WAMSI 2 - Dredging Node - Theme 2 - Synthesis Report - Predicting and measuring the characteristic of sediments generated by dredging

Brief description

Dredge plumes are formed when dredging operations suspend rock and soil particles into the water column by mechanical, scouring and mixing actions and by direct discharges from dredging equipment. The size composition and settling velocity distribution of these suspended particles depends on the in situ characteristics of the material to be dredged and on the changes to the material which occur as it is worked by the dredging equipment. Estimation of the far-field suspended sediment source terms is a challenging task, particularly at the EIA stage of the dredging project proposal, given project design, engineering and geotechnical uncertainties at that stage. The two basic approaches used are: • empirical - using source term estimates previously derived from on board and dredge plume data sets collected under circumstances and conditions similar to those anticipated for the current dredging proposal • process-based - using calibrated and validated numerical models based on an understanding of the physical processes and input data that determine the far-field source terms. The environmental impact assessment (EIA) documentation associated with 15 dredging projects in Australia (12 from WA) were reviewed to examine the practices used to estimate suspended sediment source terms for input into far-field dredge plume prediction models. The review focused on the key source term contributions from CSD and TSHD dredgers. The reviews highlighted the importance of dredge-induced sediment suspension datasets being collected according to agreed protocols and methods so that source term calculations from these data sets can be reliably ranked and compared. Overall, the number of these datasets has increased significantly in recent years. However many of these are not publicly available and their availability (and potential use) is restricted. Also, there are some relatively common dredging situations (e.g. trailing suction hopper dredging with low under keel clearance) that are not well represented by the available datasets. The acquisition of high quality datasets, from both full-scale dredging operations and laboratory experiments, also leads to an improved understanding of the physical processes involved in the generation and release of dredged material particles and the early stages of plume formation. This enables the development of process-based source models as an additional means of estimating source terms. It was not possible to recommend numerical values for source term parameters to use under local conditions based on these reviews, due to the paucity of relevant field data against which to compare estimates. A number of recommendations were made to address this, including: • adoption of standard protocols for field data collection to evaluate source terms during project implementation stage; • establishment of a dredge source term data library that could be populated over time, to cover the different types of dredges used and the various geotechnical conditions encountered in capital dredging practice in Australia; • adoption of a consistent, transparent accounting method (Becker et al. 2015, van Eekelen 2015) for reporting source term estimates for dredge plume modelling as part of EIA.

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