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In September 1992, a sand bar was constructed along the Workumerbuitenwaard at 1m below mean sea level. The sand bar- with a length of 2 km and a width of 120 m- was located 450 m from the coast and not protected by any artificial structure. Goal of the project was the eastwards movement of the bar resulting in a gradual supply of sand to the coast and expansion of the wetlands along the shore. As this project has been executed some decades ago, the case Workumerbuitenwaard serves as a historical case from which lessons can be learned.
General Project Description
Title: Sand nourishment Workumerbuitenwaard
Location: IJsselmeer area, The Netherlands
Date: 1992- 1998
Companies: Rijkswaterstaat, It Fryske Gea
Costs: € 508.000,-
Abstract: A sand bar of ca. 20 ha was created in 1992 along the coast of the Workumerbuitenwaard with the ultimate goal to increase coastal sedimentation and expansion of the coastal wetlands.
Topics: Sand nourishment, sand bar, IJsselmeer, sedimentation, coastal erosion, coastal wetlands.
Planning and Design
The initiative for the experiment was taken by the Directorate-General Rijkswaterstaat of the Ministry of Transport, Public Works and Water Management and the NGO It Fryske Gea.
The initial objective of the project was fourfold:
- Reducing coastal erosion
- Stimulating sedimentation along the coast
- Expanding existing coastal wetlands and reed vegetation
- Creating a habitat for several bird species
The sand nourishment consisted of a sand bar 450 m offshore. The top of the sand bar was meant to remain submerged soon after placement and the bar had no artificial protection.
The basic idea of the Workumerbuitenwaard project was that the sand bar would gradually erode and move shoreward, which in turn would lead to onshore sand transport and protection of the coast and the shellfish reefs. Sedimentation should further create new room for wetland and reed vegetation and a new habitat for a variety of species.
The Workumerbuitenwaard project was not successful in reaching its objectives. As expected, the nourished sand remained permanently submerged after a short period of time. The fact that the sand bar was not protected by artificial structures led to erosion and stimulated sand transport. In this particular case, however, waves transported the sand westward into deeper water, instead of eastward to the coast. Part of the sand was transported by currents to the north, where it was deposited in shallow water. Shoreward transport turned out to be rather small.
Operation and Maintenance
Monitoring data combined with field observations from 1994, 1996 and 1998 show that there has been none or little sand transport in the direction of the coast. The deeper zone directly west of the bar, however, had become much shallower. The sand bar itself had lost height under the prevailing conditions between 1992 and 1998 (see adjacent figure).
Erosion of the sand bar occurred as predicted. However, the direction of transport was different from the expectation. Instead of moving towards the coast, the sand was transported in northern direction and to deeper areas in the west. This shows that steering of dynamic processes like sand transport is difficult and that specific knowledge is needed to predict this. However, on a larger scale the sand nourishment at Workumerbuitenwaard did contribute as it led to sedimentation at other places (e.g. ‘t Gaast) along the IJsselmeer coast. If the sand bar would have been placed higher on the shoreface and if sediment-trapping groynes would have been used, more sand might have ended at the intended locations.
The sand nourishment has indirectly contributed to nature values, be it at other locations than intended. Nature and extent of the effects on specific habitat types, however, are less clear. The nourishment seems to have contributed to the development of aquatic plants in its direct surroundings, while the effects at the location of the nourishment itself seem to be very limited. The submerged sand bar hardly contributes to Natura2000 objectives for bird species. Yet, there is a positive effect on the occurrence of stonewort (green algae of the class Charophyceae) which contributes to both Natura 2000 and the EU Water Framework Directive.
Effects future developments (rising water level)
In case of rising water levels, the influence of waves and currents will increase. This can lead to further erosion of the remaining parts of the sand bar, which subsequently can lead to increased pressure on the coast.
The conclusion is that sand nourishments in this type of shallow lake environment do not automatically lead to shoreward transport and sediment supply to the coast. In this particular case, most of the sediment is being transported. Consequently, the sand nourishments at Workumerbuitenwaard did not result in shoreline accretion, nor in deposition on terrestrial areas.
The sand bar eroded as planned, but the direction of transport was different than initially predicted. This example shows that steering of wave- and current-induced sand transport is not easy and that specific knowledge is needed to predict it. Wether the level of this knowledge was sufficient at the time of the Workumerbuitenwaard nourishment is doubtful. In the meantime, science has advanced and a more recent nourishment at virtually the same location seems to behave more or less in line with the predictions.
The area where most of the sand was deposited, west of the original bar, became more suitable for water plants.
The local processes taken into account in the prediction seem to be subordinate to regional processes such as larger-scale currents and water level gradients which play a role in the sand transport near the Workumerbuitenwaard.
These lessons have been taken into account in the design for 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.O., T. Wilms, J. Cleveringa & R.C. Steijn (2010). Pilot eco@dynamiek Fryske kust. Alkyon: Marknesse (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).