Collaborative Research: Understanding the physics of flocculation processes and cohesive sediment transport in bottom boundary layers through multi-scale modeling

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Due to climate change, sea level rise and anthropogenic development, coastal communities have been facing increasing threats from flooding, land loss, water quality, and other ecosystem challenges such as harmful algal blooms. Most of these pressing problems are directly or indirectly associated with sediment transport, some related to sands, but many are due to fine-grained sediments. Fine-grained sediments are cohesive and hence they transport as porous aggregates of particles, called flocs. Through their complex structures, flocs are vehicles of organic carbon, nutrients, contaminants and sometimes they can contain sand grains. Consequently, their settling velocities are very difficult to quantify. To date, most coastal/estuarine models neglect the flocculation process and adopt a constant settling velocity to estimate deposition of fine-grained sediments, which poses a considerable limitation of their predictive capability for the various challenges addressed above. In order to understand the fundamental dynamics of flocculation and their impact on fine-grained sediment resuspension and deposition, several integrated numerical simulations and optical-based laboratory observations across different scales will be carried out, including those associated with the particle size, water turbulence motions, and bottom boundary layer. Outcomes from the proposed research will be used to better equip coastal models with sediment transport capability to tackle challenges facing the coastal communities. The research findings will be widely disseminated to the coastal modeling community through participation in conferences and collaboration with the Community Surface Dynamics Modeling System (CSDMS). The open-source code to be developed and the data from the laboratory experiments will be disseminated through CSDMS and the flocculation formulation codes to be developed will be integrated into the CSDMS modeling framework via the recently released Python Modeling Toolkit (PyMT). In addition, a clinic on flocculation modeling is planned for the CSDMS annual meeting. This project supports 2 PhD students who will receive balanced training in coastal processes, fluid dynamics, high performance computing and laboratory techniques. The project also provides partial support for an early career postdoc researcher. Two undergraduate students will benefit from this project for their research on cohesive sediments. The project also strengthens collaboration with the United Kingdom and Germany on novel observational and computational tools.The primary goal of this collaborative study is to address key challenges of cohesive sediment transport in coastal/estuarine bottom boundary layers. The study utilizes a novel particle-resolved simulation model to investigate the physics of flocculation and floc structures for heterogeneous sediments. This effort is further augmented by laboratory experiments designed to better quantify stickiness and understand flocculation of sand-mud mixtures. Using a turbulence-resolving simulation model for fine sediment transport in a wave-current bottom boundary layer, coupled with enhanced flocculation formulations to model settling velocity, the investigators will study the interplay between flocculation, resuspension and deposition of cohesive sediments in coastal/estuarine bottom boundary layers. Five hypotheses are developed to guide the experimental and modeling work which will provide insight into key small-scale processes that are difficult to resolved in coastal models. Finally, by integrating and synthesizing these research outcomes, the study will evaluate a suite of closures for the settling velocity due to flocculation, from complex to simple, to inform coastal/estuarine modeling of cohesive sediment transport at regional scale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.