United Kingdom: England: Salt Marsh Reclamation in the Blackwater Estuary

Project Stage:
Implementation

Start Date:
1995-04-08

End Date:
1995-04-08

Primary Causes of Degradation

Degradation Description

This 21-hectare site formed part of the Tollesbury and Old Hall salt marshes until it was embanked in the late 18th century and transformed into agricultural land (Boorman et al. 1997).

Reference Ecosystem Description

The Blackwater Estuary is a breeding area for the Little Tern (Sternula albifrons), and a transit point for the Ringed Plover (Charadrius hiaticula). Species that winter in the estuary region include the Pied Avocet (Recurvirostra avosetta), Black-tailed Godwit (Limosa limosa islandica), Dark-bellied Brent Goose (Branta bernicla bernicla), Dunlin (Calidris alpina alpina), Eurasian Golden Plover (Pluvialis apricaria), Grey Plover (Pluvialis squatarola), Hen Harrier (Circus cyaneus), Common Redshank (Tringa totanus), Ruff (Philomachus pugnax), and Common Shelduck (Tadorna tadorna).

Project Goals

– To retreat the line of coastal defence
– To restore salt marsh habitat for conservation purposes by breaching the existing flood embankment
– To investigate and document the re-establishment of natural intertidal processes and habitat

Monitoring

The project does not have a monitoring plan.

