Project part-financed by the European Union (European Regional Development Fund)

The Interreg IVB North Sea Region Programme


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Alkborough Managed Realignment

Effectiveness according to development targets of measure

Flood protection

The main purpose of the measure was to provide flood protection by storing water. The high water event of November 2011 was the first event to cause overtopping of the overspill weir since the opening of the scheme. Data from this event has been used to illustrate the flood storage function of the scheme (Halcrow, in prep 2012). On the flood tide the rate of water level rise within the site matches that in the wider estuary when flows are constrained with the distribution channel. Once this is overtopped the rate of water level in the site slows and begins the lag behind the main estuary. If water levels in the estuary rise above the level of the overspill weir then the amount of water passing into the site increases rapidly. This raises water levels across the whole site at a rate comparable to that seen in the main estuary. The large inflow of water from the estuary reduces peak water levels in the estuary. The actual elevations within the scheme lag behind the main estuary. As a consequence of this lag, the scheme continues to infill during the initial part of the ebb tide, until water levels drop below the level of the overspill weir.

It has not been possible to demonstrate the impacts of the scheme on water levels by comparing high tides events pre- and post-scheme. This is because there is a poor correlation between pre-scheme water levels at Spurn Point and Goole indicating that tidal heights at Goole are also affected by other parameters (e.g. wind, freshwater flow, atmospheric pressure).

A linear extrapolation of the accretion rates show that siltation will reach the present day MHWS level in all sectors, except Sector 5A, in 13 to 35 years after site opening. However, the linear regression used for this calculation will tend to under predict the amount of time since accretion rates are likely to decline as surface elevations approach MHWS. Allowing for sea level rise does significantly alter these predictions as the level of MHWS can be expected to increase in the future due to sea level rise. It is possible that future accretion rates would increase in line with sea level rise which would reduce the time for the site to accrete to this level. Over time the site will continue to accrete and thus reduces the space available for water storage and reduce the effectiveness of the site to provide flood protection.

Habitat quality

There are no specific targets for the creation of compensatory habitat; general descriptions of the quality of various aspects of the habitats are described in the sections below.

Benthic community

The 2009 third year of monitoring report (Solyanko et al 2011) records that the benthic community composition inside the realignment site has not changed greatly over time and was composed of both freshwater species and estuarine species. Flooding inside the site is spatially variable which has led to variation in accretion rates and also a higher degree of variability in the sediment characteristics. Additionally, vegetation is either colonising the site or still remains from the pre-construction period and this, together with the spatial variation in flooding and the low salinity of the water, has led to the development of a mosaic of different habitats. Vegetation is colonising areas of infrequent flooding and/or of higher elevation. Many of these habitats are favourable to species of a freshwater or terrestrial origin, which accounts for the exceptionally high number of species from the class Insecta which have a fresh water larval stage to the life cycle.
Furthermore, the site is sheltered and does not experience the same cycles of erosion and deposition as the steep, narrow mudflat outside. Therefore, the benthic community as a whole appears to be more diverse with higher biomass inside the realignment that outside on the established mudflats. However, when considering the estuarine species only, it is clear that the community inside the site is not as developed as that outside with species richness and biomass being considerably lower. Additionally, the species composition is different with communities outside being dominated by the oligochaete Heterochaeta costata with smaller number of Paranais litoralis and P. litoralis being the dominant estuarine species inside the site, generally in higher numbers. Given that most of the infaunal species found in this part of the estuary inhabit the top 1-2 cm of the sediment, it is reasonable to assume that fluctuations in community structure would follow periodic erosion and events. Overall, the benthic community in the area consists of both freshwater and estuarine species. Previous assessment of sites on the Humber estuary (Allen, 2000; Mazik, 2004; Mazik et al., 2007; Mitchell, 2008) that the communities inside and outside the realignment site are impoverished, with low species diversity and abundance compared to more saline parts of the estuary. However, community structure outside the realignment site is typical of that area of the estuary (Allen et al, 2006).

It is unlikely that a diverse and abundant estuarine benthic community will develop at the realignment site for several reasons. Firstly, the elevation of the Alkborough site exceeds 2.8 m in all areas which is unfavourable for colonisation by infauna. The site is situated in a highly turbid part of the estuary with low salinity, a combination that is stressful for most species. In addition, the irregular flooding, and sections of the site that remain under water for a long time do not reflect the mudflats outside. Unlike the external mudflats which experience regular flooding by the incoming tide, the elevation of the site and its narrow, single breach mean that flooding occurs only on spring tides. Whilst this is a useful and key feature in terms of flood defence (which is the main function of the site), it does mean that the opportunities for larvae and adult benthic species to enter the site are limited. It is also important to point out that there are few species inhabiting the natural mudflats and it would be unrealistic to expect a diverse community to develop in the realignment site. In addition, the sheltered nature of the Alkborough site favours sediment accretion and for these reasons it is likely that a vegetated saltmarsh will develop rather than a mudflat.

