The relocation of dredge material within near-shore coastal environments constitutes a major element of commercial dredging operations, with vast resources assigned to environmental monitoring and assessments. However, key parameters such as turbidity and suspended sediment concentration (SSC) are both temporally and spatially variable, requiring complex monitoring approaches. This study developed a comprehensive approach to effectively monitor dredge material relocation and the subsequent deposition in Moreton Bay, Queensland, Australia. We report continuous turbidity and velocity data obtained within a dredge disposal area over an annual cycle to address the long-term evolution of the site. This long-term mooring is equipped with a live stream to update stakeholders on environmental conditions before and after dredge activities. In addition, short-term deployments of a “near bottom monitoring system” is used to characterize the bottom layer during dredge and high-wind events. The monitoring frame consists of a Laser In-situ Scattering and Transmissometry (LISST) instrument, a high-frequency acoustic Doppler profiler and a newly designed high density water sampling system.
Our findings indicate that both sub-seasonal and seasonal fluxes contribute significantly to observed turbidity levels during dredge periods. Long-term current velocity profiling is a notable advancement for dredge monitoring methodologies, with the ability to provide net transport of material through the site. Furthermore, the influence of dredging is most prominent, with near bottom SSCs 300 mg/l higher than those observed during non-dredge periods. The development of the near bottom monitoring system also provides in-situ particle size estimates, with a significant shift in median particle diameter (D50<100 μm) directly correlated with dredge placement operations and material entrainment during elevated wind periods.
The employed methods allow for the assessment of suspended material quantities driven by both dredging and site-specific hydrodynamics. This is a clear advantage to previous monitoring efforts as it reduces the ambiguity surrounding the influence of dredge operations in the area. Furthermore, the live stream turbidity updates enable dredge operators to assess the influence of day to day operations, with the ability to adapt dredge placement operations in accordance with imposed water quality limits. Although this monitoring method is time-intensive, it has the potential to set new standards in dredge activity assessment and to help develop sustainable dredge strategies in sensitive coastal areas.