This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. 2-D sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The discrete element method (DEM) technique is coupled with the lattice Boltzmann method (LBM), for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using multiple-relaxation-time LBM (LBM-MRT) for numerical stability. In order to simulate interconnected pore space in 2-D, a reduction in the radius of the grains (hydrodynamic radius) is assumed during LBM computations. The collapse of granular column in fluid is compared with the dry cases to understand the effect of fluid on the runout behaviour. A parametric analysis is performed to assess the influence of the granular characteristics (initial packing) on the evolution of flow and run-out distances for slope angles of 0°,2.5°,5° and 7.5°. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass. The mechanism of energy dissipation, shape of the flow front, water entrainment and evolution of packing density is used to explain the difference in the flow characteristics of loose and dense granular column collapse in fluid.