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The flow field uniformity of the vertical pump intake greatly affects the pump performance, especially for axial flow pumps due to the short distance between the pipe intake and pump intake. The floor-attached vortex forms between the sump bottom and pipe intake, creating spiral-shaped velocity zones. This study employs large-eddy simulation and fine resolution grid to numerically investigate the flow characteristics near a vertical hydraulic intake, and it is found that a persistent floor-attached vortex is created near the pipe intake. The vortex wanders on the horizontal plane, and its wandering positions is according with the Gaussian distribution. The wandering floor-attached vortex not only introduce rotational flow into the pump intake but also induce quasi-periodic disturbances in its intake flow field. Analyses of the turbulent kinetic energy and Reynolds normal stresses demonstrate that the streamwise and spanwise Reynolds normal stresses dominate turbulent disturbances in the floor-attached vortex region. By applying the proper orthogonal decomposition method to its turbulent fluctuation velocity fields, it is found that large-scale coherent structures induce the in-plane displacement of the floor-attached vortex.