led to a slower recovery (Bolam et al. , 2004). The characteristics of the sediment in tidal flats and salt marshes can vary across the globe. For example, in the USA, coastal soils tend to be more peat-based than sediment-based compared to those in the UK (Atkinson, 2003). Wave action/sheltering of the coastline : How exposed the site is, and how strong the waves are, may influence the erosion rates of the sediment. Sites subject to high erosion rates may need regular placements of sediment. Alternatively, if a site composed of fine sediments is eroding, then replacement or protection by coarser material may reduce or prevent that erosion (but consider the effects of changing grain size on sediment properties and biodiversity; see above). In exposed sites, permanent or temporary breakwaters can help to disperse energy and/or trap sediment, thus controlling erosion (Zhang et al. , 2010; Pontee et al. , 2022). Note that sheltering tidal flats can facilitate seaward expansion of salt marsh vegetation (Chowdhury et al. , 2019). Sea currents can influence the migration of intertidal invertebrates (e.g. as seen in a ‘mega - nourishment’ project in the Delfland Coast, the Netherlands; Luijendijk & van Oudenhoven, 2019). Elevation: The final elevation of the deposited sediment will determine the duration and frequency of inundation and exposure of the different parts of the flat. Most of the site should lie between the level of mean low water spring and mean high water spring tides, but with some variation to support a diversity of species (see Cutts et al., 2024b).
Slope: Shallower slopes will allow for creation of a wider intertidal zone. Experience suggests a typical slope should preferably be around <0.04 or 1:1000 (WAVE, 2001).
Drainage: Well-drained marshes may be more resistant to erosion (Atkinson et al. , 2001) and can support a greater diversity of species (Wolters et al. , 2005). Poor drainage can affect the plant species that grow, favouring those tolerant of moisture and anaerobic conditions (Atkinson et al. , 2001). Pollution: Biodiversity on restored/created intertidal habitats may be negatively impacted by pollutants. These range from chemical pollutants, such as aquaculture effluents, sewage and oil, to large solid waste, such as fishing nets (e.g. Melville, 2018). A study of Indonesian tidal flats found that decapod crustaceans and oligochaetes made up a greater proportion of the macrofaunal community in areas covered by litter, whereas polychaetes dominated litter-free areas (Uneputty & Evans, 1997). If pollutants are (or are likely to be) present, consider whether they can be managed on the tidal flat and/or at the source.
Temperature: Microbenthic invertebrate recolonisation tends to be faster in tropical areas due to higher water temperatures, in comparison with other regions (Dittman, 2002).
4. Implementation
Obtaining the sediment : Dredging is common practice for maintaining navigation in infrastructure and transport corridors, such as ports and waterways (Sheehan & Harrington, 2012), and in such practices the disposal of the dredged sediment can be costly (Svensson et al. , 2022). Consequently, dredged sediment can be available at no or low cost but costs will be incurred for transportation. Sourcing sediment from nearby locations may be the most cost- effective in terms of transportation, and the sediment itself is more likely to resemble the natural
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