sediment of the focal site (Erftemeijer, 2019). This could include harbours, lagoons, or sea inlets. If material has to be sourced far offshore, this can become expensive for large sediment deposits, such as De Zandemotor in The Netherlands (Stive et al. , 2013). Dredged sediment is typically silt, sand and clay (Costa-Pierce & Weinstein, 2002). It is important to realise that the area to be recharged is accessible to the ship that will deliver the material i.e., the water depth available for the dredger to operate in when laden with the cargo (Baptist et al., 2019). Amount/height of sediment : This will vary depending on the target habitat, i.e. a tidal flat or a salt marsh, but generally the elevation of the sediment should be such that it is covered by the sea during high tide but exposed during low tide. However, if the objective is to create a nesting area for shorebirds, then consider placing the material at a height above high tide level. Of 12 intertidal recharge projects in the UK, one-off sediment placements ranged from 800 m 3 to 550,000 m 3 , while yearly placements ranged from 600 m 3 to 107,750 m 3 (Scott et al. , 2017). Moving and placing sediment : Moving large quantities of sediment requires heavy machinery and skilled labour. Sediment can be placed exactly where it is needed, or it can be spread by current and waves. In the latter case, adding the sediment at a dynamic location will allow it to spread more easily (Borsje et al. , 2012). Creating topographic diversity with the sediment (to ensure rich biodiversity) can be achieved in a number of practical ways, for example by using multiple discharge points from pipelines carrying dredged material slurry into the target area, or periodically moving the discharge point (end of pipeline) across the area to create multiple gradients in grain-size and achieving spatial variability in surface elevation (microtopography) across the flat (Erftemeijer, 2024). Consider the breeding times for birds as well as the dispersal of invertebrates as moving and placing dredged sediments cause sediment resuspension (Golder et al. , 2008; Van Der Werf et al. , 2015). Furthermore, care should be taken when using heavy machinery to spread sediment as it has been suggested this may cause too much compaction of the earth, inhibiting invertebrate colonisation (Evans et al. , 1999). 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 by deep ploughing to crack existing land drains (Mark Dixon, pers. comm.). Contamination of dredged material: Be wary that dredged material can be contaminated with heavy metals, which can be taken up by vegetation and other wildlife. Contaminated sediments are ideally to be avoided, but can, if unavoidable, be capped with clean substrate, ideally a minimum of 60 cm (Yozzo et al. , 2004). Vegetation control: Prevention/removal of colonising vegetation may be necessary if the goal is to create tidal flats (e.g. manually, chemically using herbicides, or control by fire, flooding or salinity change). Tidal flats should not be planted with mangroves, as the mudflats are typically inundated for longer periods than the mangroves can tolerate (thus typically resulting in failure of the planting efforts). Even if successful, this would be substituting one habitat for another thereby losing the specific value provided by tidal flats (Erftemeijer & Lewis, 2000). Encroachment by other vegetation, such as the prolific colonisation of opportunistic algae Agarophyton (Besterman et al. , 2020) , Ulva (Zhang et al. , 2019) and Lyngby (Estrella et al. , 2011), should also be avoided (see Cutts et al., 2024c).
62
Made with FlippingBook - professional solution for displaying marketing and sales documents online