Description
Dune stabilisation Peniche Portugal
Samenvatting (Dutch description)
Background
During storm surge events, the foot of the dunes can be eroded but the dunes act as a very flexible buffer zone, which protects the hinterland from erosion and flooding. The eroded material supplies material to the littoral budget minimising the general erosion along the entire section of shoreline. During the storm and also during more normal events, sand will be transported inland, sometimes in connection with the formation of wind alleys in the dune row. After the storm, the damaged dune will gradually be built up again, maybe slightly more inland. This means that a dune acts as a natural flexible coast protection and sea defence measures. It moves backwards parallel with the eroding coastline and at the same time it maintains its form and volume as well as a wide beach. This is a natural quasi-equilibrium situation. The erosion of dunes as a result of a severe storm surge is also referred to as dune erosion.
Dune vegetation
Barren dunes are susceptible to deflation by wind, with landward migration taking place. Vegetation plays an important role in stabilizing dunes. Different types of natural vegetation can settle on the beach and the front dune 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.
Dune stabilisation by vegetation is a sustainable protection measure, enhancing the natural protection ability of dune areas. It provides some protection against wave and storm surge attack and at the same time it preserves the natural coastal landscape, if performed moderately. Dune stabilisation requires a planned and co-ordinated effort. However, the natural protection provided by beach and dune vegetation will be impaired if the plants are damaged by undergrazing, overgrazing or if beach-users, etc. generate too much traffic. Some uses such as golf courses typically fail to allow a sand dune system to move naturally. On the other hand, sand blowing inland can cause various kinds of damage to agriculture where adaptation to this natural movement of the dune does not occur. Consequently, authorities normally tend to protect dunes by regulating their use.
Fig. 1. Marram planting and the placing of spruce fascines in wind alleys (Danish Coastal Authority[1]).
In some cases authorities have been eager to protect the dunes by planting marram grass and placing fences or fascines (placing of pine or spruce branches) in the wind alleys to trap the sand [2][3]. Larger wind alleys can also be filled artificially prior to planting. Newly planted vegetation in particular can be strengthened by using fertiliser. However, in some cases, planting marram grass has resulted in complete fixing of the dune position and an unnatural growth in height. Consequently, the flexibility of the natural dune is lost resulting in a gradual disappearance of the dune due to erosion, whereby the protection, provided by the natural dune system, is lost.
Restoring natural dune dynamics
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. Promoting natural dune development requires limiting negative interferences. Structures that obstruct sand supply to the beach by interrupting littoral drift should be removed, or otherwise compensated by sand bypassing systems or by sand nourishment of the beach or foreshore. Grazing in dune areas is prohibited in most countries, and authorities often limit public access. Such 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; boardwalks, seawalls and beachside residential homes obstruct the beach-dune continuum and urbanised hinter-dune areas restrict the environmental gradient and impede natural dune development. Natural variability is an important characteristic of natural dune development processes. This implies the exchange of sediment (with the dune, the beach and near the coast), nutrients and biota, as well as the natural cycles of accretion, erosion, growth and decay, so that greater diversity and complexity ultimately results in greater resilience of the foredune system[4]. Topographical variability contributes to subtle but valuable variations in microclimate and habitat[2].
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.
Applicability
Dune stabilisation is applicable on all coastal types where natural dunes occur. This is especially the case on moderately exposed to exposed coasts with perpendicular to very oblique wave (wind) attacks. There is ample evidence for the effectiveness of sand fences to promote dune growth in periods of strong onshore winds, provided these are not accompanied by high water levels and waves. Several examples show that sand fences can also increase the stability of the dune belt during heavy storms
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