Resuspension and clearing of dredge spoils after dredging, Cleveland Bay, Townsville, north Queensland

Field studies of the effects of dredging the 8km long Townsville navigation channel (Platypus Channel) were carried out in May 1989. The suction dredge was followed by a research vessel and the spoil plume, which formed in the lee of the dredge, was marked using a near-bottom sail drogue. Vertical profiles of temperature, salinity and suspended sediment concentration (SSC) were obtained next to the drogue. Cross sections of the suspended sediment density stratification in the water column next to the drogue and along transects perpendicular to the track of the dredge during dredging, were recorded using an acoustic sounder.In the laboratory, the settling characteristics of a composite sample of muddy silt from four areas of the harbour, that are dredged were investigated. Sediment was added to a column and subjected to turbulence. Floc sizes were measured and vertical profiles of SSC were obtained. The purpose of this research was to identify optimum conditions for dredging, that would result in the least damage to environmentally sensitive areas. Research focused on the fate of sediments, which were resuspended due to mechanical disturbance of the bottom during the process of dredging, the dredge's turbulence and the sediment released from the dredge's surface overflow. This research was undertaken to address concerns relating to the impacts of dredging and the dumping of dredge spoil in Cleveland Bay. Issues of concern included the possible effects of siltation on nearby coral reefs, seagrass beds and beaches and the potential effect of toxic metals carried in mud from Townsville Harbour.

1989年5月，针对全长8公里的汤斯维尔通航航道（鸭嘴兽航道，Platypus Channel）的疏浚效应开展了野外研究。吸扬式挖泥船（suction dredge）后方伴随科考作业船，挖泥船背流侧形成的疏浚弃土羽流，通过近底帆状浮标完成标记。研究人员在浮标旁获取了温度、盐度及悬浮泥沙浓度（Suspended Sediment Concentration，SSC）的垂向剖面数据。疏浚作业期间，借助声学测深仪（acoustic sounder）记录了浮标旁水体以及垂直于挖泥船航行轨迹的测线上的悬浮泥沙密度层化断面数据。实验室阶段，研究人员对取自该港口4处疏浚区域的浑粉砂复合样品的沉降特性开展了研究：将沉积物注入沉降柱并施加湍流环境，对絮体粒径进行测量，同时获取了SSC的垂向剖面数据。本研究的核心目标是确定疏浚作业的最优工况，以最大限度降低对环境敏感区域的损害。研究重点聚焦于疏浚作业过程中，因水底机械扰动、挖泥船湍流以及挖泥船表面溢流所再悬浮的沉积物的运移归宿。本次研究旨在回应克利夫兰湾（Cleveland Bay）内疏浚作业与疏浚弃土倾倒引发的一系列关切问题，其中包括淤积作用对周边珊瑚礁、海草床及海滩的潜在影响，以及汤斯维尔港港泥中携带的有毒金属所带来的潜在生态风险。