Bird Usage

The Alkborough realignment site is well used by a large number of birds, which include a variety of species apparently attracted by the diverse nature of the habitat (mudflat, areas of standing water, reeds and areas of terrestrial vegetation). These habitats have attracted benthivorous (mainly waders), herbivorous/omnivorous (wildfowl) and piscivorous species (cormorants, herons and grebes). Thirteen species of wader and 18 species of wildfowl were observed feeding and/or roosting on the site during the 2009-10 survey (31 species in total). Five species were lost and two new ones were gained in comparison to the 2008-09 survey. The most important sectors for both roosting and feeding waders appear to be sectors 1, 4 and 5. Sectors 1, 2, 4 and 5 are frequently flooded areas of developing mudflat, and therefore probably have the most invertebrate food items.

Greater numbers of wildfowl used the realignment site for feeding, but greater numbers of waders used it for roosting. If the Nov/Dec 2009 high wader numbers are discounted, the wildfowl are the main users of the site for both feeding and roosting. However the presence of high numbers of wading birds during these months may be indicative of the realignment site being an important stopping point on migratory routes, or as an overwintering ground where food is available readily when it may not be elsewhere due to the cold weather. The abundance of estuarine benthic invertebrates is not thought to be sufficient to support the numbers of foraging birds seen regularly on the site and it is suggested that epifaunal species (e.g. Neomysis spp.) and, for some birds, fish may be the dominant food source.

The most frequently occurring species included typical mudflat species such as redshank and dunlin, curlew, lapwing and the wildfowl shelduck, teal, wigeon, and mallard (the wildfowl are typical of freshwater areas around the estuary). Occasional large numbers of golden plover were also recorded on the site. The site still appears to be supporting several avian species of international conservation importance including avocet (Recurvirostra avosetta) which breeds within the Humber European Marine Site, golden plover (Pluvialis apricaria) and bar-tailed godwit (Limosa lapponica) which qualify as Annex 1 wintering species occurring on the Humber (English Nature, 2003).

Fish

A varied fish assemblage has been identified, together with an equally diverse epibenthic invertebrate assemblage. Data shows that the fish assemblage changes seasonally and is dominated by small fish, which is consistent with the known role of estuarine areas as nurseries for juvenile fish and hunting grounds for subadult and adult fish. This is consistent with the current understanding of fish usage of shallow estuarine areas and intertidal habitats and correlates to the abundance and variety of small prey items. All these features are likely to be linked by functional interactions operating in a similar way and showing a broad equivalence to neighbouring estuarine habitats. This emphasises the value of managed realignment schemes in restoring historical estuarine habitat losses.

General effectiveness

The primary aim of the Alkborough site is flood defence (not compensation for land claim) and therefore, any estuarine habitat created could be regarded as valuable, particularly in the upper Humber where embankments limit the extent of the intertidal area. However, the site does have a role in compensation for habitat loss due to coastal squeeze.
The flooding and sedimentation regime within the realignment site have resulted in the formation of a mosaic of habitats which is considerably more diverse than that on the natural mudflats outside the site. Habitats present include mudflat, standing water, wet grassland and reed beds and fulfil the habitat requirements of a wide range of species or fish and birds. In contrast, the benthic communities are impoverished (throughout this region of the estuary) and do not yet represent those on the mudflats outside the site and are largely composed of freshwater and terrestrial species. The high elevation of the site and the small breach reduce the frequency of tidal flooding compared to that of the natural mudflats and this is thought to limit the potential for colonisation by benthic invertebrates and favour colonisation by vegetation.

High densities of epibenthic invertebrates and small fish have been recorded inside the realignment site and are thought to be the primary food source for the birds using the area. A high proportion of the fish using the site are typically freshwater species and are thought to be utilising the range of habitats within the site which are otherwise not available within the main channel of the estuary.

Overall the Alkborough site does not necessarily compensate for habitat loss and coastal squeeze in terms of mudflat creation and infaunal community development. It does however appear to be acting as a nursery area for fish and 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.

Over time it is anticipated that the site will become well vegetated and cease to be fully inundated by high tides. This will mean that the site will not provide the 'like for like' direct habitat compensation and its ability to provide water storage during extreme events will also be reduced. The Environment Agency is currently investigating options for changing the breach configuration and altering the scheme to a regulated tidal exchange scheme.


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