Numerical modelling of dredge plumes

Chaojiao Sun, Paul M. Branson

Research output: Book/ReportOther output

Abstract

Port and coastal developments often involve large-scale dredge activities and are subject to environmental impact assessment (EIA) as part of their approval. Fundamental to EIA is the evaluation of environmental impacts and judgements about the degree to which management strategies are likely to be effective at mitigating environmental impact risk. Numerical modelling using sediment transport models (coupled to hydrodynamics and wave models) can be used to predict the ecological pressure caused by dredging activities (through increases in suspended solids, sediment smothering and light reduction). The responses of the biological assemblages to these pressure fields are then predicted by biological response models. The likely extent, severity and persistence of environmental impacts associated with dredging is then predicted. Whilst fundamental to the prediction of environmental impact, dredge plume modelling at the EIA stage is challenging due to limited availability of information such as model input data and detailed information relating to the dredging campaign is often not known. There are significant benefits to improving the modelling process, enabling proponent’s greater clarity around key data requirements and standards for model calibration and validation, and providing environmental regulators greater confidence when making decisions using modelled outputs. The goal of the Numerical Modelling Project (Dredge Science Node Project 3.4) was to improve the prediction of the transport and fate of dredge-generated sediments by developing an improved understanding of key physical processes that control the extent, intensity and duration of sediment plumes. The project sought to improve the modelling the dynamics and fate of total suspended solids (TSS) in the passive dredge plumes by developing a hindcast model of a large-scale capital dredging campaign with the goal of improving the estimation of source terms for the passive far-field model, identify and improve the estimates of important model parameters, and calculating ecologically relevant pressure fields. The modelling approach was consistent with what is typically applied in EIA. A hindcast model of a large-scale capital dredging project located in the Pilbara region (Western Australia), the Chevron Australia Wheatstone Project was developed during the study. Effort was focused on identifying the most important processes and model parameters, in particular, the bed schematization and cohesive sediment transport model parameters. A broad range of data and its analysis was used to define the sediment transport model parameters and processes. Hydrodynamic and wave models were set up using the best available bathymetry and metocean forcing for the region. The models were calibrated and validated against available wave and current data and bed shear stress climatology analysed. A sediment transport model was set up using source terms derived from the analysis of dredging logs, and sediment characteristics were based on analysis of baseline and monitoring data. Model sensitivity to different parameters was analysed model parameters that are important, difficult to establish, or interdependent were identified. A hindcast of dredge plume evolution was produced. Once calibrated, the model was applied to assess the relative contributions of both the ambient and dredging fine sediment fractions with ecologically relevant pressure fields calculated.
Original languageEnglish
PublisherWestern Australian Marine Science Institution
Number of pages82
Publication statusPublished - 14 Oct 2018

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