Sutherland D. - HR Wallingford

Introduction

The management of port developments is increasingly challenging because of the requirement for deeper channels for ship passage and the need to preserve important coastal wetlands and ecosystems. HR Wallingford, UK, has been closely involved in projects associated with these issues for a number of years dredging channels and restoring marine ecosystems. One particular site where HR Wallingford has been active is in and around the Port of Felixstowe and Harwich Harbour (the Haven Ports) on the Stour/Orwell Estuary (Figure 1). The estuary is internationally important for its wetland bird populations and the intertidal areas are protected under European legislation.

Figure 1. Locations of initial sediment recycling placements. Image credits to Spearman J., Baugh J., Feates N., Dearnaley M., Eccles D. (2014). Small Estuary, Big Port - Progress in the management of the Stour-Orwell Estuary system. ​

Habitat restoration and sediment replacement

Works to deepen the approach channel to the Haven Ports commenced in October 1998 and were completed in April 2000. Consent conditions included habitat and sediment replacement and the requirement “… to avoid any impacts as a result of the dredge on the favourable conservation status of both [the Stour and Orwell] habitats”.

Predictive flow and sediment transport modelling was used as the basis for the development of a sediment replacement strategy to offset the perceived impacts on intertidal habitats.

The modelling predicted that deepening of the harbour and the approaches would lead to trapping of material in the harbour, a reduction in sediment supply to the Stour and Orwell Estuaries and an increased rate of loss of habitat of 2.5 ha/yr. To offset this 200,000 tonnes dry solids (TDS) (annual average) was replaced in the Stour/Orwell system during dredging operations between 1998 and 2007. These operations were the first large-scale mitigation of its type in the UK. Monitoring included bathymetry surveys, LiDAR measurements (for saltmarsh habitats), bird counts, benthic ecology and fish (including shellfish) monitoring. 

Following the implementation of the sediment recycling, the fishing community became concerned by accumulations of silt at various locations within and just outside the estuary system. The evidence led to a modification in 2008 of the recycling strategy reducing sediment inputs to the estuaries significantly (a total of 50,000 TDS/yr). To date this modified mitigation appears to be successful in enhancing intertidal habitat whilst not causing adverse effects on fishery interests.

Figure 2. Saltmarsh in the Stour Estuary. Photo credits to Royal Haskoning (2013) Stour and Orwell Estuaries Annual monitoring, Analysis of saltmarsh extent, February 2013.

The benefits of the sediment replacement through dredging

Prior to the deepening of the approach channel to the Haven Ports the Stour estuary was losing intertidal area at a rate of 13 hectares per year. Since the deepening, LiDAR and bathymetric surveys in 2005, 2010 and 2015 have shown that the programme of sediment cycling has been successful in reducing both the predicted effects of the channel deepening and in offsetting intertidal erosion prior to 1998. Analysis of the extent of saltmarsh in 2005 and 2010 in the Stour and Orwell (Figure 2) was undertaken using aerial photography and site visits by Royal Haskoning DHV. After an initial loss of saltmarsh habitat between 1997 and 2005, the area of habitat has now increased following the placement of sediment in the estuaries.

The careful placement of sediment in the Stour and Orwell estuaries, supported by modelling and monitoring, has shown that this methodology can contribute effectively to restoring and maintaining marine ecosystems.

For further information contact David Sutherland, Principal Scientist, HR Wallingford, UK (D.Sutherland@hrwallingford.com).