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This building solution covers the spatially concentrated placement of (relatively) large sand volumes for coastal development. Such nourishments are placed at a specific location with the aim to gradually feed the surrounding coast. Wind, waves and currents will spread the sediments along the coast, which is a typical example of 'building with nature'. Such a nourishment will contribute to the coastal safety in the long-term while creating more opportunities for nature and recreation.
An emerged concentrated nourishment along a sandy shore, whereby part of the nourishment is dry, provides more space for recreation, water sports and beach lovers. In addition, new dunes and vegetation could develop, increasing nature value. In case of an unprotected (non-enclosed) nourishment, the shape and bathymetry of the nourishment will however change continuously under the forcing of tide, wind and waves. This requires an adaptive management strategy to anticipate on unforeseen developments. The Sand Motor Delfland is a good example of an integrated solution providing safety against flooding while facilitating nature development and recreation.
In the past, erosive coasts were protected by man-made, hard structures like dikes and dams. It was soon realised that these defences often induce erosion down-drift and so softer engineering options were introduced. Structures were built that worked with the natural deposition processes occurring at the coast. Groins and breakwaters were seen as the preferred option to build up beach and dune levels in order to offer a natural barrier to the sea. In the last 15 years alternative but nowadays quite common solutions of soft nourishment were applied. Sand was placed at the location where it was directly needed, and slowly dispersing in time, while interacting with the hard structures.
Artificial coastal sand nourishments are applied for various reasons:
In the case of structural erosion, the nourishment will have to be repeated from time to time as the erosion is an ongoing process. The interval between successive nourishments depends on the rate of erosion and on the equipment mobilisation costs. In addition, the timing of the nourishment should be carefully chosen with respect to breeding season of species (seals, birds, etc.) and nursery habitats of fish and shrimps. Generally, an interval of five years is considered acceptable (for Dutch cases). Compared to hard solutions, nourishment is a flexible solution, while costs are spread over a longer time. For conditions along the Dutch coast this makes the soft nourishments more economical than the hard solutions with the added advantage that no lee-side erosion occurs (typical for hard solutions).
Sand nourishment can be carried out at various locations in the beach profile, such as the shoreface or foreshore (underwater), beach & surf zone and in the dune zone. In alongshore direction, the nourishment can be distributed over a large distance or placed more locally (concentrated). In the latter case, the goal of the nourishment could be to feed the adjacent coast gradually. Tide, winds and waves transport the sediments and spread it along the coast. This distribution of sand occurs gradually at rates that the adaptation of nature and species can keep up with. This enables nature and species to adapt. In case of an emerged nourishment, where part of the nourishment reaches above high water level, one can speak of a concentrated nourishment feeding the surrounding coastal area: e.g. the new beach provides new services such as recreation. A very large submerged foreshore nourishment does not provide such services; thus, for additional service for coastal development it is important that part of the nourishment is emerged and accessible (either by foot or boat). In case of an emerged concentrated nourishment the length of the shoreline increases due to the nourishment providing more space for recreation, water sports and beach lovers. In addition, new dunes and vegetation could develop increasing nature value.
Beach profile (picture by Stephanie IJff)
Nourishments for coastal development attracts new services and hence new beach visitors (human and animals), which require a different management of the coast at the nourishment and its surroundings. In case of an non-enclosed nourishment, the shape and bathymetry of the nourishment will change continuously under the forces of tide, wind and waves. It is therefore important that an adaptive management strategy is adopted to anticipate the unforeseen and poorly predictable developments.
Shallow areas in front of hydraulic infrastructure act as a buffer for safety by dissipating wave energy. Due to the extra segment of water that is created by the rise in sea-level, a sediment shortage on many of such shoals will be created leading to a decrease in area. Nourishment can be an effective measure to fulfil the sediment demand of the shoals and maintain a dynamic equilibrium between erosion and sedimentation.
How to use
Different design options for sand nourishment
To determine the feasibility and design of a concentrated nourishment for coastal development, guidance is presented below. First, the feasibility of a concentrated nourishment for coastal development is assessed using a rule-of-thumb method. Subsequently, a preliminary design of the concentrated nourishment is determined using simple (interactive design) tools including parameterization. For appropriate detailing and a geometric design more advanced tools (like process-based models) are available to be applied.