Description of Project Activities:
Economic considerations necessitated a breach of the embankment, rather than its complete removal. Modelling studies undertaken by HR Wallingford revealed that creating a double breach would result in the development of a circular flow within the site. Therefore, a single breach was designed in the east of the flood embankment with dimensions of 50m wide at the embankment crest with 1:5 slopes at the base at 0.8m ODN. Prior to breaching the embankment, plots were flooded via sluices in order to raise the salinity of the sediments. Certain plots were then seeded with glasswort and seablight, while others were planted with plugs from natural salt marsh surrounding the site containing sea aster and sea lavender. On August 8, 1995, a single breach was made in the embankment to provide tidal input to the site. The wall was first lowered over a distance of ca. 1m and the tide allowed to flow into the site. Tidal velocities reached 7 cubic meters per second, which were sufficient to widen the breach to ca. 3m after 1.5 hours. Over the following days, the breach was gradually widened to its pre-determined width of 50m. A drainage ditch was cut in the base of this breach to 0.2m ODN to connect with the existing creek system. A herringbone drainage system was then constructed. The strip of salt marsh from the front of the embankment at the breach site was transplanted to the side of the creek. As a result of public pressure, existing hedgerows and trees were not removed, but were left to die naturally. A considerable fetch builds up across the site, and these hedgerows have formed natural wavebreaks. Monitoring activities followed the managed realignment, and natural plant colonization in the restoration site (i.e. community species pool) was monitored annually beginning in 1997 (i.e. the second growing season after de-embankment). Three transects of 20 m wide and 125 m long, subdivided into 2500 contiguous quadrats of 1 m2 were established. Each transect was laid out perpendicular to the new seawall and extended for 125 m from high to low elevation towards the breach.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
The number of established target species in the restoration site was less than 10 % of the maximum regional species pool in the first year after de-embankment. Our results show that it took approximately five years for species diversity in the restoration site to become similar to a local reference marsh (Old Hall and Tollesbury marshes), which contained less than 40 % of the species of the regional target species pool. Colonization of the transects started with the arrival of Salicornia spp., Suaeda maritima, Sarcocornia perennis and Spartina anglica. In the third year of restoration (i.e. an intermediate stage), the biggest increase in number of species occurred with the establishment of Aster tripolium, Atriplex portulacoides, Puccinellia maritima and Spergularia marina in the transects. The "late establishers" included Elytrigia atherica, Limonium vulgare and Spergularia media, which established four or five years after de-embankment. Salicornia was by far the most abundant species in the transects during the entire study period, followed by Suaeda in the third and fourth year after the breach. From the sixth year after the breach, Puccinellia became the dominant species of the upper part of the transects, and gradually extended seaward; whereas Spartina increased its abundance at lower elevations. The upper one metre strip of transect 1 was dominated by Spergularia marina in the fourth and fifth year after the breach. All species except Salicornia and Spartina occurred in highest abundance at the upper zone of the transects. The majority of species in the upper zone showed a sigmoidal increase in frequency of occurrence over time, either towards the maximum value of 100 % (Aster Puccinellia, Salicornia, Spergularia media, Suaeda) or towards a lower value (Atriplex portulacoides, Limonium). Spergularia marina showed an optimum in frequency of occurrence after seven, four and six years in transects 1, 2 and 3, respectively. In the intermediate zone, where five of the nine species occurred with a frequency of more than 10 % for three consecutive years, response curves for all species except Suaeda were also sigmoidal towards the maximum value of 100 %. Suaeda showed a peak after two years of restoration, after which the abundance rapidly declined, especially in transect 2. Based on abundance and percentage cover data, three different National Vegetation Classification (NVC) communities could be distinguished in the restoration site by 2001 (TableFit, Hill (1996)). The upper part of the transects changed from annual Salicornia salt marsh (SM8, (Rodwell 2000)) in the first three years of restoration via Suaeda maritima (SM9) salt marsh in transects 1 and 3 to Puccinellia salt marsh (SM13a) after six or seven years of restoration. In the intermediate and lower zones of the transects, annual Salicornia salt marsh formed the only distinct plant community. The NVC communities of the adjacent marsh included annual Salicornia salt marsh (SM8) at the lowest zone and Puccinellia salt marsh (SM13) at the two higher zones. No unexpected non-target species have established so far, indicating the suitability of the site conditions for the species from the pre-defined species pool. After eight years of restoration, 11 target species, or 32 % of the regional species pool, had established in the Tollesbury restoration site. This saturation index was low compared to other de-embankment sites of similar or younger age in north-west Europe where indices were generally above 45 % (Wolters et al. 2005b, Chapter 3). The number of species that colonized the restoration site at Tollesbury was also lower compared to two other managed realignment sites in the same estuary. The oldest of these sites, Northey Island, had already contained 12 target species after three years of restoration, whereas at Orplands, 15 different species had established within seven years after the seawall was breached (Wolters et al. 2005b, Chapter 3). On the other hand, the rate of plant colonization at Tollesbury was similar to that of an estuarine restoration site in the USA, which was opened to tidal inundation after being embanked for 70 years (Thom et al. 2002). During the period before any vegetation cover developed, results showed a net negative sediment budget for the restored sites as tidal flows eroded the areas adjacent to the breaches. Sedimentation rates were high during the summer months when the Salicornia sp. was flourishing, but the deposited material was removed during the winter when the vegetation died back. Consequently, the result for the first 3 years was zero annual accretion. However, as soon as perennial species such as Puccinellia maritime, Atriplex portulacoides, and Spartina anglica began to colonize the restored surfaces, erosion of the surface during the winter months was reduced, resulting in net annual accretion. Bird surveys and observations indicate that the site is being used by a variety of birds, including black-tailed godwit, dunlin, redshank, grey plover, brent geese and two egrets.

