Case study:Wallasea (North) Managed Realignment

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Project overview

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Status	Complete
Project web site	http://www.abpmer.net/omreg/
Themes	Habitat and biodiversity
Country	England
Main contact forename	Colin
Main contact surname	Scott
Main contact user ID
Contact organisation
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

The site on the final day of breaching (view east) (Taken for Defra, July 2006)

Project summary

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The project was promoted by Defra to create new mudflat and saltmarsh in compensation for losses of similar coastal habitats following port developments at Lappel Bank (in the Medway Estuary) and Fagbury Flats (in the Orwell Estuary). Also to enhance the coastal protection afforded the island, because its north bank was at risk of natural and unmanaged seawall breaching.

Monitoring surveys and results

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An initial five-year monitoring programme is being undertaken to describe the ecological development of the site and determine whether it meets compensation targets. This ‘site success’ monitoring includes surveys of overwintering waterbirds, benthic invertebrates, saltmarsh vegetation growth, sedimentation/erosion patterns and the settlement characteristics of the recharge area. To confirm the findings of the assessment work and test the effectiveness of the mitigation measures, this five-year programme also includes ‘impact verification’ monitoring with surveys of flow speeds, shoreline topography and channel bathymetry in the estuary.

Lessons learnt

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This project has benefited from detailed investigative work and consultations that were undertaken as part of the site selection, scheme design and impact assessment work as well as for the preparation of the monitoring programmes. However, the need to find such a large site that met all the requirements was a long-term and costly exercise. There has been a full time project manager with supporting teams including representatives from statutory and non-statutory authorities and these aspects have enabled problems to be foreseen and rapidly addressed. The team approach has also allowed innovative new design elements to be identified which should provide high value at low cost (e.g. the island features in the site and a new ‘heterogenic’ borrow-dyke design). The initial monitoring is still ongoing, and the full results from the first five-year monitoring programme have not been reported yet. However, from the interim progress reports that have been produced by ABPmer and Jacobs up to the end of 2010 (i.e. up to four years after the breaching), the following general observations have been made: Breaches and channels: The breaches and channels through them are very stable which confirms the effectiveness of the approaches that were taken to design them. Accretion: Within the site, sediment accretion has occurred relatively consistently and evenly due to the slow flows and stable internal creek/channel configurations. In the first year after breaching (2006 to 2007), accretion was around 10cm (of which about 50% is considered internally relocated materials and 50% externally imported sediments). In each subsequent year, the annual accretion is considered to be predominantly related to imported sediments and amounted to 3 to 5cm on average (specifically 5cm in both 2008 and 2009, and 3cm in 2010). Invertebrates: The accretion and the relatively stable and depositional nature of the environment has helped to promote rapid benthic invertebrate colonisation of the mudflat (approx 80ha in extent). After the first, year invertebrates abundance was at 20,000 organisms/m2 and has ranged between 10,000 and 20,000 organisms/m2 in each successive year. The benthic assemblages have been dominated by large numbers of mud snail (Hydrobia ulvae) and although the patterns of organism recruitment are clearly complex and variable, there is evidence that the assemblages are maturing over time. This is indicated by the bivalve species becoming an increasingly important component over time (representing 2%, 4%, 14% and 26% of the populations in each successive year).

Birds: The bird monitoring (by CJT Ecology) shows that over the course of the four completed years of the monitoring programme the site has been supporting very good numbers of waterbirds. This was the case even from the first winter survey (2006/07) when the site supported around 7,000 waterbirds and included good numbers (i.e relatively high in a national or international context) of many key species such as shelduck, dunlin, black-tailed godwit, ringed plover and golden plover. In the following two winters the value of the site continued to improve as the abundance of waterbirds increased to around 10,000 and then 12,000. For the most recent winter (2009/10) however the overall abundance levels declined slightly. These broad trends are strongly influenced by some major inter-annual changes in the abundance of certain species which, in turn, are likely to be influenced by the weather conditions (e.g. particularly bad weather conditions were experienced in 2009/10 when compared with other years) and/or a range of other factors that influence the natural dynamics of bird populations (e.g. breeding success, timing of migrations, national population trends and inter-annual or inter-generational changes in roosting/feeding site selection). It is of note however, that the numbers of birds using the site as a roost reduced in 2009/10 while the number of birds feeding increased. This may indicate that the habitat and the relationship with migratory birds is still maturing and developing or that under adverse weather conditions the site’s value as a feeding site as opposed to a roosting site increases. Saltmarsh: Saltmarsh coverage of elevated areas of the sites (approx 25ha in extent) has occurred relatively rapidly. On average plant coverage (i.e the amount of marsh plant compared to bare mud at any given location) has rapidly increased from less than 1% in 2007 to 6% in 2008, 60% in 2009 and finally at or around 100% in 2010.

Image gallery

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Dredge sediment recharging in November 2005

Dredge sediment recharging in November May 2006

Construction of Breach 4 (8,000m3 of m was moved during a 7h tidal window)(Taken for Defra, July 2006)

Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name North Bank of Wallasea WFD water body codes WFD (national) typology WFD water body name Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body No National/international site designation Local/regional site designations Protected species present No Invasive species present No Species of interest Dominant hydrology Dominant substrate River corridor land use Arable Average bankfull channel width category Average bankfull channel width (m) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started Works started Works completed Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources DEFRA Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design Stakeholder engagement and communication Works and works supervision Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Hydromorphology Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Social measures (incl. engagement) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information