Sand nourishment in the form of feeder beaches has been applied along Dutch coastlines for several decades. It is not surprising therefore, that there are a large number of example projects to be mentioned here.
A surplus of sand (about 21 million m³) is put into the natural system and will be re-distributed alongshore and into the dunes, through the continuous natural action of waves, tides and wind. In this way mega-nourishments gradually induce dune formation along a larger stretch of coastline over a period of one or more decades, thus contributing to the preservation and increase of safety against flooding over a longer period.
The sand motor shortly after construction
The coast of Delfland, defined as a coastal stretch of about 14 km between Hook of Holland and The Hague (Netherlands), is characterised by dunes and a net northward transport of sand, driven by predominantly southwestern winds. To maintain this part of the coast regular nourishments have been carried out of about 1 million m³ per year on average. As an alternative nourishment method, now the Sand Engine has been placed based on principle of spatially concentrated nourishment for coastal development. It concerns a pilot experiment which will be extensively monitored and investigated.
- under development
In winter 2010 a beach nourishment was conducted between the port of Scheveningen and the jetty (pier). A volume of 1.5 Mm³ of sand was placed on the beach. The sand was borrowed beyond the 20 m depth contour line at a distance of about 15km. The sand was pumped through a floating pipeline of 2 km long and disposed near the beach. The nourishment resulted in a shift of the shoreline of 40 - 70 m as the area close to the port breakwater was not nourished. This 'jump' in the shoreline caused the waves from the northwest to nicely refract and create great surfing conditions.
Scheveningen beach, before (left) and after (right) the sand nourishment
In October and November of the year 2009, a pilot project was executed at the coast of Ter Heijde, a small village along the Holland Coast. The project comprises the construction of three so called sand groynes. Sand groynes are concentrated nourishments, constructed from the shoreline in seaward direction appearing in the form of small peninsulas. The groynes contain about 200.000 m³ of sand and serve as an input of sand in the coastal system. The sand in the groynes is anticipated to be uniformly redistributed over a stretch of coast of about 2.5km by the impact of waves and currents. The pilot project was executed as a part of the reinforcement program of the Delfland Coast as one of the Zwakke Schakels (in English: Weak Links) along the Dutch coast.
Sand groynes in Delfland
Sylt (Germany) is a sandy barrier island in the North Sea. It is the most northern of the German Wadden Sea islands. The coastal zone is characterised by sand dunes, by sand dune cliffs at the west coast (up to 35 m high, flattening towards the south) and by sand beaches in front of the cliffs, that break some of the oncoming wave energy. Nonetheless, the 40 km west coast is very vulnerable to erosion.
Sand nourishment started in 1972 and has been repeated at irregular or regular intervals. Hard structure protection of the west coast has been found ineffective in the long run. Even some negative effects have been noticed. Partly because of the hard measures taken, the central part of Sylt is reasonably stable. However, the hard measures on the long run will fail. Nourishments are now needed to protect these hard coastal constructions. Furthermore, the nourishments are effective in stopping the coastline from receding. At Westerland, the nourishments have to be repeated every six years. See the for more details. report
Sand Engines IJsselmeer
Along the Frisian coast of the IJsselmeer a need for strengthening of lake dikes in the light of rising sea levels and the need of adapting lake water levels became apparent. It was decided to explore the application of small sand engines at two locations (see
Natural nourishment - Bornrif at Ameland
The Bornrif is an attached bar at the northwestern edge of the Wadden Sea barrier island, Ameland. Ameland is located in the northern part of Netherlands and together with another Wadden island, Terschelling, it forms the Amelander inlet. The Bornrif is a dynamic feature influenced by the Ameland estuary dynamics, tides and wave driven currents; however, other aspects as wind, vegetation and bioturbation also play an important role in its development.
Impression of the Bornrif at Ameland
In 1993 the Bornrif was shaped as a hook (like a plunging wave), which was later copied in the design shape of the Sand Engine Delfland. Furthermore, the eco-morphological evolution of the Bornrif shows large similarities with the numerical model predictions for the Sand Engine. The evolution of the Bornrif was studied in detail (Achete, 2011).
The table below summarises the characteristics of the cases.