Modelling suspended sediment transport on Australias North West Shelf.

The distributions and movements of sediments on the North West Shelf have a major influence on the ecosystem, primarily through their effect on primary production and benthic habitat distributions. They are also highly relevant to human activities on the NWS, particularly construction and maintenance of offshore structures, dredging, and other coastal developments involving habitat modification. This study provides a general characterisation of the distributions of suspended and bottom sediments based on available data. These data confirm the existence of a high turbidity zone extending from the coastline to around the 20 m isobath. Bottom sediments also show a marked cross shelf zonation, with gravel and sand dominating the inner shelf, and finer sand, silts and clays dominating the less energetic outer shelf. A three dimensional sediment transport model was developed to estimate major resuspension, deposition, and suspended transport patterns over the NWS. The simulations covered the three years 1996, 1998, and 1999. They successfully reproduced the winnowing of fine sediments (i.e. very fine sand, silts and clays) in the relatively high-energy environment of the mid shelf, and their subsequent export onto the continental slope. The model also predicted a persistent strip of elevated suspended sediment concentrations along the inner shelf. Near surface suspended sediment concentrations in this zone typically increased by an order of magnitude from the neap to spring tide. Long-termed average sediment fluxes were directed southwest along the shelf and off the shelf. While there were significant uncertainties associated with these fluxes, the results suggested that sediment losses from the shelf exceeded the mean annual river loads by at least a factor of three over the simulation period. The model also predicted major sediment transport events during tropical cyclones. Modelled off shelf loads during Tropical Cyclone Bobby far exceeded both annual river loads and annual loads during non-cyclone conditions. These simulations also demonstrated how cyclone induced ocean eddies could capture large quantities of suspended sediments and carry them offshore to be deposited within highly localised patches in the deeper ocean. While there were various limitations in the model formulation, its predictions were broadly consistent with the limited available observations. It is suggested that further progress in sediment modelling on the NWS should be based on extended data sets for sediment processes, such as grain size distributions, flocculation, consolidation, and river loads, as well as improved wind and wave products, and inclusion of bedload transport.

西北陆架（North West Shelf，后文简称NWS）沉积物的分布与运移对该区域生态系统具有重要调控作用，其影响主要通过改变初级生产（primary production）与底栖生境（benthic habitat）分布实现。同时，沉积物的分布与运移也与NWS海域的人类活动密切相关，尤其涉及近海构筑物的建设与维护、疏浚作业，以及其他涉及生境改造的沿海开发项目。
本研究基于现有观测数据，对NWS海域的悬浮沉积物与底质沉积物的分布特征进行了系统性概括。现有数据证实，存在一条从海岸线延伸至约20米等深线（isobath）的高浊度带。底质沉积物同样呈现显著的跨陆架分带特征：内陆架以砾石与砂质沉积物为主，外陆架水动力条件较弱，以细砂、粉砂与黏土沉积物为主。
本研究构建了三维沉积物输运模型，用于模拟NWS海域的主要再悬浮（resuspension）、沉积及悬浮输运过程格局，模拟时段涵盖1996、1998及1999年三个完整年份。模型成功复现了中陆架强水动力环境下细颗粒沉积物（含极细砂、粉砂与黏土）的簸选（winnowing）过程，以及这些沉积物随后向大陆坡（continental slope）的输运过程。该模型同时预测，内陆架沿线存在一条持续存在的悬浮沉积物浓度升高带。该区域近表层悬浮沉积物浓度通常会从小潮（neap tide）期至大潮（spring tide）期升高一个数量级。
长期平均沉积物输运通量（sediment flux）整体沿陆架向西南方向及向外海方向输运。尽管该输运通量存在显著不确定性，但模拟结果显示，在模拟时段内，NWS海域陆架的沉积物流失量至少是多年平均河流输入量的3倍以上。该模型还预测了热带气旋（tropical cyclone）引发的大型沉积物输运事件：热带气旋“鲍比（Tropical Cyclone Bobby）”期间的模拟跨陆架输运量，远高于年度河流输入量及非气旋期的年度输运量。本次模拟还揭示了热带气旋诱导的海洋涡旋如何捕获大量悬浮沉积物，并将其携带至外海，最终在深海区域形成局地性沉积斑块。
尽管该模型在构建过程中存在诸多局限，但其预测结果与有限的现有观测数据整体吻合度较高。研究建议，未来NWS海域的沉积物模拟研究应基于更完善的沉积物过程观测数据集（如沉积物粒度分布、絮凝作用（flocculation）、固结作用（consolidation）及河流输入量等），同时优化风场与浪场数据，并纳入推移质输运（bedload transport）过程。