Treffer: A Numerical Modeling Framework for Flocculation and Cohesive Sediment Transport in the Wave Bottom Boundary Layer.

Flocculation, a critical process in coastal and estuarine systems, plays a significant role in sediment transport, nutrient cycling, and ecological health. This study develops a cohesive sediment transport modeling framework tailored to the wave bottom boundary layer under dilute and equilibrium conditions, explicitly incorporating flocculation effects via a Population Balance Equation (PBE). Using Direct Numerical Simulation, six baseline cases, each with a distinct sediment concentration profile resulting from a constant settling velocity and critical erosion shear stress, are generated to drive the PBE flocculation model for given floc yield strength and stickiness. Results reveal that flocculation significantly influences sediment concentration profiles promoting three distinct stages, well‐mixed, transition to lutocline, and well‐developed lutocline. At low concentrations with well‐mixed profiles, cohesive floc properties are less significant, and turbulence is a main flocculation driver. In contrast, as concentration increases, cohesive floc properties become crucial, facilitating lutocline formation. The analysis also highlights limitations of depth‐averaged settling velocity as a parameterization. It is suitable for well‐mixed and transitional profiles but fails in well‐developed lutoclines, where empirical formulations that explicitly incorporate turbulent shear rate and sediment concentration better capture variability. This study underscores the necessity of incorporating flocculation effects into sediment transport models to enhance predictions of sediment dynamics in wave bottom boundary layers. Plain Language Summary: In coastal and estuarine waters, tiny sticky particles often clump together to form larger ones called flocs, a process known as flocculation. This process, driven by turbulence, particle concentration, and natural organic matter, strongly affects how sediments move, how nutrients cycle, and how healthy these ecosystems remain. In our study, we created a new model to better understand flocculation near the seabed, where waves stir up sediments. Using high‐resolution computer simulations, we tested different sediment levels and found that at low concentrations, turbulence mainly controls flocculation, while at higher concentrations, the stickiness of the particles takes over, creating a dense sediment layer near the bottom (a lutocline). We also showed that a simple average method commonly used to predict how fast sediments settle only works under certain conditions, and empirical approaches that consider turbulence and sediment concentration are needed when lutoclines form. Overall, our work highlights the importance of including flocculation in sediment transport models to improve predictions of coastal and estuarine dynamics. Key Points: A new modeling framework incorporating flocculation for cohesive sediment transport in wave‐bottom boundary layers is developedHigh‐cohesion flocs may encourage the formation of a sharp negative concentration gradient and its displacement toward the bedDepth‐averaged settling velocity scales well at low concentration, while Teeter's formulation promises to capture variabilities [ABSTRACT FROM AUTHOR]

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