An assessment of the potential impact of dredging activity on the Tamar Estuary over the last century: Bathymetric and hydrodynamic changes

The Tamar Estuary, S.W. England, is a commercial and military port and the site of a major naval dockyard. Parts of the estuary require regular maintenance dredging and some capital dredging has been undertaken. The estuary is also assigned Special Area of Conservation (SAC) status under the European Habitats Directive and is an important site for migrating wildfowl and wading birds. As part of an assessment undertaken for a consortium of the port operators and regulatory authorities, we have analysed the ecological, physical, chemical and socio-economic impacts of dredging on the Tamar Estuary ecosystem. This paper focuses on the physical changes through the analysis of historical survey data and the use of hydrodynamic modelling. The objective was to evaluate whether dredging has influenced the estuary morphology or hydrodynamic regime and thus impacted on the ecological habitat. Bathymetric survey data collected between 1895 and 1968 were compared with multi-beam echo sounder data obtained in 2001. The paper survey records were digitised, geo-referenced and adjusted to the modern datum and units. The accuracy and limitations of this approach are discussed. Both sets of data were contoured and analysed using a GIS package. Comparisons between 1895 and 2001 hydraulics were made using a 1-D hydrodynamic and sediment transport model set up for the bathymetry at those times but 'driven' using a single, common, year-long data set for tides and river flows. The results suggest that although the deep channel in the region that is mainly south of the Tamar road bridge, is on average 0.5 m deeper in 2001 than it was in 1895, this has had a negligible effect on the area of intertidal mud and, thus, on habitat available for wading birds, and minimal impact on the large-scale hydrodynamics of the estuary.