Exo Environmental have been working for the past year on the management of a maintenance dredging project, which has the added benefit of providing an opportunity to restore lost areas of saltmarsh. The harbour requires dredging to remove in excess of 100,000m³ of accumulated sediment, thereby clearing the navigation channels. The surrounding area, which is under the protection of several national and international designations, is recognised for saltmarsh and associated habitats. Yet many of the marshes are declining or in unfavourable condition, while coastal saltmarsh plays an important role in coastal defence and carbon sequestering. Given that the habitat is disappearing rapidly and is of such importance, the UK government has set targets to create new areas while achieving no net loss, therefore saltmarsh enhancement is a nationally important aspect of this project.
Dredging projects such as this can change the environmental quality, therefore monitoring was undertaken to establish an understanding of the baseline conditions and thence to assess environmental change and potential impact caused by the dredging works. Monitoring also allows for the assessment of compliance with permit, licence, legal and contract requirements and to help calibrate and validate the hydrographic model generated which will be used to predict the effects of the dredging activities.
The project has reached the end of the baseline monitoring stage, allowing a good understanding of the local environment and the measurement of chemical, physical and biological parameters, of water, sediment and ecology. Four stations were set up in the harbour area to measure tidal stream data and water quality, which collected data for 14 days at each location. Each station combined an Aquadopp® complete profiler system with an In-Situ TROLL 9500 multi-parameter water quality sonde.
A ‘van Veen’ grab sampler was used to collect samples of the sediment from the bed at five different locations to allow for bed material characterisation, following homogenisation of the sediment by the University of East Anglia laboratory. This information was used as input date for the bed friction as part of the hydrodynamic model.