Dune stabilisation
Dunes are a natural coastal feature on moderately exposed and exposed sandy coasts. Dunes are formed by sand from the beach, which is blown inland by the wind – see the article Dune development.
Coastal dunes are built and maintained by sand blown inland from the beach. Effective dune stabilisation therefore requires both an adequate supply of dry beach sand and measures that retain this sand in the dune. Vegetation, sand fences and fascines reduce wind speed near the surface, trap wind-blown sand and help repair or strengthen the foredune. Their effectiveness is greatest where the beach is sufficiently wide to supply sand and to limit frequent wave attack at the dune foot. Where the beach is narrow or structurally eroding, maintaining or restoring beach width and sediment supply is generally the first measure, complemented by planting, fencing or fascines. However, natural variability is an important characteristic of natural dune development processes. Stabilisation should be limited to the areas where it is required for coastal protection, control of sand drift or repair of human damage.
Contents
Background
During storm surge events, dunes can be eroded but the eroded sand generally remains in the active coastal zone. This beach-dune sand system therefore forms a flexible buffer zone that protects the hinterland from flooding and contributes to minimizing shoreline retreat. During storms and also under less severe conditions, sand will be transported inland, sometimes in connection with the formation of wind alleys in the dune row. After a storm, a surviving dune can gradually recover if sand is returned to a sufficiently wide upper beach and onshore winds transport it back to the dune [1]. Recovery is generally much slower than storm erosion and may remain incomplete on a narrow or sediment-deficient beach. Where the beach–dune sediment budget is approximately balanced, the beach supplies sand for dune recovery and the dune belt can adjust gradually to shoreline movement. Where the beach is persistently narrowing or losing sand, the dune will generally retreat or diminish unless sediment is added. The erosion of dunes as a result of a severe storm surge is referred to as dune erosion. The article Dune erosion gives a more detailed description of dune erosion processes and the article Shoreline retreat and recovery deals with after-storm recovery processes.
Sand supply from the beach
Where dune degradation is caused by a shortage of beach sand, the sediment deficit must be addressed first (Fig. 1). Dune stabilisation depends primarily on the condition of the beach. Wind can build or repair a dune only when suitable sand is available on the beach and the upper beach remains dry for sufficient periods. A wide dry beach also reduces the frequency with which waves reach and erode the dune foot. The relevant width is especially the dry upper beach or backshore that is exposed to wind during most tidal stages, rather than the total width uncovered only at low tide. See also Dune development and Dune erosion.
Vegetation, fences and fascines trap wind-blown sand but do not compensate for a persistent beach sediment deficit. On a narrow or structurally eroding beach, they may be destroyed by waves before substantial dune growth occurs. Depending on the site, this can require beach or shoreface nourishment, sediment bypassing or modification of structures that interrupt littoral sediment transport, before local dune stabilisation can succeed[2]. See also Shore nourishment and Sand by-pass systems.
Dune vegetation
Unvegetated dune surfaces are susceptible to wind erosion. Sand may be redistributed within the dune, carried inland or lost from exposed erosion corridors, depending on wind direction and dune morphology. Vegetation plays an important role in stabilizing dunes. A variety of natural vegetation species can establish on the beach and the foredune and are adapted to retain drifting sand. An overview of sand binding plants that are adapted to saline, nutrient-poor and harsh hydro-sedimentary conditions can be found in the article Shore protection vegetation. Beach and foredune vegetation typically require regular sand burial for healthy growth, resulting in the formation of deep, layered root structures. Flume experiments and field observations show that such root systems, especially dense fine root structures, contribute more to erosion control than the above-ground biomass[3][4]. Newly planted vegetation can begin trapping wind-blown sand relatively quickly, but its full contribution to resistance against wave erosion develops only after a dense root system has formed.
Dune stabilisation by vegetation is a sustainable protection measure, enhancing the natural protection ability of dune areas (Fig. 2). Vegetation traps wind-blown sand and its roots can reduce sand loss during wave attack. However, vegetation cannot compensate for insufficient beach sand volume or prevent major erosion when severe waves directly attack the dune.
Human use must be managed because trampling, vehicles and grazing damage vegetation and create erosion corridors. Sand blown into the hinterland can cause serious disruption to economic activities taking place there. Consequently, authorities normally tend to protect dunes by regulating their use.
Porous fences and fascines (pine or spruce branches) reduce near-surface wind speed and cause transported sand to be deposited. They are effective only where the beach supplies wind-blown sand. They should be placed above the zone of frequent wave attack; otherwise they may be undermined or destroyed before a stable dune develops[6][7]. By trapping wind-blown sand before a storm, fences can increase the volume of the foredune available to absorb erosion. In this way sand fences can reduce the risk of dune collapse during heavy storms[8]. The fences themselves may be damaged or removed when directly attacked by waves.
Larger wind alleys can be filled with compatible sand before planting, but access and wind concentration must also be controlled; otherwise the gap is likely to reopen. Plant establishment on artificial or severely degraded dunes is sometimes stimulated with fertilizer, but only where necessary and with regard to possible effects on native dune vegetation. Closely spaced fences and dense planting can retain most transported sand near the seaward dune edge, producing a high, narrow ridge instead of a broader dune. Such a ridge may remain vulnerable to wave erosion if the beach in front continues to narrow. Fixing the dune position cannot prevent its eventual loss where the shoreline continues to retreat and the beach sediment deficit is not corrected. Under such conditions, repeated wave erosion can progressively narrow the dune.
