site after four years coincidentally with the appearance of a saltwater clam Macoma balthica (Atkinson et al. , 2004).
Vegetation: The initial increase in the amount of surface water on a site may eliminate breeding habitat for specialised birds, but the eventual re-establishment of salt marsh vegetation can re-create ideal breeding conditions (Brawley et al. , 1998). Proximity to man-made structures: Man-made structures could deter some birds by obstructing their view of predators (Erftemeijer, 2023). One study found shorebird species diversity at a restored site to be lower closer to man-made structures (Armitage et al. , 2007).
4. Implementation
Breaching coastal defences: The width of a single breach can range from 20 m to 150 m (Thom et al. , 2002; Mazik et al. , 2010; Elliot, 2015) but in some cases multiple breaches are dug (Hughes et al. , 2009). The number of breaches needed will depend on the tidal range and the bathymetry of the area to be flooded (Hand Winterwerp, pers. comm.). Breaches could be located where there is an existing sluice (Mark Dixon, pers. comm.). It is recommended to start excavation during neap tide, as the tide ebbs, with the final breaching undertaken in one ebb tide cycle on one day only (Mark Dixon, pers. comm.). Once the final breach is made there is no going back, so it is recommended that the material is loaded and carted at the same speed as it is excavated from the breach, and that there are escape routes for machinery to get off site as the tide comes in (Mark Dixon, pers. comm.). If breaches are too narrow, tidal exchange can be restricted, limiting the amount of sediment entering the site (Williams & Orr, 2002), while larger breaches can be more costly. The size and number of breaches will influence the frequency and depth of inundation, and the amount of sediment entering a site. Be aware that scour holes can form around breaches at their base, caused by fast flowing water (Whitehouse, 2006). Extreme weather events caused by climate change could affect breaches, for example with the increase in typhoons in China (Huang et al. , 2022; David Melville, pers. comm.) Modifying culverts or other openings : Culverts channel water through or under a barrier or obstruction, such as a road. Culverts can be removed or their diameter increased to allow more tidal exchange (Streever & Genders, 1997). Where culverts have allowed sufficient tidal flow, their diameter has ranged from 0.75 – 2.10 m (Barrett & Niering, 1993; Burdick et al. , 1996; Brawley et al. , 1998; Roman et al. , 2002; Buchsbaum et al. , 2006; Wolters et al. , 2008). Culverts or other openings can be used for regulated tidal exchange by placing tidegates on them (Ausden, 2007). This consists of a hinged door that opens in a seaward direction, allowing the flow of water to be self-regulated as the force of the incoming tide pushes the gate closed. Floats can be used to open the gate based on changes in water level (e.g. Ridgway & Williams, 2021). Electronically operated gates can also be used (Ausden, 2007). Drainage: Channels can be constructed to improve drainage and provide foraging habitat for fish and invertebrates (Olmstead & Fell, 1974; West & Zedler, 2000). Alternatively, it is suggested that a natural drainage system can develop following deep ploughing to crack existing land drains (Mark Dixon, pers. comm.).
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