The influence of flocculation in turbid plumes from mechanical and hydraulic dredging

IF 2.1 3区 地球科学 Q2 OCEANOGRAPHY Continental Shelf Research Pub Date : 2024-06-01 DOI:10.1016/j.csr.2024.105263
Andrew M. Symonds , Paul L.A. Erftemeijer , Rachel E. White , Federico Pastorelli
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Abstract

The process of flocculation in dredge plumes was investigated by field measurements using Laser In-Situ Scattering and Transmissometry (LISST), Optical Back Scatter (OBS) sensors and an Acoustic Current Doppler Profiler (ADCP) during dredging campaigns in Port Curtis, Queensland (Australia). Suspended sediment characteristics differed significantly between natural ambient suspended sediment and sediment suspended by dredging as well as between sediment suspended by mechanical and hydraulic dredgers. LISST measurements enabled distinction of the suspended sediment by origin and showed that the majority of sediment particles naturally in suspension were aggregated flocs made up of multiple smaller particles. Primary particles in natural suspended sediment had a median particle size (d50) of 8 to 14 μm, while in-situ flocs had a d50 of 52 to 99 μm. There were both microflocs and macroflocs present in the natural suspended sediment, with the microflocs being dominant (representing 67%–80% of the flocs). Mechanical dredging resulted in the release of individual particles, microflocs and macroflocs, with the potential for a comparable percentage of macroflocs as in natural suspended sediment (11%–27% compared to 20%–33% in natural suspended sediment). Macroflocs released by mechanical dredging were likely to have originated from the seabed. During overflow from the hopper, the suspended sediment released from the hydraulic dredger had smaller flocs compared to the natural suspended sediment, with a reduction in the d50 of in-situ flocs from 91 to 67 μm and a reduction in the proportion of macroflocs from 32% to 24%. These results indicate that larger macroflocs present on the seabed had been broken up by the hydraulic dredging, with most of the suspended sediment remaining in the form of smaller microflocs, suggesting that the flocs were not completely disaggregated by the shear-stress generated during hydraulic dredging. Ongoing aggregation of suspended sediment was shown to occur in the sediment suspended by both dredging methods within 20–25 min of the plumes being generated. This study demonstrates the importance of considering flocculation in predictive dredge plume modelling studies to prevent significantly underestimating the settling velocity and substantially over-estimating the extent and intensity of dredge plumes if flocculation is not accounted for.

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机械和水力疏浚产生的浑浊羽流中絮凝物的影响
在澳大利亚昆士兰州柯蒂斯港进行疏浚作业期间,通过使用激光原位散射和透射测量法(LISST)、光学背散射(OBS)传感器和声学海流多普勒剖面仪(ADCP)进行实地测量,对疏浚羽流中的絮凝过程进行了研究。自然环境中的悬浮沉积物与疏浚造成的悬浮沉积物之间,以及机械疏浚船和水力疏浚船造成的悬浮沉积物之间,悬浮沉积物的特征存在很大差异。通过 LISST 测量,可以按来源区分悬浮沉积物,并显示自然悬浮的沉积物颗粒大多是由多个较小颗粒组成的聚集絮团。天然悬浮沉积物中原始颗粒的中值粒径(d50)为 8 至 14 微米,而原位絮凝体的 d50 为 52 至 99 微米。天然悬浮沉积物中既有微絮状物,也有大絮状物,其中以微絮状物为主(占絮状物的 67%-80% )。机械疏浚会释放出单个颗粒、微絮凝体和大絮凝体,其中大絮凝体所占比例可能与天然悬浮沉积物中的比例相当(11%-27%,而天然悬浮沉积物中的比例为 20%-33%)。机械疏浚释放的大絮凝物可能来自海床。与天然悬浮沉积物相比,水力挖泥船从料斗溢出时释放的悬浮沉积物中的絮凝物较小,原位絮凝物的 d50 从 91 微米降至 67 微米,大絮凝物的比例从 32% 降至 24%。这些结果表明,海底较大的大型絮凝体已被水力疏浚分解,大部分悬浮沉积物仍以较小的微絮凝体形式存在,这表明絮凝体并未完全被水力疏浚过程中产生的剪应力分解。在两种疏浚方法产生羽流后的 20-25 分钟内,悬浮沉积物会不断聚集。这项研究表明,在预测性疏浚羽流建模研究中考虑絮凝的重要性,以防止在未考虑絮凝的情况下大大低估沉降速度,以及大大高估疏浚羽流的范围和强度。
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来源期刊
Continental Shelf Research
Continental Shelf Research 地学-海洋学
CiteScore
4.30
自引率
4.30%
发文量
136
审稿时长
6.1 months
期刊介绍: Continental Shelf Research publishes articles dealing with the biological, chemical, geological and physical oceanography of the shallow marine environment, from coastal and estuarine waters out to the shelf break. The continental shelf is a critical environment within the land-ocean continuum, and many processes, functions and problems in the continental shelf are driven by terrestrial inputs transported through the rivers and estuaries to the coastal and continental shelf areas. Manuscripts that deal with these topics must make a clear link to the continental shelf. Examples of research areas include: Physical sedimentology and geomorphology Geochemistry of the coastal ocean (inorganic and organic) Marine environment and anthropogenic effects Interaction of physical dynamics with natural and manmade shoreline features Benthic, phytoplankton and zooplankton ecology Coastal water and sediment quality, and ecosystem health Benthic-pelagic coupling (physical and biogeochemical) Interactions between physical dynamics (waves, currents, mixing, etc.) and biogeochemical cycles Estuarine, coastal and shelf sea modelling and process studies.
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