Vegetation: It is suggested that existing terrestrial vegetation should be cut back or removed before flooding, to prevent rapid die off post-breaching, which can pollute adjacent tidal systems (Mark Dixon, pers. comm.). Furthermore, chemical application on vegetation should stop six months prior to breaching to prevent a ‘pulse’ of chemicals into tidal waters (Mark Dixon, pers. comm.). Capturing fresh water: Constructing low-level bunds or excavating lagoons to capture any fresh water that is coming into the site pre-breaching can considerably improve the value of a site for birds by providing fresh water to drink and preen in (Mark Dixon, pers. comm.). Counter walls: Tidal inundation can be limited to the target area by constructing new defences or ‘counter walls’ (e.g. Reading et al., 2008). Consider using curved wall faces to better reflect wave energy (Mark Dixon, pers. comm.). Existing footpaths can be diverted onto counter walls (Mark Dixon, pers. comm.). The tide may naturally reach higher ground in some parts of the site, in which case counter walls may not be needed (e.g. Leeds, 2016). Counter walls can be expensive to build and maintain, so allowing the tide to move to higher land can keep costs down (Mark Dixon, pers. comm.).
Case Study: Meddat Marsh, Nigg Bay reserve, Scotland
Meddat Marsh in the Nigg Bay RSPB reserve was the first salt marsh created through managed realignment in Scotland. Over one third of the salt marsh in Nigg Bay had been lost between 1946 and 1977. The sea wall that was built in the 1950s had no salt marsh in front of it, which meant it was constantly being eroded by wave action, resulting in high maintenance costs. Meddat Marsh was purchased by the RSPB from a local landowner to be used as the realignment site. In 2003, the existing sea wall was breached. Two breaches 20-m-wide were dug using a mechanical digger across sites of relict channels, which allowed fast incoming tide but slow outgoing tide. Secondary defences were built to prevent the neighbouring land from being flooded. The rationale for breach design: Using two breaches provided sufficient inundation whilst retaining some sheltered conditions for vegetation to establish and for creeks to develop. Removing the entire sea wall was too expensive and would have provided no shelter. Allowing the sea wall to breach naturally was an option, but strategic placement of the breaches was preferred. The site was repeatedly monitored before and after breaching the seawall. Salt marsh plants began to colonise within six months. Within 10 years, salt marsh vegetation was dominant and covered the majority of the site; mud-dwelling invertebrates that are eaten by birds were recorded, 25 species of waterbirds were recorded with up to 2,000 individuals using the site, and 20 – 30 cm of sedimentation occurred. Overall, this created 20 ha of salt marsh and 5 ha of pioneer salt marsh and tidal flat habitat, increasing these habitats by 23% in the Nigg Bay reserve. The sea wall required no maintenance during this time.
Sources: Elliot (2015); Video: Restoring salt marsh (youtube.com/watch?v=aiOl8bjctAw)
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