The Impact of Maintenance Dredging on Shallow Water Environments

Abstract:

Maintenance dredging is the process of regularly removing accreted sediment to maintain optimal conditions, such as dredging navigation channels for nautical depth to allow continued safe passage of vessels. This process can release vast amounts suspended sediment within a marine system, either through disposal of the sediment, or dredge equipment dragging along the seabed resulting in unintended effects such as smothering and burial of benthic organisms. If the sedimentary particles mobilised are predominantly cohesive in nature, they could potentially flocculate into larger, faster settling, lower density flocs throughout the water column, significantly impacting the resultant sediment transport processes in the area. This study focuses on the impacts that maintenance dredging has on a range of shallow water environments, such as shorelines and ports in the short term immediately after the dredging and long term, in the months that follow. This has been achieved through the assessment of case studies of maintenance dredging operations reported around the world to determine the different sedimentary, biological, contaminant-based and disposal related impacts to the local environments due to dredging. Assessments have been supported by relevant supplementary data to provide additional context and drawing conclusions. This study has found that key impacts on the environment were directly attributed to the removal of sediment from the seabed and the resulting plume rather than contaminants and trace metals released from the seabed into the plume such as cadmium and zinc. This has far-reaching implications, with one plume assessed observed transiting up to 2.4km from the dredge site. This was especially true for the effects on the native biological community, where the reduction of light levels due to a plume were observed reducing populations of sensitive sea grasses Zostera capricorni and Halophila ovalis, and the demersal fish Menticirruhs americanus.