*Article* **The Hydrodynamic Dispersion Characteristics of Coral Sands**

#### **Xiang Cui 1,2, Changqi Zhu 1,\*, Mingjian Hu <sup>1</sup> , Xinzhi Wang <sup>1</sup> and Haifeng Liu <sup>1</sup>**


Received: 10 July 2019; Accepted: 24 August 2019; Published: 28 August 2019

**Abstract:** Dispersion characteristics are important factors affecting groundwater solute transport in porous media. In marine environments, solute dispersion leads to the formation of freshwater aquifers under islands. In this study, a series of model tests were designed to explore the relationship between the dispersion characteristics of solute in calcareous sands and the particle size, degree of compactness, and gradation of porous media, with a discussion of the types of dispersion mechanisms in coral sands. It was found that the particle size of coral sands was an important parameter affecting the dispersion coefficient, with the dispersion coefficient increasing with particle size. Gradation was also an important factor affecting the dispersion coefficient of coral sands, with the dispersion coefficient increasing with increasing d10. The dispersion coefficient of coral sands decreased approximately linearly with increasing compactness. The rate of decrease was −0.7244 for single-grained coral sands of particle size 0.25–0.5 mm. When the solute concentrations and particle sizes increased, the limiting concentration gradients at equilibrium decreased. In this study, based on the relative weights of molecular diffusion versus mechanical dispersion under different flow velocity conditions, the dispersion mechanisms were classified into five types, and for each type, a corresponding flow velocity limit was derived.

**Keywords:** coral sands; porous media; model test; dispersion; mechanical dispersion; molecular diffusion
