sand jets and particle clouds are found in many engineering applications, such as marine bed capping, dredging material disposal, and discharge of wastewater. a coaxial jet is composed of an inner jet surrounded by an annular jet and is often employed as an effective method to mix two different flow streams. swirling jets form when an azimuthal velocity is added to the axial flow. adding a swirling motion or carrier fluid can influence the hydrodynamics of flow and enhance mixing efficiency. understanding the dynamics of sand particles and ambient water is important for proper design and optimizing coaxial and swirling jets in engineering systems. the coaxial sand-water or swirling impinging process can be used as an effective way to optimize the releasing condition and to enhance the placement of materials in ambient currents. in the first part of this thesis, detailed laboratory experiments were conducted to study the dynamics of gravity-driven finite amount of sand particles (i.e., particle cloud) vertically released downward into stagnant water. the importance of nozzle diameter, sand particle mass, and particle size in spreading and mixing of particles in water was studied, and the main parameters controlling particle motion and mixing were identified by the aspect ratio and stokes number. the axial and radial profiles of sand concentration and velocity were measured by an accurate and robust optical probe (pv6). practical formulations to estimate the decay rate of sand concentration and velocity were developed to assist the analysis and design of dredge disposal. parameters were calculated inside particle clouds to better understand the evolution of particle clouds in stagnant water.