Description • Monitoring • Effectiveness of the scheme • Further information

Orplands Sea Wall, Blackwater Estuary, Essex

Description

The site forms part of the St Lawrence Bay and is situated on the southern side of the Blackwater estuary in Essex . The site includes two retreat areas - tidal mudflats and an area of saltmarsh to seaward - and has two distinct parts; Site A and Site B, which are next to each other along the coast, but separated by a counterwall. The two sites had different pre-inundation characteristics:

• Site A was rough grassland before it was reopened to tidal flooding, it had not been ploughed and the line of the original major creeks can still be seen as shallow, linear depressions.
• Site B was used for cereal production and clay pipe land drains had been installed approximately 1m below the land surface to lower the water table in the field.

Map of the Orplands study areas. Area 1 - mudflats seaward of the managed retreat sites; Area 2 - Orplands A retreat site, Area 3 - Orplands B retreat site, Area 4 - mudflat seaward of the control saltmarsh; Area 5 - saltmarsh (Environment Agency).

The seawall protected 40 hectares of agricultural land which was divided into SSSI grazing marsh and land in arable production. Land levels ranged from -1m OD to above +4m OD. The original line of the sea wall was constructed in the 18 th century and until the 1950’s was fronted by saltmarsh and a high level mudflat foreshore. Sea wall condition deteriorated as the fronting saltmarsh eroded and, despite extensive previous maintenance works, it was estimated in 1994 that in excess of £600k would be required to sustain the defence over a 20 year period; at 15k per hectare it was determined that it was uneconomic to invest in conventional sea wall construction and repair techniques.

Breaching took place in April 1995 and involved demolishing the seawall to allow normal tidal exchange to take place. Breaching took place over a period of 4 days during neap tides. A series of nine meandering vertically sided 'creeks', one metre deep, were excavated within the site to facilitate tidal flow over the area.

Two new set back walls were constructed to the north and south of the site to protect adjacent areas from saline intrusion and flooding. Flapped pipes were laid on the site to allow freshwater drainage onto the new saltings. Material from the breach was re-deposited landward to act as a buffer to wave energy penetrating through the breach.

Tall vegetation at the rear of the site was left to decay naturally and provide organic material to the site and also to act as sediment trap for fine sediments.

 

Orplands sea wall post breach; looking westwards along seawall from eastern end of site, showing badly eroding saltmarsh outside of the breached sea wall (to the right) and retreat site to the left of the picture (Environment Agency).

Monitoring

As Orplands was the first deliberate managed realignment site in the UK, a comprehensive pre- and five year post-construction monitoring program was undertaken to assess the effects of the scheme. The following items were monitored within the realignment site plus a control site:

Vertical accretion / erosion rate on both foreshore and salting.

• Lateral erosion / accretion rates.
• Various physical parameters including cohesive strength and organic content.
• Various chemical parameters including pH, redox potential and nutrient content.
• Various hydrographic parameters, including wave energy and current speeds.
• Changes in saltmarsh morphology in terms of creek lengthening and areas of die back.
• Changes in plant vigour in terms of % species cover, plant height and plant community.
• Composition and abundance of benthic invertebrates, fish, mammals and birds.

Effectiveness of the scheme

Annual monitoring by the Environment Agency has shown that saltmarsh vegetation has developed on the higher elevations of the sites and is dominated by pioneer Salicornia spp. communities, with areas of intertidal mud becoming established on lower areas. The higher elevation of Site B (former arable land) led to the colonisation of Salicornia with larger expanses of mud developing on the lower Site A. This, in turn, led to the build up of soft muddy sediments at the seaward edge of the retreat site, which has been colonised by invertebrates. Mobile species, those that have a planktonic larval phase (such as Nereis and other polychaetes) and Hydrobia have colonised these muddy sediments; while bivalves and other species that have no planktonic larval phase, such as oligochaetes, have either not colonised or did not appear for a number of years.

The poorer groundwater drainage of Site A, which had not been ploughed or drained since enclosure, produced highly anoxic conditions. It is thought these conditions prevented plant colonisation below 2.5m ODN. For the same reason, bivalve mollusc species, though present in substantial numbers in the nearby mudflats, failed to colonise the same anoxic mudflats;

The speed of development of the waterbird assemblage followed that predicted from the changes in benthic fauna. The build up of fine muddy sediments led to the assemblage being initially dominated by Redshank, Dunlin and Grey Plover, which feed predominantly on Hydrobia and Nereis (the two main invertebrate species to colonise in the first few years following the breach). Knot was initially absent but their colonisation coincided with the appearance of the bivalve Macoma on the sites, a preferred prey item, approximately three years after the breach. Species such as Oystercatcher and Turnstone, which feed on molluscs and crustaceans, were very scarce on the sites during winter, although common on the surrounding estuary. In the short-term, it is unlikely that the retreat sites will develop in such a way that populations of these two species would be supported. The large molluscs, preferred by Oystercatchers, were virtually absent at least five years after the breach and it may take some years for these to colonise, if they ever do. However, experimental introduction of these species may speed up the colonisation process.

There are still extensive areas of bare mud and pioneer plant communites on the Orplands site, and in the south-west corner of the site there is a transition from saltmarsh to non-tidal grassland not interrupted by a seawall; a transition that is rather unusual in East Anglia because of the extensive land claim that has taken place on the upper edge of most saltmarsh sites in this region.

Further information

Atkinson, P.W., S. Crooks, A. Grant and M. Rehfisch (2001). The success of creation and restoration schemes in producing intertidal habitat suitable for waterbirds. English Nature Research Report No. 425.

Environment Agency (1999). Results of post breach monitoring of Orplands coastal realignment site. Environment Agency, Peterborough.