Case study:Alkborough tidal defence scheme

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Project overview

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Status	Complete
Project web site	http://www.environment-agency.gov.uk/research/planning/108978.aspx
Themes	Flood risk management, Social benefits
Country	England
Main contact forename	Volker
Main contact surname	Stevin
Main contact user ID
Contact organisation	Environment Agency
Contact organisation web site
Partner organisations	Natural England, Associated British Ports, North Lincolnshire Council, Department for Environment, Food and Rural Affairs (Defra), Yorkshire Forward, Heritage Lottery Fund, European Union (EU)
Parent multi-site project
This is a parent project encompassing the following projects	No

Alkborough area

Project summary

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Alkborough Flats are on the south bank of the Humber Estuary, on the eastern side of the confluence between the River Trent and the River Humber. The site is around 440 hectares of agricultural land, of which approximately 375 hectares are protected by a flood embankment constructed in the late 1950s. The Alkborough tidal defence scheme increases the level of flood protection to an area stretching from the Humber Bridge to Goole up the tidal River Ouse and as far as Keadby Bridge on the tidal River Trent. The scheme features include a 20 metre-wide breach of the existing flood bank, a 1,500 metre length of lowered embankment or spillway and a new section of floodbank to protect assets at the edge of the site. The scheme has been designed to: reduce the impact of sea level rise; safeguard the internationally important nature conservation interest of the Humber by creating new wetland habitat; provide a focus for education and access opportunities for local communities.

Monitoring surveys and results

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A monitoring programme funded in the first three years via this project, examining the physical and biological development of the newly created habitat at Alkborough Flats, began in summer 2007. Although the project had an existing programme of monitoring in place, this was primarily targeted towards the delivery of extreme water level reductions and the establishment of the basic habitat types. This project has meant that we have been able to develop the most comprehensive monitoring programme across the widest range of parameters of any managed realignment site in Europe. The design work for the monitoring programme was agreed by a team made up of key stakeholders and partner organisations. Work on agreeing the programme started in November 2006 with a workshop meeting and was agreed in early 2007. This includes a topographic and sedimentological survey, fish and benthic community analysis and analysis of spatial patterns in the distribution of feeding and roosting birds. The monitoring report attached as Appendix B1.5 details the findings of the second survey, conducted in 2008/2009. The key findings at Alkborough are: 1.Sediment accretion within the realignment site is continuing but at a slower rate than was initially recorded in 2007/8. Much of the site is now at an elevation of 3.1 m or more. Accretion levels on the mudflat outside the site can be considered stable with the exception of the infilling of a particularly low lying section of the mudflat where significant accretion has taken place. 2.The sediments both inside and outside the realignment site are described as fine-coarse silts, with fine sands being present in some areas, and are typical of the area. No changes in particle size were noted over time although water content and sediment organic content decreased inside the site between 2007 and 2008. 3.The benthic communities are impoverished (low diversity, abundance and biomass) and reflect the low salinity and high turbidity of this part of the estuary. Outside the site, the infaunal species and their abundances are typical for this area with Heterochaeta costata and Paranais litoralis being the dominant species. Inside the realignment site, the community is dominated by invertebrate species typically associated with freshwater and terrestrial environments. However, infaunal estuarine species (predominantly P. litoralis) are present and widespread throughout the site. 4.High tidal elevation and reduced flooding frequency appear to favour colonisation by vegetation. Whilst a significant amount of perennial rye grass (Lolium perenne) still exists, a total of 6 salt tolerant species were found including Phragmites australis, Aster tripolium, Atriplex prostrata, Ranunculus sceleratus, Puccinellia maritima and Rumex obtusifolius. Of these species, P. australis was the most common and widely distributed and was generally found along the edges of old agricultural drains. 5.The conditions within the site have led to the development of a mosaic of generally sheltered habitats, not present on the established mudflats outside the site, which have attracted a variety of bird, fish and epifaunal invertebrate species. Many of the fish species present are typically associated with freshwater and were not found in samples outside the site although a number of euryhaline fish were also found. Commonly occurring species included flounder and eel and the size distribution of flounder suggests that the site is acting as a nursery for this species. This is a valuable function of the site considering the historic habitat loss elsewhere in the estuary. The high densities of Neomysis integer caught suggest that this

species is an important food source rather than the benthic invertebrates which are not thought to be present at sufficient densities to support the number of birds using the site. 

6.Of the birds present, 16 species of waders use the site for feeding, 5 of which also use the site for roosting. Of these, lapwing, curlew, redshank and dunlin were the most frequently recorded. 18 species of wildfowl used the site for feeding, with 13 also roosting. Shelduck, teal, wigeon and heron were the most frequently occurring species. Particularly high numbers of golden plover were noted during November 2008. A number of the species using the site are of conservation significance and classed as Annex 1 species under the Habitats Directive. There is a broad association between bird distribution and habitat type with golden plover and curlew generally feeding in the infrequently flooded grassy areas, away from most other species. However, the majority of bird activity is in one sector which contains a wide variety of habitats. It is therefore difficult to draw conclusions about habitat preferences or use for individual species. 7.The Alkborough site has the dual aim of flood defence and compensation for habitat loss due to coastal squeeze. That is, replacement of mudflat habitat. Given the site design, the elevation and the restricted flooding, it is unlikely that this will be achieved in the long term and there is already evidence that the site will become vegetated. However, the site does appear to be acting as a nursery area for fish and is a significant feeding and roosting area for birds. In this respect, the development of the site appears to have been beneficial to this region of the estuary which is otherwise largely characterised by narrow mudflats with species poor communities and, in most areas, little vegetation.

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Catchment and subcatchment Select a catchment/subcatchment Site Edit site Name Alkborough 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 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 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 Funding for the scheme has come from the Environment Agency via the Department for Environment, Food and Rural Affairs (Defra,) Yorkshire Forward, the Heritage Lottery Fund and the European Union (EU) through the Interreg programme. The Alkborough Flats project is one of five being funded by the EU through the Interreg programme to promote new approaches to the impacts of sea level rise. Associated British Ports (ABP) have also contributed by allowing some of their land to be used as part of the scheme. 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