|BwN Building Blocks||BwN Toolbox||Pilots and cases||BwN Knowledge|
The project It Soal is executed along the Frisian IJsselmeer coast located near the Workummerbuitenwaard between 1995 and 1997. It consists of a constructed longitudinal groyne and two sandbars. Main purpose of the project was to create rest,- moult,- and foraging habitat for wading birds. The dyke and sandbars also functioned as zoning structure to divide nature and recreational areas. The project is successful in reaching its objectives. The groynes contributed to retain the sediment as well as to the zonation of nature and recreation.
General Project Description
Title: It Soal
Location: Along the Frisian IJsselmeer coast, located near Workumerbuitenwaard (the Netherlands)
Date: 1995- 1997
Companies: It Fryske Gea, Rijkswaterstaat
Costs: € 622,000
Abstract: It Soal was executed between 1995 and 1997 with the aim to create habitat for waterfowl and zoning of nature and recreational areas. The project encompassed the construction of a breakwater and two sandbanks and is considered to be a success.
Topics: Breakwater, sandbank, waterfowl, zonation
View pilot location in a larger map.
Planning and Design
After the experience with an unprotected sandbank on the Workumerbuitenwaard (see case Workumerbuitenwaard), It Fryske Gea and Rijkswaterstaat decided to execute a sand nourishment protected by a breakwater. The latter, 750 m long, was built in 1995. Sand was nourished in 1997, in the form of two emerged shore-parallel sandbanks in the lee of the breakwater.
The goals of the project were:
1. to increase the nature values in the area by creating rest-, moult- and foraging space for wading birds (e.g. white spoonbill, pied avocet), northern shoveller, stilt walkers, geese and terns;
2. to separate nature area It Soal from the adjacent recreation area It Soal (located southwest of the nature area).
On the coast of the Workumerbuitenwaard a 750 m long recurved breakwater was constructed in 1995, starting perpendicular to the coast and ending parallel to the coast. In the sheltered area between the breakwater and the shore two sandbanks (total volume 15,000 m3, footprint 9 ha, emerged part 0.6 ha) were constructed in 1997. The breakwater prevented the sandbanks from washing away. At the shore a hill was constructed offering people a view over the area.
The project was successful in reaching its objectives. The breakwaters helps retain the sediment and the project contributes to Nature2000 (bird species) and the EU Water Framework Directive (macrofauna, macrophytes and fish).
Operation and Maintenance
Results related to morphology
At It Soal there is a considerable wave- and current-induced north-bound longshore sediment transport. The breakwater shelters the sandbanks from this transport process. Fifteen years after construction, the sandbanks still exist, with approximately the same size. This rather stable situation indicates that the sandbanks are not part of the longshore transport process. The project protects the shore from heavy storms, but other influences on the morphodynamica in the area have not been observes.
Effects of rising water levels on morphology
Since the mean waterlevel in the IJsselmeer is stable since the construction of It Soal, there is no experience with higher water levels. The project is an example of how valuable nature can develop in shelterd areas with bare sandbanks. Higher water levels will permanently submerge the sandbanks and lead to more wave and current action, hence more morphodynamic activity, until a new equilibrium has been reached.
Results related to ecology
Compared to more recent projects, It Soal has contributed more to reaching specific nature and water quality goals. This indicates that nature development takes its time after implementation of the engineering works.
It Soal includes a number of habitats, such as sandbanks, stoneworts and pondweed. The breakwater itself is a habitat for bullheads and zebra mussels. As compared with the rest of the IJsselmeer, the number of waterfowl has remained the same or decreased less rapidly. The project has resulted in habitats of both larger diversity and size; from a nature development perspective the project is therefore a success.
For breeding birds the artificially regulated water level at It Soal is a problem since the banks are submerged during the breeding season and shoreline vegetation hardly develops. Breeding higher up on the banks is not always an option because of the higher vegetation density there.
The project also had some negative effects on fauna, which manifests in; a decrease in southern pochards, fish eating birds and specific waterfowl.
Effects of rising water levels on ecology
If the mean water level is raised and no extra measures are taken (heightening the sandbanks and breakwater), the sandbanks are likely to be permanently submerged. Aquatic plants will only survive if the bank height is increased along with the rising water level; if not, the aquatic vegetation will probably move to higher parts of the land.
As the water level rises, aquatic plants have a decreasing ability to trap sediments in between their roots and to stimulate sedimentation by decreasing the flow velocity. As a result, the flow velocity will increase and erosion processes will be enhanced.
Sand nourishments along the Frisian IJsselmeer coast are valuable if they:
- Do not lead to the destruction of valuable existing nature
- Contribute to the creation of a larger area of sheltered shallow zones and related vegetation
- Lead to higher diversity of land- water gradients
- Lead to a higher amount of bare emerged sandbanks which are important to a variety of bird species.
- Exist kong enough to achieve an almost stable situation with the above features.
- Man-made structures (in this case a recurved breakwater) can help to reach the goal of creating terrestrial habitats and reaching nature values.
- Such structures can help create shelter and long-term stability of sandbanks in othwerwise for the Frisian coast- relatively dynamic circumstances.
- These lessons have been taken into account in the design off the Sand Engine IJsselmeer coast.
- Bak A., W.M. Liefveld, H.A.M. Prinsen & F. van Vliet (2007). Evaluatie Natuurontwikkelingsprojecten IJsselmeergebied. Bureau Waardenburg bv in opdracht van Rijkswaterstaat IJsselmeergebied (in Dutch).
- Folmer E., T. Wilms & J. Cleveringa & R. Steijn (2010a). Pilot eco-dynamiek Fryske kust. Building with Nature (in Dutch).
- Groot, A., Lenselink, G., Vlieger, B. de & Janssen, S. (2010). Morfologische, ecologische en governance principes voor ecodynamisch ontwerpen: Toegespitst op de 'Bouwen met Natuur' pilots Friese IJsselmeerkust (in Dutch).
- Lauwaars, S. & M. Platteeuw, 1999. Een groene riem onder het natte hart: evaluatie van natuurontwikkelingsprojecten in het IJsselmeergebied. Ministerie van Verkeer en Waterstaat: Lelystad (in Dutch).
- Sarink H. & J. Balkema, 2008. Kenniskaarten IJsselmeergebied. Op weg naar een beleidskader (in Dutch).