In the USA, dredging results in the yearly accumulation ofmillions oftons ofsediment that has to be disposed of. It has become a recent practice to place this material in shallow areas to create tidal wetlands. Basically, the dredged material is pumped onto shallow areas hydraulically. If the site is exposed to waves, it must be protected by small breakwaters often made of riprap or low-lying geotextile tubes. The dredged material is then given time to dewater and consolidate. Tidal creeks are created to allow the muddy substrate to become exposed at low tide. To create a salt marsh, the grass Spartina alternifora is then planted mechanically, after being harvested from natural areas or grown in a nursery. Within a few years the created saltmarsh appears similar to a natural marsh but may lack some of the biological diversity and community maturity. In the short term, the degree of success depends on whether the attributes of the dredged material are similar to those of nearby natural saltmarshes. Over longer periods, the evolution of created saltmarshes shows much unexplained variability; some marsh attributes (e.g., below ground plant biomass) continue to develop over many years, while in other marshes these attributes grow quickly during the first few years and then stabilize.
These created saltmarshes generally do not capture the biological productivity (e.g., shrimps and fish, though there are a few welcome exceptions) of natural salt-marshes. However, such created wetlands are still much preferable, and more attractive, than sea disposal of the dredged mud and the pollution it creates. There is still a need for scientific studies to improve created saltmarshes by improving plant establishment, and by linking biological communities with geomorphogical features and tidal drainage patterns. The exposure of created saltmarshes to wave erosion remains a critical factor to their relative success or failures over time; however the long-term (periods > a few years) success remains little studied.
When possible, the created saltmarshes should be located behind large, shallow mud banks because they help protect the saltmarshes by dissipating wave energy.
In the southeast coast of England, UK, the preferred method of creating wetlands is by managed retreat (Figure 2). Much of the low-lying farmland is reclaimed saltmarshes, and it is protected by sea walls. These sea walls may need to be raised against a future sea level rise. This option is very expensive. Instead, the sea walls can be removed or breached, smaller sea walls relocated further inland, and saltmarshes are recreated in front of them. These dissipate wave energy and thus provide a protective buffer for the sea walls that may be built smaller. The ecological advantage is that these salt-marshes, like the natural ones, provide organic matter for fish and intertidal fauna, and they are also an important habitat for birds. There are still difficulties in establishing the plant vegetation, and this may be due to the weathering of the soils after they were reclaimed for farming over the last 400 years. Planting the vegetation with attached soil from natural saltmarshes helps to alleviate this problem. A suitable tidal drainage creek network must also be created that replicates the natural one, whereby the plants are submerged typically no more than 10 times per month.
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