Settling and Drag of Aquatic Plant Seeds in Still Water

Flotation and settling of aquatic plant seeds are key processes for dispersal, colonization and succession of riparian and lacustrine vegetation. Studying these processes is crucial for understanding plant community regeneration, guiding wetland restoration and maintaining aquatic ecosystem stability. Despite significant progress in nonspherical particle settling research, current drag coefficient models were largely developed for sediment or volcanic particles and lack validation for biological seeds across a wide range of flow conditions. Furthermore, the quantitative link between easily measurable shape descriptors—such as the Corey Shape Factor ((Formula presented.)) and sphericity ((Formula presented.))—and the drag coefficient ((Formula presented.)) of aquatic plant seeds has not been systematically clarified. To address this gap, systematic flotation and settling experiments on the seeds from four typical aquatic plants (Nelumbo nucifera, Nymphaea spp., Acorus calamus and Nymphoides hydrophylla) in still water were carried out, revealing distinct settling patterns driven by seed morphology and density. The main novel findings of this study are as follows: (i) the introduction of a unified predictive model for (Formula presented.) that incorporates (Formula presented.) and (Formula presented.) as simple, measurable shape descriptors; (ii) the identification of a critical hydrodynamic transition at particle Reynolds number (Formula presented.) = 10, leading to a piecewise model structure that significantly improves prediction accuracy across different flow regimes; and (iii) rigorous model development using a large literature dataset of 395 settling measurements ((Formula presented.) range 0.03–10,000) and validation on an independent dataset of 59 measurements. The proposed model demonstrates high predictive accuracy and broad applicability, providing a reliable tool for simulating seed dispersal and supporting wetland vegetation restoration and aquatic plant population management.