Case study:Wallasea (North) Managed Realignment

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Location: 51° 36' 0", 0° 50' 0"
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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 Defra, Environment Agency, CJT Ecology
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


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)
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Catchment and subcatchment



Site

Name North Bank of Wallasea
WFD water body codes
WFD (national) typology Intertidal
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

Reach length directly affected (m)
Project started 2006/01/01
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

Supplementary funding information

Funded mainly by Defra who commissioned an extensive site investigation/selection programme as well as all the legal, public consultation and scheme-build elements of the project. The landowner, Wallasea Farms Ltd, assisted throughout and was responsible for the submission of the Planning Application and, post-construction, will be responsible for site maintenance.



Reasons for river restoration

Mitigation of a pressure Flood risk management
Hydromorphology
Biology
Physico-chemical
Other reasons for the project


Measures

Structural measures
Bank/bed modifications
Floodplain / River corridor
Planform / Channel pattern
Other
Non-structural measures
Management interventions Managed realignment
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

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

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional links and references

Link Description

Supplementary Information

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