Description • Monitoring • Effectiveness of the scheme • Further information

Shotley and Trimley Habitat Enhancement Schemes, Orwell Estuary

Description

In October 2004 The Port of Felixstowe completed the extension of an additional 270m of quayside at its Trinity III berths. The scheme also involved dredging of the main approach channel to enable larger ships with deeper draughts to berth at the newly extended quay. At the same time, the Stour and Orwell Estuaries SPA is currently being degraded in the lower Orwell due to underlying erosion of the intertidal areas that have been ongoing since the 1920s. In parallel with this the seawalls at Trimley and Shotley were both in a very poor state of repair, thereby threatening the integrity of the freshwater sites behind them due to the likelihood of a breach.

As well as mitigating for the effects of the project on the adjacent SPA the scheme was also seen as an opportunity to improve the stability of the existing lower Orwell flood defences and to provide intertidal habitat of enhanced value for feeding waterfowl. More specifically, it was hoped that 3ha of saltmarsh habitat would develop following the completion of the habitat enhancement.

Shotley

On the Shotley foreshore (see picture below) the schemes involved the construction of clay bunds around Shotley Marina, and backfilled with silt. Further north, two areas of existing gravel were topped up (see North Shotly Case Study) with silts and another scheme was constructed based on bunds created using in-situ material backfilled with 0.3m of silts.

Shotley habitat enhancement scheme

Trimley

On the Trimley side of the Orwell, schemes were implemented that utiised both clay and gravel for the bunding. Following the removal of silt from the approach channel, gravel was dredged from the new approaches and placed in the quay construction zone and onto the Trimley foreshore in order to construct the habitat enhancement bunds, including a gravel island for nesting little terns. Approximately 22,000m³ of gravel was placed on the Trimley foreshore for the bunds and 80,000m³ stored for the gravel island, which covers approximately 1 hectare above high water.

Trimley habitat enhancement scheme

Monitoring

A monitoring strategy was prepared by Posford Duvivier Environment (PDE) and agreed with CEFAS. The monitoring comprised the following:

• Monitoring of saltmarsh vegetation at three replicate quadrats at each sampling station, with two sites established per 100m stretch of foreshore
• Monitoring of the level and gradient of the bund and mudflat at 5 transects at Trimley and 10 transects at Shotley.
• Monitoring of benthic invertebrates at five sampling stations within each habitat enhancement scheme and 3 reference stations on the foreshore adjacent to each scheme. 3 replicate cores taken at each station.
• Monitoring of waterfowl between February and November as part of the estuary-wide low water counts.

Effectiveness of scheme

Surveys of the habitat enhancement bunds have been carried out on behalf of HHA in November 2003, March 2004, July 2004 and March 2005.

In terms of saltmarsh development, colonisation has occurred at both sites with small patches of Salicornia recorded at Shotley and localised Spartina colonisation recorded at Trimley.

Topographic surveys of both sites have revealed marked differences in the behaviour of the bunds. Whereas the Trimley placements have remained relatively stable the clay bunds surrounding Shotley have been highly mobile with the backfilled material remaining soft. The original backfill level of this area was about 2.4m CD which has reduced by up to 0.6m in places. The reductions in elevation here are likely to have been associated with both consolidation and mass erosion/loss of the placed material. The original North Shotley scheme levels have fallen by up to about 0.3m, with the centre of the site remaining high

Benthic monitoring of invertebrates show that in November 2003 (one month after completion of the bunds) the benthic community was, in general, very impoverished. For example in many of the cores only 1 to 2 species were found, which is far less than for cores taken from the adjacent muddy foreshore. However, this finding is to be expected given the timing of the mud placement (one month earlier). Results from the previous monitoring surveys are still being analysed and are due to be reported towards the end of 2005.

Further information

Posford Duvivier Environment (2001) Trinity III Terminal (Phase 2) Extension. Compensation, Mitigation and Monitoring Agreement. Unpublished report to Port of Felixstowe.