Dredging operations and other port activities may damage the marine environment by re-mobilising organic and heavy metal pollutants stored within older sediments deposited in channels and harbour basins. These sediments tend to absorb chemicals such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). Cost-effective techniques are needed for surveying, sampling and modelling such sediments, as an input to dredging management and the identification of the most appropriate sediment treatment and removal techniques.
The aim of H-SENSE was to develop predictive sedimentological models for the management of harbour activities with regard to silting, evaluation of pollution, dredging procedures and traffic management. Specific objectives were to:
- identify sediment sources, transport pathways and deposition in harbours;
- characterise the relation between sediment components and transfer mechanisms (particularly bottom turbulence);
- develop a generic model of sediment development and pollutant concentrations.
Through experimental work in the harbours of Göteborg (Sweden), Bergen (Norway) and Ventspils (Latvia), H-SENSE has made recommendations on routine and specialist techniques for surveying sediment - including methodological developments resulting from this project.
Geochemical databases were established for the three test harbours. These included the vertical distribution of sediment as well as spatial variations, since dredging and ship turbulence can affect sediment at considerable depth and thereby re-mobilise old contaminants. A new system for comparing contaminated sediments with different composition was proposed, since current classifications used by most harbour authorities are not sensitive to the mobility and toxicity of elements in different phases.
Three modelling approaches were developed and compared for the spatial prediction of clay distribution, harbour bed conditions, zinc pollution and sediment thickness. Further work will be needed to take these tools into general application. GIS modelling may reduce the cost of monitoring schemes currently used by harbour authorities.
Hydrographic surveying and the application of sediment data are important for improved dredging management and environmental protection. Many harbour sediments contain industrial and domestic pollutants, which could be released as a result of harbour operations or expansion. H-SENSE has provided methods permitting the most cost-effective sampling techniques and the most appropriate sediment management strategies to be applied.
The uncertainties in the distribution of sediment contamination, and in the significance of the various chemicals in given concentrations, favour the application of a risk-based approach to environmental protection. The generic modelling developed by H-SENSE provides a basis for the audit trail that is critical to such risk-based management.
One of the main barriers to sediment modelling is the lack of suitable input data. H-SENSE recommended that harbours review their procedures and strive to construct archives containing the type of data shown to be valuable for modelling. Routine monitoring in connection with maintenance dredging, as well as specific construction and remediation projects, will, if planned appropriately, provide the basic requirements. This would substantially reduce the costs of modelling.