Background to major recent and proposed port development in the Harwich Haven
The Harwich Haven, in England, is formed by the confluence of the estuaries of the River Orwell (Suffolk) and River Stour (Essex) (see Figure 1). The area has a long maritime history, with the last century seeing significant development of port facilities, most notably at Felixstowe and Harwich. In terms of volume of container handling, the Port of Felixstowe is the largest container port in the UK and has a total berth length of over 4,100m. Harwich International Port currently has a total berth length of over 1,600m.
The scope of the discussion in this paper focuses on those major port expansion and capital dredging projects that have been undertaken in the Harwich Haven area since 1998, in particular the deepening of the dredged approach channel by the Harwich Haven Authority (HHA) in 1998-2000. This capital dredging project comprised dredging approximately 18Mm3 of material and deepened the channel by 2m to -14.5m Chart Datum (CD). In addition, in 2003 construction commenced on a 270m extension to the Trinity III Terminal at the northern end of the Port of Felixstowe to create additional deep-water berthing facilities. Capital dredging of approximately 900,000m3 of material was associated with this extension. This construction work was completed in late 2004.
Further more, there are cur rently two proposed port developments in the Harwich Haven; the Bathside Bay Container Terminal and the Felixstowe South Reconfiguration. Both proposals involve reclamation of intertidal and subtidal areas and capital dredging of the approaches and berths. These proposals were both the subject of local Public Inquiries in 2004. In December 2005, the Bathside Bay Container Terminal proposal was granted a ‘minded to approve’ decision and in February 2006 the Felixstowe South Reconfiguration was fully approved.
Designated sites within the Stour and Orwell estuary system
There are a number of sites designated for their nature conservation importance within the Stour and Orwell estuary system. Both estuaries are designated as separate Sites of Special Scientific Interest (SSSI) under the Wildlife and Countryside Act 1981. These two SSSIs form the Stour and Orwell Estuaries Special Protection Area (SPA) (classified under the ‘Wild Birds Directive’) and Ramsar site.
Environmental studies in the Stour and Orwell estuary system
As a consequence of the presence of major ports in the Stour and Orwell estuary system and the various proposals for development, the estuary is one of the most intensively studied systems in the UK and, arguably, in Europe. In particular, knowledge of the hydraulic processes and sediment budget of the estuary has been built up over a number of decades, resulting in well calibrated and validated numerical models of the system. Such knowledge is fundamental in that it provides the ability to predict the likely effects of development on the hydraulic and sediment regime of the estuary system with a high degree of confidence. These predictions are crucial in enabling detailed environmental assessment of the potential impacts of development on, for example, the morphology of intertidal areas to be undertaken and for informing the development of practicable mitigation measures to offset the predicted impacts of development, where required. Such environmental effects and mitigation measures have been reported through the Environmental Impact Assessment (EIA) process that has been undertaken for each of the proposed developments in Harwich Harbour.
In view of the designated status of the estuary system and the nature of the environmental impacts predicted to arise from each of the port developments and capital dredging projects described above, each project has, as part of EIA, also been subject to ‘appropriate assessment’ in accordance with Regulation 48 of the Conservation (Natural Habitats &c.) Regulations 1994. The purpose of appropriate assessment is to assess the implications of a proposed development for the designated status of any relevant ‘European sites’ (e.g. SPA), as defined through the sites conservation objectives (which derive from the reasons, or qualifying features, for which the site was originally classified). In essence, it is the appropriate assessment process which has driven the need to develop appropriate mitigation and compensatory measures in light of the various predicted effects of development on the habitats and species for which the Stour and Orwell estuary system is designated.
Harwich Haven approach channel deepening
In 1998, work commenced on a project to deepen the approach channel into the Harwich Haven from its existing depth of -12.5m CD to -14.5m CD to improve navigation. The capital dredging would give rise to approximately 18Mm3 of sediments, comprising a mixture of mud, sand and gravel and clay, most of which was unusable (clay) and was disposed of offshore in a designated disposal ground. Royal Haskoning (previously Posford Haskoning and Posford Duvivier), in association with HR Wallingford, undertook an EIA for the proposed channel deepening [HR Wallingford, Posford Duvivier Environment, (1998), ‘Harwich Haven Approach Channel Deepening – Environmental Statement’, January 1998.].
Overview of key predicted impacts
The environmental studies concluded that the channel deepening would give r ise to a number of effects on the hydraulic and sedimentary regime of the Stour and Orwell estuary system. It was predicted that the effect of the channel deepening on tidal propagation would result in an increase in the level of the low water mark on spring tides (i.e. there would be a decrease in the tidal range) which, when the slope of the intertidal throughout the system was taken into account, notionally equated to the decreased exposure of approximately 4ha of intertidal area. In terms of the area of intertidal exposed for feeding waterbirds, this effect is viewed as a loss of intertidal area in the system; in reality, it is the conversion of intertidal habitat to shallow subtidal for a majority of tides. It is not possible to mitigate this effect. In addition to the above, it was predicted that the rate of intertidal erosion within the estuary system would increase by approximately 2.5ha per annum. This effect was predicted to arise as a result of the trapping of muddy material in the deepened channel which would previously have been transported further into the estuary system and been deposited onto the intertidal areas. The trapped material would then be dredged during routine maintenance dredging campaigns (undertaken on an approximately 12 week basis by the HHA) and disposed at a designated site offshore. The maintenance dredging would, therefore, represent a mechanism by which fine material is lost from the estuary system. It should be noted that the Stour and Orwell estuary system is currently an eroding system, with an estimated background rate of intertidal erosion in the order of approximately 10ha per annum overall.
In order to mitigate the predicted increase in the rate of intertidal erosion of approximately 2.5ha per annum, a number of different approaches were explored, as summarised below (collectively termed ‘sediment replacement’). Over time, and based on the results of trial placements of sediment and monitoring, the overall mitigation strategy has evolved into an approach that is considered to be the most effective (relying substantially on water column recharge; see below). The locations that are currently licensed for the placement of maintenance dredgings are shown in Figure 2.
• Subtidal placement of fine material
Fine sediments that are dredged during the maintenance dredging campaigns are placed on the seabed and act as a feed of material into the estuary system.
• Water column recharge
Maintenance dredgings are discharged from the dredger at certain defined placement locations within the estuary system adjacent to intertidal areas. Placements are made under specific tidal conditions that encourage material to disperse over intertidal areas. This represents a novel approach that has been proven to be successful.
• Increased overflow during maintenance dredging
During maintenance dredging, overflow can be increased above the normal rate. This represents a further method of returning fine material within the estuary system.