Description • Monitoring • Effectiveness of the scheme • Further information

North Trimley Marsh, Orwell Estuary

Description

In October 1998, the Harwich Haven Authority (HAA) commenced works to deepen the approach channel to the Haven Ports. As a condition of the various consents for the scheme a mitigating and monitoring package was agreed with English Nature, the Environment Agency and Defra to minimise the adverse impacts of the scheme on the Stour and Orwell Estuaries SPA. As a result, in November 2000 16.5 ha of intertidal habitat was created through managed retreat at North Trimley Marsh.

The site was created on former arable land. The western perimeter is made up of the seawall which formerly separated the Orwell estuary from the farm land. A new counter-wall was created around the north, eastern and southern perimeter of the land.

One of the required criteria for the site was that no more than 30% of its area should develop into saltmarsh. At least 70% of the retreat area was, therefore, designed to be below the level of +3.5m CD, as saltmarsh was estimated to develop at levels higher than +3.5m CD. The entrance (i.e. the breach in the sea wall) was designed at +1.5m CD so that the site would completely drain at low tide.

Maintenance dredgings (fine mud) from the channel in the lower Orwell estuary were pumped into the site to provide a suitable substratum for colonisation of estuarine flora and fauna. Approximately 35,000m³ of dredged material was dredged by a trailing suction hopper dredger and pumped via a floating pipeline through the breach to a floating pontoon where it was pumped onto the site. The material was deposited in four locations at high water, from where it was spread out over the site. The dredged material formed a layer approximately 30 to 50cm deep above the underlying soil.

Trimley Marsh Managed Realignment Site (Posford Haskoning, 2004)

Monitoring

Monitoring has been carried out on North Trimley Marsh since November 2000 and has involved the following:

Benthic invertebrates

Cores (11cm diameter by 15 cm deep) are taken from 12 stations. The sampling stations are mostly on the middle and lower shore, roughly evenly distributed over the site. Each station is marked with a labelled stake and the position recorded with GPS. Two replicate cores are taken from each sampling station (within an area of approximately 2m²), giving a total of 24 samples over the whole site.

Particle size analysis

Single process particle size analysis is carried out on 8 of the samples and double process on the remaining 4. As the majority of the samples comprise silt (less than 63μm), the single process analysis is undertaken for the <63μm fraction only.

Vegetation

Vegetation monitoring is undertaken using quadrat sampling. Fixed quadrats, each covering an area of 2m by 2m, were established at regular intervals around the perimeter of the site (where vegetation may be expected to colonise) and marked at two diagonal corners using stakes. The position of each quadrat was recorded using a GPS. Within each quadrat the percentage cover of each species is recorded and target notes made.

Waterfowl

The waterfowl usage of the site is monitored by means of counts undertaken around low water at monthly intervals throughout the over-wintering period (November to February) by the Suffolk Wildlife Trust. These low water counts form part of the ornithological monitoring of the whole of the Stour and Orwell estuary system being undertaken by Suffolk Wildlife Trust on behalf of the HHA. Counts of the rest of the estuary are taken at the same time, with the managed retreat site counted as a separate sector. Each count is started 1.5 hours before low water and continued, where necessary, to 1.5 hours beyond low water. All species present are identified and counted.

The second, third and future years of monitoring have been designed to be less intensive than the first. In the first year, benthic and vegetation surveys were carried out at roughly 3-monthly intervals up to June 2002. Since this time, surveys have been carried out approximately every six months.

Effectiveness of the scheme

The site has become more ‘natural’ over the first three years of monitoring. This is mainly due to the mudflats becoming ‘smoother’ in appearance, with more small creeks developing and the vehicle tracks (left from construction) evening out.

Monitoring carried out in the second year of monitoring (post breach) showed distinct changes in the benthic community of the site. Initially, during the first year of monitoring, the benthic community was characterised by a high degree of evenness between the numbers of each of the different species that make up the community and the community was dominated numerically by Macoma balthica. As the site has developed there have been marked increases in both biomass and number of individuals per core. However, the type of community has also changed. Between September 2001 and September 2003, the community was dominated by extremely high numbers of Hydrobia ulvae.

Around 1.6 ha of saltmarsh has developed around the fringe of the site and along the facing of the new sea wall. The saltmarsh community is mainly dominated by the pioneer species Salicornia, and appears to be behaving naturally, following distinct seasonal variation in growth and die-back. Overall, the saltmarsh is developing well and remains within the original target for its extent. The site continues to support high numbers of birds, particularly Dunlin and Ringed Plover.

Further information

Posford Haskoning (2004). Trimley Managed Retreat Site. 2nd Annual Monitoring Report (2002 - 2003). Unpublished report for the Harwich Haven Authority.