Hydrodynamic behavior of wave responses and wave forces induced by piston-modal resonance in the narrow gap formed by a ship section in front of a bottom mounted terminal is investigated based on the OpenFOAM® package. Numerical simulations suggested that the free surface piston-modal oscillations in the narrow gap have closely relationship with the vertical velocity along the gap bottom, implying the velocity flow around the gap bottom is able to affect the wave amplitude in the narrow gap, significantly. The horizontal wave forces can be decomposed into static water forces due to the difference of water level besides the box and the dynamic water forces due to the velocity flow. The amplitudes of static water forces are larger than that of horizontal wave forces, implying that the dynamic water forces can counteract part action of the static water forces. The dynamic wave forces approach to the maximal values at resonant frequencies, indicating that the extreme flow velocity around resonant frequency is useful for reducing the horizontal wave forces.