Coastal Erosion

Coastal Erosion

Reducing Coastal Erosion Several methods may be used to prevent coastal erosion - to increase the height and stability of existing dunes, or to repair damaged dunes and thus encourage sand accumulation. Newly blown sand will deposit around obstructions such as grass clumps and debris. Conditions conducive to natural dune formation, including plantings of native vegetation or installation of structural barriers can be provided to start and accelerate sand accumulation. Planting of native salt resistant vegetation such as Ammophila arenaria - Marram Grass, is the primary method for erosion prevention, dune construction, improvement and repair. Physical structures, such as fencing or Willow hedges can help trap sand and stabilise dunes, and are often used in conjunction with vegetation establishment.
Protection by planting Marram Grass Marram Grass (Ammophila arenaria) is the dominant sand fixing species. It is a native, perennial, rhizomatous grass. It has a natural ability to emerge from sand if covered, by way of the stems elongating, which enables the plant to grow above the deposited sand. New roots are produced in the layer of freshly deposited sand. Full sand holding capacity can be achieved in as little as 2 to 4 years. Marram grass can grow in height from 0.6 to 1.2 m / year. The grass is most vigorous on seaward slopes, where it is buried regularly by windblown sand. At these sites, new healthy white roots develop in the fresh layer of sand. The growth becomes less vigorous when the sand accumulation diminishes. Marram grass can be propagated from seed and can also be multiplied by planting small sections of rhizome. Naturally occurring Marram grass is usually a protected species in a coastal environment and excavation of rhizomes is not possible.
Traditionally, barerooted Marram Grass was planted manually in bundles of six plants, at a spacing of 4 bundles per square metre. With the introduction of Cell Grown plants establishment has become much easier and cheaper, with the large root plug containing upwards of 3-4 rhizomes per plant, therefore only four single plants are required per sq m. Being easily handled and planted, and being full of moisture, the success rate of the planting in this environment is near 100% . All year round planting is an option, also allowing working in better weather. In particularly exposed sites a method to aid survival of the marram grass is to install small bundles of reed or dead willow branches between the grasses. These bundles are usually planted in a grid of 0.5m x 0.5m. They are inserted into the ground 20 cm deep with about 30 cm remaining above the sand surface. This provides shelter for the marram plants in the early stages. The branches naturally decay, and will disappear after a short number of years, leaving a natural marram planting. Planting Marram grass on the windward side of reed or willow screens has proven to be the most efficient, effective and labour saving method for establishment. The sand builds up on the grass allowing it to establish more roots, hence improving growth rate and survival.
Use of other species Other salt tolerant species such as Buckthorn, Wild roses, Willows, Broom and Gorse will also perform well in a coastal erosion environment. In coastal inland areas tree species such as Lodgepole pine, Corsican pine, Holly, Birch and Alder will thrive. Plant Availability Plants are available almost all year round and are generally contract grown. Local seed provenances are also available to order. Prior to despatch all stock is well watered and fertilized with 80 kg N, 20 kg P, and 20 kg K per ha of a slow-release fertilizer that is active for up to 18 months. This ensures rapid establishment and sustained healthy growth. Physical protection via Wind screens and Osier hedges Cheviot Trees produce many willow structures, the lighter of which are ideal for insertion into sand dunes, thus trapping sand. Sections can be supplied ready made or material can be supplied loose for on-site construction. To physically prevent blowing of sand, one of the commonest methods is the installation of a wind screen. This can be an artificial, vertical screen / fence, or a ‘hedge’ of branches from some brushwood. Willow is often used. A screen may be placed parallel to the dune foot, catching sand blown from the beach into the dunes. In this way the dune foot is improved. When a screen is also placed on top of the dune, the dune will become higher. Screens at right angles to the dune foot will trap the sand which is blown parallel to the dunes. These perpendicular screens act like groynes and will catch sand, but as a consequence, they may also cause a lee-side erosion. A height of 1.50m above ground surface is recommended for dune building structures. In areas where sand conditions are poor for dune building, a height of 0.75 m is appropriate. The fencing is supported with wooden posts driven into the ground at 3.0m intervals. Wooden posts should be hardwood or treated softwood. The minimum practical length for posts is 2.0m, a length of 2.50m is optimum. Wooden posts should be no less than 10cm diameter. Secure the fencing material by fastening it to each post with four ties of galvanised wire and weave the material between the posts, so that every other post has fencing on the seaward side. If the base of a sand fence is placed at ground level, dunes will be built over the structure. If the base is elevated just 10-15cm above the ground, dunes will be built on the downwind side of the structure, and the fencing can be retrieved for reuse as the dunes are formed. In this case place the fences 1.50 to 3.0m seaward of the area to be restored. For further information or advice please don't hesitate to contact us.