By applying a methodology useful for analysis of complex fluids based on a synergistic combination of experiments, computer simulations, and theoretical investigation, a model was built to investigate the fluid dynamics of granular flows in an intermediate regime where both collisional and frictional interactions may affect the flow behavior. In Part I, the viscoelastic behavior of nearly identical sized glass balls during a collision have been studied experimentally using a modified Newton's cradle device. Analyzing the results of the measurements, by employing a numerical model based on finite element methods, the viscous damping coefficient was determined for the glass balls. Power law dependence was found for the restitution coefficient on the impact velocity. In order to obtain detailed information about the interparticle interactions in dense granular flows, a simplified model for collisions between particles of a granular material was proposed to be of use in molecular dynamic simulations, discussed in Part II.