演 講 公 告

講題：海岸環境中的黏性沉積物傳輸——絮凝作用的重要性
Cohesive sediment transport in coastal environments – significance of flocculation dynamics 
 

主講人：美國德拉瓦大學土木與環境工程系　徐天健教授

Department of Civil, Construction, and Environmental Engineering, and Center for Applied Coastal Research, University of Delaware 

時間：3月25日（星期三）下午2點 - 3點30分

地點：中央大學科學一館S-325教室

摘要：

　　Flocculation describes the process when fine-grained sediments (clay and silt) are mixed with seawater, they become cohesive and form larger aggregates. The cohesion can also come from Extracellular Polymeric Substance (EPS) due to high organic content. Flocculation is important as it controls the settling velocity of cohesive sediment and allows porous aggregate to carry organic carbon, pollutant, and nutrients.

　　Through laboratory experiments and multi-scale numerical modeling, this talk will discuss several major research findings. Firstly, through comprehensive laboratory jar tests and settling column experiments, we report evidence that bio-cohesion significantly reduces the variability of settling velocity of cohesive sediments. This finding provides important evidence for practical (or simple) modeling of fine sediment settling velocity in environments with high organic content. Secondly, we study how organic carbon can form various oil-mineral aggregates and dictate the fate of spilled oil. Thirdly, through numerical modeling of flocculation with population balance equations and boundary layer sediment transport, we reveal that the cohesion properties of flocs play an important role in determining the vertical profile of cohesive sediment in the current and wave bottom boundary layer.

　　Through collaboration with Dr. Zhi-Cheng Huang, our analytical methods allow further interpretation of field data collected at Datan Algal Reef that bio-cohesion causes sediments to be flocculated and the resulting settling velocity is about 0.5 mm/s with relatively low variability across different floc size classes. These findings further inspire new field work planned for 2026~2027 to study the effect of flocculation on the vertical distribution of cohesive sediment in the bottom boundary layer in the Delaware River Estuary with emphasis on seasonal and spring-neap variabilities.