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

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.

疏浚作业通过机械、冲刷混合作用以及疏浚设备直接排放，将岩土颗粒悬浮于水体中，由此形成疏浚羽流。这些悬浮颗粒的粒径组成与沉降速度分布，取决于待疏浚物料的原位特性，以及疏浚设备作业过程中物料发生的变化。

远场悬浮沉积物源项的估算极具挑战性，尤其是在疏浚项目提案的环境影响评价（Environmental Impact Assessment, EIA）阶段，彼时项目设计、工程与岩土工程相关的不确定性尚未明确。目前常用的两种基本方法为：
• 经验法：借鉴此前在与当前疏浚提案预期工况相似的条件下，基于现场及疏浚羽流数据集推导得到的源项估算结果；
• 基于过程的方法：依托对决定远场源项的物理过程与输入数据的认知，使用经过校准与验证的数值模型。

本次研究对澳大利亚境内15个疏浚项目（其中12个来自西澳大利亚州）的环境影响评价文档进行了综述，旨在考察用于估算远场疏浚羽流预测模型输入所需悬浮沉积物源项的实践方法。综述重点关注了绞吸式挖泥船（Cutter Suction Dredger, CSD）与耙吸式挖泥船（Trailing Suction Hopper Dredger, TSHD）的关键源项贡献。

综述结果凸显了按照统一协议与方法采集疏浚诱发沉积物悬浮数据集的重要性，唯有如此，方能基于这些数据集可靠地对源项计算结果进行排序与对比。总体而言，近年来此类数据集的数量已显著增长，但其中多数并未公开，其可获取性（及潜在应用）受到限制。此外，现有数据集未能充分覆盖部分较为常见的疏浚工况，例如龙骨下净空极低的耙吸式疏浚作业。

通过现场实船疏浚作业与室内试验获取高质量数据集，还有助于深化对疏浚物料颗粒产生与释放、羽流形成初期阶段所涉及物理过程的认知，进而推动基于过程的源项模型开发，作为估算源项的补充手段。

由于缺乏可用于比对估算结果的相关现场数据，本次综述无法针对当地工况推荐适用的源项参数数值。

为此，研究提出了多项解决方案，包括：
• 在项目实施阶段采用标准协议采集现场数据以评估源项；
• 建立疏浚源项数据库，随时间推移逐步充实内容，覆盖澳大利亚大型疏浚作业中使用的各类挖泥船与遭遇的各类岩土工程条件；
• 采用统一且透明的核算方法（Becker等，2015；van Eekelen，2015），作为环境影响评价的一部分，用于报告疏浚羽流建模所需的源项估算结果。