Water Quality Risk Assessment for Dredging Operations, Plymouth Sound, UK

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Courtesy of Oracle

As part of a formal Environmental Impact Assessment for a port development project in Plymouth Sound, UK, the potential risk of a deterioration in water quality from capital dredging was informed by the application of a probabilistic risk approach. In the absence of UK statutory sediment quality guidelines (SQGs) to assess acceptability of dredging operations, non-statutory guidelines are routinely used for comparison. This is not considered a wholly satisfactory approach since non-statutory guidelines are not developed with the particular characteristics of UK waters in mind. An alternative approach has made use of empirical relationships for determining the partitioning of contaminants between the solid and dissolved phases where predicted dissolved contaminant concentrations were compared against statutory Environmental Quality Standards (EQSs) for marine water quality. This method made use of a probabilistic distribution of model parameters and assisted the regulatory authority to approve the project on the basis of a minimal risk of deterioration in water quality.

This paper discusses not only the technical aspects of probabilistic modelling but also, how such risk analysis can inform regulatory consenting procedures.

ABP Plymouth is one of 21 ports owned by Associated British Ports Holdings Group PLC within the UK. Located on the southwest coast of England (Figure 1) its primary business is to support passenger and commercial haulage movements to and from the European Continent through ferries operated by Brittany Ferries.

The procurement of a new ferry, the ‘Pont Aven’ by Brittany Ferries which was designed to be larger than the existing vessels, and hence requires greater manoeuvring area to maintain navigational safety, meant that various marine construction works would need to be completed to accommodate safe passage to and from her berth in Millbay Docks, Plymouth.

The works included capital dredging in Millbay Docks totalling about 40,000m³ of fine sediments to deepen the existing estuary bed to -7.6m Chart Datum (CD). The project would allow Brittany Ferries to meet demand for greater capacity on routes to both France and Spain and maintain the status of Plymouth as a major international ferry port. The capital dredging works were schedule within a three month winter window, although this had to be extended to allow for downtime due to ad-verse weather conditions.

In the UK, capital projects in the marine environment are subject to a range of environmental legislation and consents proce-dures. Application for marine consents including those under the Food and Environment Protection Act (FEPA) 1985 and Coast Protection Act 1949 (CPA) are required from government authorities before proceeding with construction activities be-low mean high water and the disposal of dredge material at sea (in the case of FEPA) and also to ensure safe navigation (in the case of CPA). Depending on the nature of the project consent is likely to be dependant upon completion and submission of a formal Environmental Impact Assessment (EIA) and, where European designated nature conservation sites are present, completion of an Appropriate Assessment (AA) in accordance with the European Union Habitats Directive (92/43/EEC).

Successful negotiation of the consenting procedures relies not only on fulfillment of the various assessment requirements in accordance with the relevant legislation to ensure that effects on the environment are acceptable, but more so on the ability to agree the findings of scientific investigations with the many statutory and non-statutory authorities who are stakeholders to the process. Failure to agree upon the results of environmental impact assessments with stakeholders can result in significant additional project costs from additional procedural requirements such as public inquiries and subsequent delays resulting in a high likelihood of refusal of any proposals.

The re-suspension and bioavailability of sediment bound contaminants in the water column is often an important consid-eration by regulators and statutory authorities when determining the acceptability of capital and maintenance dredging opera-tions. The risks associated with such operations are routinely a determining factor when stipulating developmental controls such as seasonal restrictions, monitoring thresholds and the overall project acceptability.

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