevent this migration and cause coastal squeeze[16]. Monitoring and management are therefore needed to maintain the width, elevation and vegetation condition required for the protective function.

Rehabilitation and creation

Degraded salt marshes can recover when the causes of degradation are removed and suitable tidal, sedimentary and morphological conditions are restored. Managed realignment can create new intertidal habitat on formerly embanked land by breaching the seaward defense. Breaching does not itself improve flood safety: a landward defense then becomes the primary flood defense, while development of salt marsh in front of it can reduce wave loading and maintenance requirements. High-water levels are often not significantly reduced by the restored marsh and can sometimes even be amplified[17].

Creation or rehabilitation of salt-marsh foreshores for shore protection is therefore feasible only where elevation, sediment supply and hydrodynamic conditions permit marsh establishment, persistence and recovery after disturbance. See Estuarine habitat rehabilitation measures and Principles of coastal habitat restoration.


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Related articles

Wave damping by vegetation
Biogenic reefs of Europe and temporal variability
Ecology and management of European biogenic reefs
Dynamics, threats and management of salt marshes
Seagrass meadows
Salt marshes
Spatial and temporal variability of salt marshes
Mangroves
Shore protection vegetation
Biogeomorphology of coastal systems
Climate adaptation measures for the coastal zone
Artificial reefs
Ecological enhancement of coastal protection structures
Blue carbon revenues of nature-based coastal protection
Estuarine habitat rehabilitation measures
Theseus Official Deliverable 2.1 - Integrated inventory of data and prototype experience on coastal defences and technologies


References

  1. ↑ Jordan, P. and Froehle, P. 2022. Bridging the gap between coastal engineering and nature conservation? A review of coastal ecosystems as nature-based solutions for coastal protection. Journal of Coastal Conservation 26: 4
  2. ↑ Ward, L.G., Kemp, W.M. and Boynton, W.E. 1984. The influence of waves and seagrass communities on suspended particulates in an estuarine embayment. Mar. Geol. 59: 85–103
  3. ↑ Verduin, J.J. and Backhaus, J.O. 2000. Dynamics of plant-flow interactions for the seagrass Amphibolis antarctica: Field observations and model simulations. Estuar. Coast. Shelf Sci. 50: 185–204
  4. ↑ Granata, T.C., Serra, T., Colomer, J., Casamitjana, X., Duarte, C.M. and Gacia, E. 2001. Flow and particle distributions in a nearshore seagrass meadow before and after a storm. Mar. Ecol. Prog. Ser. 218: 95–106
  5. ↑ Paul, M. 2018. The protection of sandy shores – Can we afford to ignore the contribution of seagrass? Mar. Pollut. Bull. 134: 152–159
  6. ↑ 6.0 6.1 Chastel, T., Botten, K., Durand, N. and Goutal, N. 2020. Bulk drag coefficient of a subaquatic vegetation subjected to irregular waves: Influence of Reynolds and Keulegan-Carpenter numbers. Houille Blanche 106: 34–42
  7. ↑ Bradley, K. and Houser, C. 2009. Relative velocity of seagrass blades: Implications for wave attenuation in low-energy environments. J. Geophys. Res. 114: F01004
  8. ↑ Koch, E.W., Sanford, L.P., Chen, S-N., Shafer, D.J. and McKee Smith, J. 2006. Waves in seagrass systems: Review and technical recommendations. US Army Corps of Engineers, ERDC TR-06-15
  9. ↑ Sierra, J.P., Gracia, V., Castell, X., García-León, M., Mösso, C. and Lin-Ye, J. 2023. Potential of transplanted seagrass meadows on wave attenuation in a fetch-limited environment. J. Mar. Sci. Eng. 11: 1186
  10. ↑ Möller, I. 2006. Quantifying saltmarsh vegetation and its effect on wave height dissipation: Results from a UK East coast saltmarsh. Estuar. Coast. Shelf Sci. 69: 337–351
  11. ↑ Möller, I., Spencer, T., French, J.R., Leggett, D.J. and Dixon, M. 1999. Wave transformation over salt marshes: A field and numerical modelling study from North Norfolk, England. Estuar. Coast. Shelf Sci. 49: 411–426
  12. ↑ Bouma, T.J., De Vries, M.B., Low, E., Peralta, G., Tánczos, I.C., van de Koppel, J. and Herman, P.M.J. 2005. Trade-offs related to ecosystem engineering: A case study on stiffness of emerging macrophytes. Ecology 86: 2187–2199
  13. ↑ Zhang, M., Dai, Z., Bouma, T.J., Bricker, J., Townend, I., Wen, J., Zhao, T. and Cai, H. 2021. Tidal-flat reclamation aggravates potential risk from storm impacts. Coast. Eng. 166: 103868
  14. ↑ Vuik, V., Suh Heo, H.Y., Zhu, Z., Borsje, B.W. and Jonkman, S.N. 2018. Stem breakage of salt marsh vegetation under wave forcing: A field and model study. Estuar. Coast. Shelf Sci. 200: 41–58
  15. ↑ Fagherazzi, S. 2013. The ephemeral life of a salt marsh. Geology 41: 943–944
  16. ↑ Kirwan, M.L., Walters, D.C., Reay, W.G. and Carr, J.A. 2016. Sea level driven marsh expansion in a coupled model of marsh erosion and migration. Geophys. Res. Lett. 43: 4366–4373
  17. ↑ Kiesel, J., Schuerch, M., Möller, I., Spencer, C. and Vafeidis, A. 2019. Attenuation of high water levels over restored saltmarshes can be limited: Insights from Freiston Shore, Lincolnshire, UK. Ecol. Eng. 136: 89–100




The main authors of this article are Prinos, Panayotis, Koftis, Theocharis, Firth, Louise, Davies, Clare, Davies, Andrew, Hawkins, Stephen, van Belzen, Jim, Skov, Martin and Bouma, Tjeerd
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