If suitable sand-binding vegetation species for dune protection are not available, for example, due to climatic or sedimentary conditions, the use of artificial surrogates can be considered. Laboratory experiments show that coconut mats inserted horizontally in the dune face can be effective to mimic the sand-binding function of natural root systems. However, since natural materials are susceptible to biodegradation, the sustainability of such measures is a concern[9].
Restoring the conditions for dune stabilisation
Urgent interventions to restore or reinforce dunes can be necessary in some cases. In most cases, however, better results are achieved by restoring the conditions for natural dune development. A priority measure, already discussed above, is to ensure sufficient beach width for sand supply to the dunes.
Newly planted and actively accreting foredunes should be protected from grazing and trampling until vegetation is well established. Further restrictions may also regulate the traffic in the dunes, e.g., ban on motor traffic, paved walking passages in areas near parking lots and fencing natural dune areas (especially fragile newly planted zones). Mechanical beach clearing can destroy vegetation and incipient foredunes. Seawalls and buildings can interrupt sediment exchange between the beach and dune. Public access should be concentrated on designated paths or elevated boardwalks designed to minimize trampling and interference with wind-blown sand transport. Urbanized hinter-dune areas restrict the environmental gradient and impede natural dune development.
Excessive fixation can suppress natural dune processes and should be avoided where dune mobility is compatible with safety and land use. Exchanges of sediment, nutrients and biota between the dune, beach and nearshore, as well as the natural cycles of accretion, erosion, growth and decay, promote greater diversity and complexity. This ultimately results in greater resilience of the foredune system[10]. Topographical variability contributes to subtle but valuable variations in microclimate and habitat[6].
Restoration of the conditions for natural dune development may conflict with the interests of beach tourists and residents (e.g., obstructed sea view due to high dunes or vegetation). A long-term strategy for coordinated management of land within the coastal zone therefore requires public consultation and participation to adequately address the social, economic and cultural aspects involved.
Artificial hard dune core
Where failure of a narrow dune would expose low-lying land to flooding and sufficient space or sand for a wider dune is unavailable, a buried hard core can provide residual protection after the outer sand cover has eroded. In the Netherlands, a hard artificial dune core has been applied in places where a single coastal dune row protects land below sea level. In one case (coastal village of Katwijk), the hard dune core was designed to also serve as a parking lot for beach tourists. Examples of artificial dunes serving as sea defense structures are discussed in the article Climate adaptation measures for the coastal zone.
At some sites, the sandy dune overlies weaker or more easily scoured deposits, such as peat or soft clay. These layers can become exposed when the sandy dune front is eroded during severe long-lasting storms or storm clusters (rapid storm sequences). The washing away of these erodible layers can lead to dune collapse. Such inhomogeneous dunes can also be strengthened by incorporating an artificial hard core. Dunes along the microtidal Baltic coast in Poland are internally reinforced with gabions and geotextile mattresses[11]. These reinforcement materials account for only about 1% of the dune's volume. Replacing the eroded sand cover by nourishment retains the appearance and part of the habitat function of the dune, although the buried reinforcement limits natural morphological adjustment during severe erosion. A buried hard core is flood-defense reinforcement rather than dune stabilisation in the ecological or geomorphological sense.
Related articles
References
- ↑ Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
- ↑ Pye, K., Blott, S.J. and Guthrie, G. 2017. Advice on Options for Sand Dune Management for Flood and Coastal Defence. Natural Resources Wales (UK) Evidence Report 207
- ↑ Figlus, J., Sigren, J.M., Feagin, R.A. and Armitage, A.R. 2022. The unique ability of fine roots to reduce Vegetated Coastal dune erosion during wave collision. Front. Built Environ. 8:904837
- ↑ de Battisti, D. and Griffin, J.N. 2020. Below-ground biomass of plants, with a key contribution of buried shoots, increases foredune resistance to wave swash. Ann. Botany 125: 325–334
- ↑ Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.
- ↑ 6.0 6.1 NSW 2001. Coastal Dune Management: A Manual of Coastal Dune Management and Rehabilitation Techniques. New South Wales, Department of Land and Water Conservation, Australia, http://www.environment.nsw.gov.au/resources/coasts/coastal-dune-mngt-manual.pdf
- ↑ USACE 2008. Coastal Engineering Manual. Part V, Ch. 7. Coastal Engineering for Environmental Enhancement pp. V.7.17-V.7.21.
- ↑ Harris, M.E., Ellis, J.T. and Barrineau, P. 2020. Evaluating the geomorphic response from sand fences on dunes impacted by hurricanes. Ocean and Coastal Management 193: 105247
- ↑ Ahrenbeck, L., Lojek, O., Schattmann, J., Mehrtens, B., Schweiger, C., Kosmalla, V., Schürenkamp, D. and Goseberg, N. 2025. Surrogate root system modeling—A hybrid dune reinforcement. Coastal Engineering 202, 104835
- ↑ Doyle, T.B. and Woodroffe, C.D. 2023. Modified foredune eco-morphology in southeast Australia. Ocean and Coastal Management 240, 106640
- ↑ Rozynski, G. 2023. Coastal protection challenges after heavy storms on the Polish coast. Continental Shelf Research 266, 105080
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