Factors limiting recovery of the ecosystem:
Atriplex portulacoides and Limonium were the only two species suggesting a maximum abundance of less than 100 % had been reached at the highest elevation zone within the study period. Although Atriplex has good reproductive ability, producing between 20 and several hundred seeds per plant and capable of spreading vegetatively at a rate of 1.3 cm per year, the species and especially its seedlings is intolerant to waterlogging (Chapman 1950). The prevalence of poorly drained soils may therefore explain its restricted abundance at Tollesbury, especially since Puccinellia, a species that tolerates waterlogged soils, was the first perennial species to reach 100% abundance. Generative reproduction in Limonium may be limited as the plants rarely flower until the third year (Boorman 1967) and viability of the seeds is very low compared to other salt-marsh species (Hutchings & Russell 1989). This species is a poor competitor for light (Boorman 1967) and its abundance may be limited by the presence of other perennial species. In general, the sigmoidal increase in abundance of perennials in our study suggests that their population dynamics are mainly determined by clonal growth, possibly as a result of limited germination and seedling establishment due to high soil salinities (Shumway & Bertness 1992). It was originally hypothesised that species abundance in the species pools and dispersal traits would be among the most important factors for plant recolonization. Nevertheless, the discriminant analysis indicated that salt tolerance was more important in determining the sequence of species establishment than abundance of the species in the local species pool or dispersal traits. For example, Triglochin maritimum was still absent from the restoration site after eight years, even though it was present in 12.2 % of the plots on the adjacent marsh. The Ellenberg value for salinity describes this species as being characteristic of salt meadows and upper marshes subject to occasional tidal inundation, or consistent brackish conditions (Hill et al. 1999). Other species present (though with a frequency of less than 2 %) on the adjacent marsh but absent from the restoration site include Armeria maritima, Festuca rubra, Juncus maritimus, Plantago maritima and Seriphidium maritimum. The Ellenberg values for salinity of these species are all below six, indicating that these species are slightly salt tolerant and generally occur on upper marsh sites not inundated by all tides. With less than 10 % of the restoration site at an elevation above the level of mean high water spring tide these species have little chance to establish. There has been some criticism of the provision of a breach rather than the total removal of the embankment. Although the remaining flood embankments are not to be maintained and will eventually fail, this may take several decades. In the meantime, the tidal flows within the site are constrained and the difference in elevation between the breach and the minimum surface of the site means that the lower parts of the site act as a sump at low tide. This, in turn, means that sedimentation within the site will tend to be concentrated within the "˜sump' area, and higher areas will receive commensurately less material. Thus, sedimentation will undoubtedly influence the rate and extent of recovery at the site. Another factor that will determine the success of the managed realignment at Tollesbury is the scarcity of salt marsh species in the community species pool. The site was first embanked more than two centuries ago, and the halophytic species disappeared from the established vegetation. Hence, target species have to disperse into the restoration site from a local species pool (i.e. adjacent salt marsh) or regional species pool (i.e. all salt marshes within a biogeographically uniform region) (Zobel et al. 1998). This is a slow and uncertain process, and managed-realignment sites elsewhere in northwestern Europe have seen only 26 to 64% of species from the regional species pool successfully re-establish themselves (Wolters et al. 2005b).

Socio-Economic & Community Outcomes Achieved

Key Lessons Learned

Observations have shown that the first species colonizing the Tollesbury restoration site are highly salt tolerant with two of the three species also being abundant in the local species pool. These species occupy the lowest elevation zones, which are inundated most often and which may increase the chance of a diaspore being transported from the source area into the restoration site. Species that are still absent are characteristic of higher marsh zones which are occasionally inundated. Their absence is mainly explained by the sparse availability of suitable space for establishment, possibly in combination with low abundance in the local species pool and reduced opportunity of dispersal by tidal water due to their occurrence at higher and less frequently flooded elevations. The difference between intermediate and late establishers appears to be related mainly to a combination of abundance in the local species pool and flotation time. Sowing or planting these species may help to increase their rate of establishment.

Long-Term Management

Monitoring of this site is ongoing.

Sources and Amounts of Funding

This project was funded through the MAFF’s Flood and Coastal Defence research programme. Subsequent monitoring activities have been conducted with the support of DEFRA.

Other Resources

Natural England
http://www.english-nature.org.uk/livingwiththesea/project_details/good_practice_guide/habitatcrr/ENRestore/CHaMPs/EssexEstuaries/index.htm

DEFRA
http://www.defra.gov.uk/

Primary Contact

Organizational Contact