suggested that dredge islands require deposits every three to seven years (Golder et al ., 2008).
It has been suggested that feeding shorebirds are attracted to standing fresh water on dredge islands, if it contains lots of mosquito larvae (Parnell et al ., 1986). Incorporating ponds into dredge island design may benefit shorebirds. If islands are being created close to existing sites used by shorebirds, work should avoid the breeding season, when birds are particularly sensitive to disturbance (Golder et al ., 2008). The presence of infrastructure, such as airports or wind turbines, which introduce a bird collision risk, can restrict the feasibility of island creation (Climate-ADAPT 2023). Be wary that dredged material can be contaminated with heavy metals and persistent organic pollutants (POPs), which can be taken up by vegetation and other wildlife. Contaminated sediments can be capped with clean substrate, ideally to a minimum thickness of 60 cm (Yozzo et al. , 2004).
As an indicative cost of island creation, a single 200 x 325 m island off the north coast of France cost €8 million (about US$ 8 million; February 2024 conversion). in 2005 (Aulert et al ., 2012).
Case Study: Marker Wadden, Markermeer, The Netherlands
Marker Wadden is a man-made archipelago consisting of five islands in the Markermeer lake in the Netherlands. The islands cover 1,300 ha (excluding two under construction in 2023). The islands are used recreationally but there are strict rules for visitors. There are jetties, walkways and bird observation points for visitors. The Markermeer lake was created in 1976 when a dike was built connecting the two coastal cities Enkhuizen and Lelystad, leaving Markermeer cut off from open water. A lack of current led to a build-up of silt at the bottom of the lake. The silt is disturbed by wind-induced waves, leaving the water turbid with disastrous consequences for the food web. As a way to restore the ecological balance, five islands were created with dredged material, the goal being to halt ecosystem decline and restore native biodiversity.
How the islands were created
Construction began in May 2016. A total of 30 million m 3 of sediment (sand, silt and clay) was dredged from Markermeer lake to create the islands. Equipment used included a cutter suction dredger Edax, a spray pontoon, three crane vessels and earth-moving equipment. Firstly, ring dikes were built out of sand, into which the dredge material was sprayed. This material was deposited in layers, allowing the sediment to consolidate before adding a new layer until the islands were above sea level. The larger, sandier sediment particles settled closest to the spray nozzle, while the finer, siltier sediment spread further away, creating texture at the bottom of the lake. The top layer of sediment dried to form a crust. Sand hills were built in the water at the back of the islands creating a gradual transition from land to
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